Free Access
Issue
A&A
Volume 620, December 2018
Article Number A140
Number of page(s) 66
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201834105
Published online 11 December 2018

© ESO 2018

1. Introduction

Luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs) are galaxies with infrared (IR) luminosities exceeding 1011 L and 1012 L, respectively. LIRGs and ULIRGs are normally found to be gas-rich galaxy mergers, as tidal torques can funnel material from kpc scales to the innermost regions of the galaxy and trigger intense star formation and/or AGN activity (e.g., Hernquist 1989; Sanders 1999; Di Matteo et al. 2005), the latter more significantly so with increasing IR luminosity (e.g., Valiante et al. 2009; Petric et al. 2011; Alonso-Herrero et al. 2012).

These objects, common at redshifts 1−3 where the peak of star formation in the Universe is observed, represent a very important stage in galaxy evolution (e.g., Casey et al. 2014). The scenario proposed by Sanders et al. (1988) and Hopkins et al. (2005), for example, indicates that after a complete obscuration phase of the merger, ULIRGs in a late stage of the interaction would later disperse or consume the gas and probably evolve into an obscured type II quasar (QSO), and eventually into an exposed QSO. This process will ultimately lead to the formation of an elliptical galaxy, and accounts for the growth of the central supermassive black hole (e.g., Sanders et al. 1988; Hopkins et al. 2009).

In agreement with this scenario, recent studies of pairs of galaxies have found that the fraction of dual AGN grows with decreasing separation between companions (e.g., Ellison et al. 2011; Satyapal et al. 2014, 2017; Silverman et al. 2011; Koss et al. 2012). More specifically, in a sample of LIRGs and ULIRGs, Stierwalt et al. (2013) found an increase in the fraction of composite systems with merger stage. Satyapal et al. (2014) found larger fractions of IR-selected AGN with respect to optically selected AGN in mergers, which is likely due to the increase of obscuration. Evidence of an excess of AGN with high obscuring column densities in mergers are also found in recent works (e.g., Díaz-Santos et al. 2010; Kocevski et al. 2015; Del Moro et al. 2016; Lanzuisi et al. 2015; Ricci et al. 2017).

X-ray observations are an ideal tool for analyzing the properties of the inner regions of such obscured objects, because the gas and dust have a higher transparency than at larger wavelengths. Previous studies of small (e.g., Franceschini et al. 2003; Ptak et al. 2003; Teng et al. 2005) and larger (e.g., Teng & Veilleux 2010; Iwasawa et al. 2011; Ricci et al. 2017) samples of ULIRGs have highlighted the potential of X-rays in distinguishing the contribution of AGN and starburst and the ability to detect enshrouded AGN.

One of the recent works, C-GOALS I (Chandra-GOALS I, Iwasawa et al. 2011), is an X-ray study performed with the Chandra X-ray Observatory (Chandra, hereafter, Weisskopf et al. 2000) of a complete sample of LIRGs within the Great Observatories All-Sky LIRG Survey (GOALS, Armus et al. 2009). GOALS is a multi-wavelength study of the brightest IR galaxies in the local Universe, a low-redshift subsample of the 60 μm flux selected IRAS Revised Bright Galaxy Sample (RBGS, Sanders et al. 2003). The GOALS galaxies, all at z < 0.088, are perfect laboratories for multi-wavelength studies of LIRGs with a level of detail that only the observation of local galaxies allows. The arcsecond resolution provided by Chandra can offer information of individual galaxies within mergers, and help distinguish previously undetected or unresolved AGN, in particular, complementing studies of LIRGs and ULIRGs at harder X-rays (e.g. Ricci et al. 2017).

The C-GOALS I paper presents data obtained by us and others with Chandra and represents the X-ray component of the multi-wavelength survey for the most luminous IR GOALS sources. This work, C-GOALS II, extends the X-ray study to a subsample of the lower luminosity range of GOALS galaxies. These data were obtained during Chandra cycle 13 (PI: Sanders), combined with available archival data. The extension of the X-ray sample is motivated by the interest in reaching completeness in all wavelengths for the GOALS sample, and by the opportunity of comparing results derived at different IR luminosity ranges. The sample contains galaxies at earlier merger stages, contributing to the expansion of previous studies into the domain of the less luminous LIRGS. In particular, Iwasawa et al. (2011) observed a deviation in the correlation between IR and X-ray luminosities in nearby star-forming galaxies (e.g., Ranalli et al. 2003; Grimm et al. 2003; Mineo et al. 2014) for the galaxies in the C-GOALS I sample. The IR luminosities of galaxies in the C-GOALS II sample fall into the range where this change of behavior should occur, and are ideal to further study the reasons for and possible implications of such deviation.

The C-GOALS II sample is described and compared to the C-GOALS I sample in Sect. 2. The observations and data reduction are described in Sect. 3. Results, including all X-ray images, fluxes, spectra, and radial surface brightness profiles, are presented in Sect. 4, while derived properties and discussion of the X-ray and IR luminosity correlation are presented in Sect. 5. Finally, we summarize our conclusions in Sect. 6. Notes on individual objects can be found in Appendix A, and X-ray contours, detailed images in the 0.5−2 and 2−7 keV bands, along with radial surface brightness profiles for each source, can be found in Appendix B.

2. Sample

GOALS (Armus et al. 2009) is a comprehensive study of 201 of the most luminous IR-selected galaxies in the local Universe. The sample consists of 179 LIRGs and 22 ULIRGs, 85 of which are systems that contain multiple galaxies. GOALS is drawn from the IRAS RBGS (Sanders et al. 2003), with a luminosity threshold of LIR ≥ 1011 L. The RBGS is a complete sample of galaxies, covering the whole sky, that have IRAS 60 μm flux densities above 5.24 Jy and Galactic latitude |b|≥5°.

Iwasawa et al. (2011) studied a subsample of GOALS, C-GOALS I (hereafter, also CGI), which is complete in the higher IR luminosity end of the GOALS sample (log(LIR/L)=11.73−12.57). It contains 44 systems in the redshift range z = 0.010−0.088. The new sample, C-GOALS II (hereafter, also CGII), is an incomplete subsample of the lower luminosity section of GOALS, and includes all sources in the log(LIR/L)=11.00−11.73 range with available Chandra data, as of January 2016. It is comprised of 63 systems, 30 of which contain multiple galaxies. The redshift range of the new sample is z = 0.003−0.037. The distribution of IR luminosities and distances of the two samples is shown in Fig. 1. Table 1 gives basic parameters for all the objects in the C-GOALS II sample. We note that names and positions refer to the IR detected systems. Decomposition into individual galaxies is taken into account in Sect. 4.

thumbnail Fig. 1.

Distribution of luminosity distance (left panel) and IR luminosity LIR(8−1000 μm) (right panel) for the 44 objects of C-GOALS I (Iwasawa et al. 2011), the 63 objects of C-GOALS II, and the 201 systems of the full GOALS sample (Armus et al. 2009). The vertical dashed line represents LIR = 1012 L, the boundary between LIRGs and ULIRGs.

Open with DEXTER

Table 1.

Basic parameters of the objects in the C-GOALS II sample.

Figure 1 also shows the incompleteness of CGII, comparing it with the full GOALS distribution of distances and luminosities. Of the 63 systems within CGII, 31 were observed through the same proposal, which was drawn to be representative of all possible merger stages. For the remaining 32 systems, data were taken from the archive according to availability. The proposal for which observing time was awarded varies in each case, and all target different scientific goals (e.g., study of AGN, SFR, and X-ray binaries). For this reason, we do not expect our subsample to be biased toward a certain type of object, merger stage, or luminosity within the parent GOALS sample.

3. Observations and data reduction

Thirty-one systems were observed with Chandra in cycle 13 (PI: Sanders) with a 15 ks exposure on each target, carried out in imaging mode with the ACIS-S detector in VFAINT mode (Garmire et al. 2003). For the remaining 32 objects studied in this work, Chandra data were obtained from the archive. Exposure times for these targets varied from 4.88 to 58.34 ks, all taken with the ACIS-S detector in either FAINT or VFAINT mode. Table 2 shows the observation log for the whole CGII sample, as well as the total source counts in the 0.5−7 keV band for each object, obtained from the data analysis. The counts were derived for individual galaxies, and summed together when an object within the CGII sample contained more than one galaxy.

Table 2.

Chandra observation log for the objects in the CGII sample.

The data reduction was performed using the Chandra data analysis package CIAO version 4.7 (Fruscione et al. 2006), and HEASARC’s FTOOLS (Blackburn et al. 1995). The cosmology adopted here is consistent with that adopted by Armus et al. (2009) and Iwasawa et al. (2011). Cosmological distances were computed by first correcting for the three-attractor flow model of Mould et al. (2000) and adopting H0 = 70 km s−1 Mpc−1, ΩV = 0.72, and ΩM = 0.28 based on the five-year WMAP results (Hinshaw et al. 2009), as provided by the NASA/IPAC Extragalactic Database (NED).

4. Results

Results of the X-ray analysis of the Chandra data are presented in Table A.1. For each galaxy we present the background-corrected ACIS-S X-ray soft band (S, 0.5−2 keV) count rate and X-ray hard band (H, 2−7 keV) count rate, the hardness ratio or X-ray color, estimated X-ray fluxes and luminosities in both soft and hard band, and the logarithmic ratio of each X-ray band to the IR luminosity listed in Table 1, LIR(8−1000 μm). X-ray color, or hardness ratio, is computed as HR = (HS)/(H + S), using the bands defined previously.

Individual galaxies belonging to the same GOALS system (i.e., contributing to one single IRAS source) are identified by using the same GOALS number in the first column. Source names shown in the second column are used throughout this work; see Appendix A for a clarification on the identification of each component.

The hard X-ray flux (FHX) listed in Table A.1 is in the 2−7 keV band, where Chandra is more sensitive; and the listed hard X-ray luminosities (LHX) refer to the 2−10 keV band. Spectral fitting to derive the fluxes is performed in the 2−7 keV range as described in Sect. 4.4, and the fitted models are later used to estimate the luminosity up to 10 keV, in order to compare the derived results to those of previous works, in which the 2−10 keV band is used.

Although significant intrinsic absorption in dusty objects such as LIRGs is likely present, X-ray luminosities were estimated by correcting only for galactic absorption. The X-ray spectra of our galaxies are complex, containing multiple components, with different degrees of obscuration, as explained in Sect. 4.4. As the estimated absorbing column density values are heavily model dependent, we did not use them to correct the luminosities listed in Table A.1.

As mentioned in Sect. 2, many of the LIRGs in the CGII sample are composed of multiple galaxies, which are associated with a single GOALS object, as IRAS is unable to resolve them. All spatially resolved components in the Chandra data are presented separately, their count rates, fluxes, and luminosities were computed individually. In order to obtain the X-ray to IR ratios listed in Table A.1, the IRAS flux associated with each object must be appropriately separated into the corresponding contribution of each component. This separation was carried out according to the best possible estimate available for each source, as listed in Table 3. The most accurate estimation would be derived by obtaining the separate contribution of each component from the far-infrared (FIR) emission. When possible, this was done using Herschel photometric data (Chu et al. 2017). However, 14 of the multiple systems in the sample are unresolved by Herschel, and thus the MIR Spitzer MIPS 24 data were used for this purpose.

Table 3.

IR fractions.

For four systems in the sample the individual components remain unresolved at MIR wavelengths, and other determinations were used, as specified in Table 3 and Appendix A.

Only objects that were detected in X-rays and contribute to at least 10% of the IR luminosity of the IRAS source were analyzed and are presented in this work. This cut means that out of the 63 GOALS systems in the sample, 84 individual galaxies are studied in CGII. No galaxy contributing < 10% to the IR has a strong X-ray emission, but in cases in which the source is detected in the Chandra data, it is specified in Appendix A. For all galaxies in pairs that are not included in the analysis, their contribution to the IR luminosity of the bright component is taken into account.

4.1. X-ray images

We show how the X-ray radiation is related to the optical and IR emission, by comparing the 0.4−7 keV brightness contours with HST, SDSS, or IRAC images according to availability, in this order of preference. Appendix B shows X-ray contours overlaid on HST-ACS F814W (I-band) images (Evans et al. in prep.) for 27 objects, overlaid on SDSS DR-12 i-band images (Alam et al. 2015) for 18 objects and overlaid on IRAC channel 1 images (Armus et al. 2009; Mazzarella et al., in prep.) for the remaining 18 objects.

The contours are taken from a 0.4−7 keV image, smoothed using a Gaussian filter with a dispersion of 1 arcsec, with the exception of NGC 5135, shown in Appendix B, for which a smoothing of 0.5 arcsec was used in order to preserve the two X-ray central peaks.

Eleven contour levels were defined, divided into ten equal logarithmic intervals, in the four different surface brightness ranges shown in Table 4. “Interval 1” was used for the majority of the sample. In order to outline lower surface brightness features in some sources, 11 contour levels starting at a lower surface brightness values were taken, as “Interval 2” or “Interval 3”. For a few systems, a higher lower surface brightness limit was taken in order to eliminate noisy features in the contours, defined as “Interval 4”. For the bright objects NGC 1068 and NGC 1365, 21 contour levels were used instead, in order to reflect the X-ray morphology appropriately. Appendix B contains information on which optical or IR image was used to overlay the X-ray contours on, and also on the Interval that we used for X-ray contour ranges.

Table 4.

X-ray contour ranges.

As the hard-band emission from all objects is generally more peaked and less intense than the soft-band emission, the contours mostly trace soft X-ray emission from the sources. For this reason, in sources for which one or more clear hard X-ray peaks are seen, these are marked with a green cross. We define a hard X-ray peak as point-source emission that clearly stands out from the rest of the photon distribution in the unsmoothed images. In cases where many point-like sources that are clearly not associated with any central source in the galaxy are present in an image, we opted not to mark them all individually. For a more detailed description of the X-ray emission in both bands, Appendix B also presents the smoothed and unsmoothed images in the 0.4−7 keV band, and smoothed images in the soft (0.5−2 keV) and hard (2−7 keV) bands, for all objects. An example of one of these images is shown in Fig. 2.

thumbnail Fig. 2.

X-ray images and surface brightness profiles for NGC 2146. North is up and east to the left. Similar figures for all 59 objects in the CGII sample are presented in Appendix B. Upper left: X-ray (0.4−7 keV) brightness contours (magenta) with marked hard X-ray peaks (green crosses) overlaid on optical/IR images. Upper right: Radial surface brightness profiles in the 0.5−2 keV band (open squares) and the 2−7 keV band (filled squares). Profiles have been centered using the brightness peak in the hard X-ray band, when clearly originating in the nucleus. We refer to Appendix A for ambiguous objects. Bottom: From left to right, unsmoothed and smoothed images in the 0.4−7 keV band, and smoothed images in the soft (0.5−2 keV) and hard (2−7 keV) bands. The pixel size is ∼0.5″ × 0.5″. The scale bar in the bottom left image represents 5″.

Open with DEXTER

4.2. X-ray spectra

Figure C.1 presents the X-ray spectra for all sources. Spectral data are shown separately for each object with more than one resolved component. Instead of showing the usual count rate spectra, which are data folded through the detector response, we present the Chandra ACIS spectra corrected for the detector response and converted into flux density units. This has the advantage of presenting the spectral properties without the need of spectral fitting, and facilitates comparison with other multi-wavelength data from GOALS. The flux density range for all spectra was set to be the same, two orders of magnitude, for consistent comparison. An example of one such spectrum is shown in Fig. 3.

thumbnail Fig. 3.

X-ray flux density spectra for MCG-03-34-064, obtained from the Chandra ACIS. Flux density in units of 10−14 erg s−1 cm−2 keV−1.

Open with DEXTER

This presentation introduces some uncertainty, particularly when a spectral bin is large enough, within which the detector response varies rapidly, for instance, for galaxies with only few counts. It should also be taken into account that even though these have been corrected for the detector effective area, the energy resolution of the detector is preserved, and therefore they are independent of any spectral model fitting; that is to say, they are not unfolded spectra.

It should also be noted that the spectra in Fig. C.1 are for display purposes only, and all physical quantities determined were obtained through spectral fitting of the count rate spectra, with the appropriate detector responses.

4.3. AGN selection

The AGN classification in the sample was performed using different criteria in both X-rays and IR. Any galaxy that met any of our selection criteria, described below, was classified as an AGN and is listed in Table 5.

Table 5.

Sources with an AGN signature in IR or X-rays.

The X-ray selection was performed using two different methods: an X-ray color selection, and detection of AGN spectral features.

The X-ray color, or hardness ratio, gives the relative intensity of emission in hard and soft bands (in counts). A high HR indicates strong emission above 2 keV, which is often associated with the presence of an obscured AGN, that is, column density, NH in the range of 1022 − 1024 cm−2. The threshold for AGN selection was chosen as HR > −0.3, as it was for the CGI sample (see Iwasawa et al. 2011).

Figure 4 shows the hardness ratio of all sources in the sample as a function of their hard X-ray luminosity. AGN selected through all criteria described in this section are plotted with filled squares, while all absorbed AGN are marked with open circles. Most AGN in the sample have an HR below the threshold, as many are absorbed or not selected through X-rays.

thumbnail Fig. 4.

Hardness ratio as a function of the 2−7 keV luminosity for all sources in the CGII sample. All AGN from Table 5 are plotted as filled squares, and those in which absorption features are fit (labeled A in the table) are marked with an open circle. The dashed line shows the −0.3 boundary, above which sources are selected as AGN (unless evidence points toward a lack of AGN presence, see Appendix A).

Open with DEXTER

Some AGN are missed by this HR selection because absorption in the nucleus is significant and soft X-ray emission coming from external starburst regions is strong. Such galaxies can still show a hard-band excess in their spectra, and if fitting them with an absorbed power-law with a fixed 1.8 photon index yielded a high enough absorbing column density, we classified them as absorbed AGN (see Sect. 4.4.2). Each of these cases is listed in Table 5 and discussed individually in Appendix A.

When absorption is even stronger, only reflected radiation can be observed in the hard band, and therefore sources appear weak, their HR being even smaller. A clear signature of a highly obscured AGN is a the detection of a strong Fe Kα line at 6.4 keV, which is also used as a criterion for AGN selection. We set a threshold of 2σ for the detection of the iron line in order to classify a source as an AGN. Sources selected through this criterion are listed in Table 5, and details on the iron line fits can be found in Sect. 4.4, Table 6.

Table 6.

Fe Kα line fits.

The IR selection was performed by means of the detection of the [Ne v] 14.32 μm line over kpc scales, which traces high-ionization gas. The ionization potential of [Ne v] is 96 eV, which is too high to be produced by OB stars. Therefore, detection of this line in the integrated spectra of galaxies is a good AGN indicator (see Petric et al. 2011, and references therein).

Another possible indicator is when the equivalent width of the 6.2 μm PAH feature is lower than 0.1 μm. Polycyclic aromatic hydro-carbons (PAHs) are either destroyed by the radiation originating from the AGN, or their features are diluted in the spectra by the strong MIR continuum it creates; this results in a low value of the EW (see Stierwalt et al. 2013, and references therein).

With the X-ray criteria alone, we found that 21 galaxies host an AGN. This represents (25 ± 5)% of our sample. With the addition of IR criteria, 5 other galaxies are classified as AGN, resulting in a total AGN fraction of (31 ± 5)% for the 84 individually analyzed galaxies in CGII. Galaxies selected as AGN are presented in Table 5, along with optical classifications and whether or not they meet our X-ray and IR selection criteria.

Two sources in the sample met the selection criteria, but we opted to not classify them as AGN, for reasons explained in Appendix A: IRAS F17138−1017 and IRAS F16399−0937 (S), which meet the HR criterion.

Table 5 also lists the contribution of the AGN to the bolometric luminosity for all sources classified as AGN. The contribution of the AGN to the MIR luminosity was derived by Díaz-Santos et al. (2017) for all GOALS galaxies, employing up to five Spitzer/IRS diagnostics. Applying corrections based on spectral energy distribution (SED) templates of pure starbursts and AGN sources, they derived the fractional contribution of the AGN to the overall bolometric luminosity (as in Veilleux et al. 2009).

Figure 5 shows the HR of all sources in the sample as a function of the fractional contribution of the AGN to the bolometric luminosity, AGNbol. Sources with a fraction larger than 0.2 can be considered to have an energetically significant AGN. X-ray selected AGN, through any of the three criteria mentioned above, are highlighted as filled symbols. All marked AGN below the HR = −0.3 threshold show signs of obscuration, as they have been selected through any of the other two X-ray criteria. In the full C-GOALS sample, 19 of 32 X-ray selected AGN lay below AGNbol < 0.2. Therefore, more than half of the AGN detected through X-rays are not easily selected through the described combination of MIR diagnostics.

thumbnail Fig. 5.

Hardness ratio as a function of the fractional contribution of the AGN to the bolometric luminosity (as derived from MIR data, Díaz-Santos et al. 2017) of the source, in red for CGI sources and black for CGII. X-ray selected AGN from Table 5 are plotted as filled squares.The horizontal dashed line shows the HR = −0.3 threshold. The vertical dashed line shows the value above which the AGN is energetically significant.

Open with DEXTER

4.4. X-ray spectral fitting

The 0.4−7 keV Chandra spectra of the CGII galaxies present similar properties to those of the CGI sample, that was analyzed by Iwasawa et al. (2011): a mostly emission-line dominated soft X-ray band, and a hard X-ray power-law. As has been discussed by these authors, both the spectral shape and the morphology of the emission (see images in Appendix B) suggest a different origin for the soft and hard X-rays, and therefore the two were analyzed separately.

A few objects in the sample (IRAS 18090+0130 (W), IRAS F06076−2139 (S), IC 0860, and NGC 7591) were not fit because they have an excessively low count number, of the order of ≲25 cts, in the full 0.4−7 keV band. For these sources, only the count rates and HR were computed, and results on fluxes and luminosities are not presented in Table A.1.

For the majority of our sources, which have few counts, we made the fit through C statistic minimization instead of χ2 minimization.

4.4.1. Soft band (0.4–2 keV) fitting

Starburst galaxies, when not dominated by a luminous AGN, have soft X-ray emission originating in hot interstellar gas (∼0.2−1 keV), which is shock-heated by supernovae explosions and stellar winds from massive stars. Emission from hot gas can generally be fit with a standard thermal emission model, with a solar abundance pattern, for instance, mekal (Mewe et al. 1985; Kaastra 1992; Liedahl et al. 1995). However, in our data, such a simple model does not agree with many of the observed emission line strengths and provides an unsatisfactory fit in most cases. A better fit can be obtained either with a modified abundance pattern that is richer in α elements, or through the overlap of more than one mekal at different temperatures.

The hot gas within a starburst region is expected to be enriched by α elements, which are produced in core-collapse supernovae. Metal abundances should deviate from a solar pattern, as has been found for star-forming knots in nearby galaxies, such as the Antennae (Fabbiano et al. 2004). At the same time, the extended soft X-ray emitting gas is expected to be multi-phase: the shocked gas swept away by a starburst wind seen at outer radii is free from absorption, while the hotter gas at inner radii may have some absorption of the interstellar medium (e.g., Strickland & Stevens 2000). A temperature gradient can be approximated at first order as two mekal models with different temperatures. One model would fit the most external, colder gas component (at T = T1), which is located far away from the nucleus, and therefore is less strongly absorbed by the interstellar material. The other model would fit the inner, hotter gas (at T = T2), which is obscured by the denser material in the central region of the galaxy.

Ideally, the data should be modeled using more than one mekal component, with different temperatures and absorbing column densities, and with non-solar metal abundances. However, given the quality of the data, this would imply severe overfitting. As we are interested in probing the level of obscuration in the C-GOALS sources, we opted to model the data using two mekal models as defined above, which both have solar abundance patterns.

The results obtained through this fitting, the parameters of which are listed in Table A.2, show that it is possible to satisfactorily fit the sources with high enough number counts using this model, which is to be expected if part of the emission truly originates in a denser, inner region. However, we note that this model is not clearly superior to a single mekal component with non-solar abundances, as was used by Iwasawa et al. (2011) on the CGI sample; and that most of the analyzed sources do not have good enough data quality to determine a clear best fit between the two models.

The distributions of the obtained parameters for the full CGII sample are shown in the histograms presented in Fig. 6. The temperature associated with the colder mekal component (T1) used to model the 0.5−2 keV emission of each source presents a narrower distribution than that associated with the hotter component (T2). The distribution of T1 has a median value of 0.38 ± 0.03 keV and an interquartile range of 0.32−0.63 keV. The distribution of T2 has, as expected, a higher median value of 0.97 ± 0.18. The interquartile range is 0.77−1.2 keV range, with a long tail extending up to T2 ∼ 4.5 keV. We note that even though the two distributions overlap (i.e., some T1 values are higher than some T2 values), for each single source T2 > T1.

thumbnail Fig. 6.

Left panel: distribution of mekal model temperatures, where T1 is the temperature of the external, colder gas component and T2 is the temperature of the internal, hotter gas component, for the CGII sample. Right panel: distribution of absorbing column densities associated with the inner, hotter gas component, for the CGII sample.

Open with DEXTER

Figure 6 also shows the distribution of column densities, NH, absorbing the hottest mekal component. The median of the distribution is (1.1 ± 0.2)×1022 cm−2, with an interquartile range of (0.8−1.4)×1022 cm−2.

A few sources, named in Sect. 4.4.2, were modeled in the full 0.4−7 keV band with a single power-law; and therefore no values for T1, T2, and NH were derived for them. In addition, NGC 6285 and IC 2810 (SE) were fit with a single mekal component in the 0.4−2 keV range, and NGC 7752/3 (NE) and ESO 440-IG058 (N) were fit with a single mekal component in the full 2−7 keV range. These sources are not included in the histograms shown in Fig. 6, or in the averages described previously.

4.4.2. Hard band (2–7 keV) fitting

In the hard X-ray band, where the emission from hot interstellar gas and young stars significantly decreases, X-ray binaries dominate the emission in the absence of an AGN. Their emission can be fit by a simple power-law. The photon index, Γ, is the slope of a power-law model that describes a photon spectrum, defined as dN/dEE−Γ photons cm−2 s−1 keV−1. Derived values for Γ for all sources in the CGII sample are listed in Table A.2.

This fit was only performed for galaxies that had at least 20 cts in the 2−7 keV range. Galaxies with a lower count number were fit while imposing a fixed power-law photon index of 2 (average spectral slope found for a sample of local starburst galaxies, Ranalli et al. 2003), leaving only the model normalization as a free parameter. This limit was set in order to obtain meaningful constraints for the spectral slope. It is lower than the one fixed for the CGI sample (50 cts) since many of the sources in the current sample are much fainter, as expected given their lower IR luminosities.

A few objects within the sample show a clear, steep flux increase at energies ≥3−4 keV (see, e.g., MCG−03−34−064 in Fig. C.1), which is a sign of the presence of an absorbed AGN (see, e.g., Turner & Miller 2009). In such cases, which all have a count number higher than 20 cts, we fit an absorbed power-law imposing a fixed photon index of 1.8 (a typically expected value for the photon index of an AGN, see, e.g., Nandra & Pounds 1994). This left the absorbing column density, NH, as a free parameter. This model was preferred when the fit yielded values NH ≳ 1023 cm−2, and it was statistically better than a simple power-law fit. In such cases, we classified the source as an absorbed AGN.

A few sources in the sample (NGC 5331 (N), IRAS F16399−0937 (S), ESO 550−IG025 (S), MCG+12−02−001 (W), CGCG 049−057, UGC 02238, NGC 4418, and ESO 343− IG013 (N) and (S)) are clearly best-fit with a single power-law in the full 0.4−7 keV band, and the Γ parameter shown in Table A.2 corresponds to that fit.

4.4.3. Iron Kα lines

The Fe Kα line is a frequently used reliable diagnostic of heavily obscured AGN. As we described in Sect. 4.3, we used it as one of our X-ray AGN selection criteria. The cold iron line seen in some of the CGII sources was fit with a Gaussian model centered at 6.4 keV. Six sources in the CGII sample have such a line fit with a significance above 2σ, which is the threshold we set to consider a detection.

A more conservative and frequently used threshold to consider a line as detected in the data is a 3σ significance. If we had imposed this more restrictive criterion, only NGC 1068 and MCG−01−34−064 would have detected Fe Kα lines in the sample. The threshold was lowered because of the low signal-to-noise ratio for all sources in the CGII sample. However, we note that lowering it to 2σ does not change the fraction of selected AGN within the sample, as all sources with a line detection also meet other selection criteria. We still consider the presence of this line at 2σ to be relevant information, which can give support to other AGN determinations, and therefore included it in the analysis.

Parameters of the fit for these six sources are shown in Table 6, including the line energy, intensity, and equivalent width with respect to the continuum. The detection of these lines has been previously reported based on other X-ray observations (Koyama et al. 1989; Band et al. 1990; Mazzarella et al. 2012; Gilli et al. 1999; Levenson et al. 2002; Ricci et al. 2014; Risaliti et al. 2009).

4.5. X-ray luminosities and correlation with LIR

Figure 7 shows the distribution of derived luminosities in the soft and hard band, presented in Table A.1, compared with that of those obtained for the CGI sample of Iwasawa et al. (2011). For CGII, the distributions peak at log(LSX)∼40.6 erg s−1 and log(LHX)∼40.9 erg s−1, which is slightly lower than the peak of both bands for CGI sample, at log(LX)∼41.1 erg s−1. The median logarithmic values for the soft- and hard-band luminosities are listed in Table 7. CGII has lower X-ray luminosity values than CGI, as expected, reflecting a correlation between IR and X-ray luminosity that is seen in both the CGI and CGII samples (see Figs. 8 and 9).

thumbnail Fig. 7.

Distributions of soft-band X-ray luminosity, 0.5−2 keV (left panel), and hard-band X-ray luminosity, 2−10 keV (right panel), for the individual galaxies of CGI and CGII.

Open with DEXTER

Table 7.

Statistical X-ray properties of the sample.

thumbnail Fig. 8.

Plots of soft (left panel, 0.5−2 keV) and hard (right panel, 2−10 keV) X-ray luminosity vs. IR luminosity, where the X-ray luminosity is corrected only for Galactic absorption. X-ray selected AGN, shown in Table 5, are shown in black. When multiple objects are present in a source, their IR luminosity is divided, as shown in Table 3.

Open with DEXTER

thumbnail Fig. 9.

Plots of soft (left panel, 0.5−2 keV) and hard (right panel, 2−10 keV) X-ray luminosity versus FIR luminosity derived as in Eq. (1), where the X-ray luminosity is corrected only for Galactic absorption. Data used by Ranalli et al. (2003), along with their derived correlation, are shown in blue. CGI and CGII data (for galaxies without an AGN) are plotted in red and black squares respectively. When multiple objects are present in a source, their IR luminosity is divided as shown in Table 3. All sources containing AGN, as listed in Table 5 or classified as AGN by Iwasawa et al. (2011) have been removed both from the plot and from the fits. The red, dashed line shows our best quadratic fit for the C-GOALS + Ranalli et al. (2003) data. Grey, dashed lines (left panel) show theoretical lines of obscuration for NH = 0.5, 1.0, 2.0, 5.0 × 1022 cm22, as described in Sect. 4.5.

Open with DEXTER

The origin of this correlation is in the presence of star formation in the galaxies. FIR luminosity measurements detect the energy absorbed by the dust of the interstellar medium from young, bright stars; and thus are a good estimator of the total star formation rate (SFR; e.g., Kennicutt 1998). In galaxies with a considerable amount of star formation, such as starburst galaxies, emission in other wavelengths can also be related to young and massive stars, such as X-ray luminosity (e.g., X-ray binaries emission and supernova remnants, SNRs). Therefore, it has been suggested that if a good correlation between X-ray luminosity and IR luminosity exists in galaxies, the SFR can be directly inferred from the X-ray luminosity. Compatible correlations have been found in previous works for local star-forming galaxies with IR luminosities lower than those of LIRGS (e.g., Ranalli et al. 2003; Grimm et al. 2003; Mineo et al. 2014).

Figure 8 shows the X-ray luminosity as a function of the IRAS IR luminosity. The data show a moderate correlation, with a typical spread of more than one order of magnitude when only sources without detected X-ray AGN presence (open squares) are considered. Sources that contain X-ray AGN typically lie above the trend, adding scatter to the correlation. Sources with multiple components are separated into their respective contributions, and plotted separately, as it has been shown that when they are plotted summed into one single source, the correlation becomes less clear (Iwasawa et al. 2009). Their total (8−1000) μm IRAS luminosity, as in Table 1, is separated into the different contributions using the percentages indicated in Table 3.

As we found in the CGI galaxies, X-ray selected AGN tend to be more luminous in X-rays than the rest of the sample, especially in the hard band. However, the values for log(LX/LIR) are higher for the CGII sample than for CGI, as listed in Table 7. This result means that while our sample is less bright both in X-ray and in IR than the CGI sample, we find a higher relative X-ray to IR luminosity. Removing X-ray selected AGN from both samples gives lower ratios, which we also list.

Comparing these average values to the log(LX/LIR)∼ − 3.7 found by Ranalli et al. (2003) for local star-forming galaxies with lower SFR, we find a large discrepancy. However, their IR luminosity does not include the 1−40 μm range, which may contribute a non-negligible amount of power, in particular for IR-warm AGN-dominated systems. Therefore a direct comparison needs to introduce a correction. Furthermore, only at radio and FIR wavelengths are the most intense starbursts transparent (e.g., Condon 1992), so that from their detected flux the SFR can be accurately estimated.

IR luminosities derived by Ranalli et al. (2003), hereafter LFIR, follow the expression

(1)

from Helou et al. (1985).

We used this expression to derive LFIR for all non-AGN objects in the CGI and CGII samples, again accounting for the contribution of multiple components following Table 3. As listed in Table 7, log(LX/LFIR) is similar to −3.7 for the CGII objects, but not for the galaxies in CGI. A direct comparison between the distribution followed by objects analyzed by Ranalli et al. (2003), as well as their derived correlation, and GOALS objects is shown in Fig. 9.

The best-fit correlations derived by Ranalli et al. (2003) are

(2)

(3)

which correspond to the blue dotted lines plotted in Fig. 9. Galaxies in the GOALS sample with LFIR ≲ 8 × 1010 L follow Eqs. (2) and (3), but those with higher LIR have a systematically lower X-ray luminosity than predicted.

This behavior suggests that a better fit would be obtained with a quadratic relation in log scale. Using the least-squares method, we obtain a best fit for the C-GOALS + Ranalli et al. (2003) data:

(4)

(5)

where x = log(LFIR). This fit is plotted as a red dashed line in Fig. 9. Below FIR luminosities of ∼3 × 1044 erg s−1 (i.e., ∼8 × 1010 L), the quadratic fit overlaps the linear correlation. Above this value, the X-ray luminosity increases far more slowly with FIR luminosity. This effect is stronger in soft X-rays. We note that when we fit a single power-law to the full data sample, we did not recover a relation that was compatible, within the errors, with Eq. (2). A power-law fit also yielded a larger χ2 value than the fits given by Eqs. (4) and (5).

As soft X-rays are easily absorbed by moderate column densities, we show the effect that obscuration could have on the Ranalli et al. (2003) correlation. In order to do so, we took an average spectrum that is characteristic of the galaxies within our sample: a double-component mekal model with temperatures T1 = 0.38 and T2 = 0.97, the median values derived from our soft X-ray analysis. According to our model, the inner component of T = T2 can have considerable absorption, and fitting yields values in the range NH = 0.1−2.5 × 1022 cm−2. We assumed different column densities, NH = 0.5, 1.0, 2.0, 5.0 × 1022 cm22, and absorbed the hotter component. We used the model to calculate the decrease in flux caused by the different column densities, and considering that the linear correlation derived by Ranalli et al. (2003) has no intrinsic absorption, we plot the “absorbed” equivalent correlations in Fig. 9. NH = 5.0 × 1022 cm−2 absorbs more than 99% of the emission of the inner component in the 0.5−2 keV range, meaning that higher column densities would result in no change in the emission, that is, only the emission of the outer, unabsorbed component remains.

4.6. Radial profiles

Radial profiles for all sources (except for IRAS F06076−2139 (S), for which not enough counts are detected in the Chandra data) were characterized in two different ways. In the first method, we computed the soft X-ray half-light radius () for the 0.5−2 keV band as the radius within which half of the total number of counts is emitted. In the second method, surface brightness profiles were computed and provided in Appendix B, also shown in Fig. 2 for NGC 2146. These profiles were computed in the soft 0.5−2 keV band, shown as open squares, and the hard 2−7 keV band, shown as filled squares.

Profiles were centered using the hard X-ray peak that corresponds to the nucleus of the galaxy, which typically corresponds to the near-infrared nucleus. When no clear central emission in the (2−7) keV band was detected, the profiles were centered using IR images. For all galaxies whose radial profiles were centered using IR images, a comment has been added in Appendix A.

The values for , both in arcsec and in kpc, are provided in Table 8. While this value can give an idea of the size of the central more intensely emitting region of a galaxy, we note that for sources without an extensive diffuse emission (e.g., NGC 3221, which is mostly composed of point-sources), it might not have a physical meaning. Other sources show non-axisymmetric morphology, most likely associated with extended starburst winds (e.g., UGC 08387, NGC 6286 (SE), NGC 2146, NGC 4194, NGC 1365, and NGC 0838). See Appendix B for detailed images of the morphology of the X-ray emission in all sources

Table 8.

Half-light radius

Because of the pixel size of the Chandra CCD, the smallest radius within which counts can be computed is limited to 0.5″. Very compact sources can have more than half of their detected counts within this region, making the estimation of impossible. This is the case for six of our sources, for which an upper limit is provided. It would also be the case for the vast majority of sources when the hard band emission were considered, which is the reason we do not provide values of .

Figure 10 shows a histogram of all half-light radii presented in Table 8. Sources with only upper limits derived are included, as they all fall below , which is the bin size. The distribution of has a median of 1.0 ± 0.1 kpc, with an interquartile range of 0.5−1.9 kpc. This shows that most sources within CGII have a compact X-ray distribution, with half of the emission being generated within the inner ∼1 kpc. VLA 33 GHz studies of the 22 brightest LIRGs and ULIRGs in the C-GOALS sample find half-light radii below 1.7 kpc for all sources, meaning that the emission is also compact in other wavelengths (Barcos-Muñoz et al. 2017).

thumbnail Fig. 10.

Histogram of half-light radii for the sources in the CGII sample.

Open with DEXTER

All values of are upper limits, as any amount of obscuration in the sources (likely important in most, as seen in Fig. 9), which is concentrated in the inner regions, will result in an apparent decrease of compactness.

Figure 11 shows the distribution of half-light radii as a function of the FIR luminosity. The size of the most intensely X-ray emitting region shows no clear correlation with the overall IR luminosity, although CGI sources tend to have higher X-ray radii. Within the CGII sample, sources that contain an AGN, as listed in Table 5, plotted as filled squares, tend to be compact. (83 ± 7)% of them have . This is in agreement with previous results from a study of the extended MIR emission in GOALS using Spitzer/IRS spectroscopy, where it was found that progressively more AGN-dominated galaxies tend to show more compact MIR emission (Díaz-Santos et al. 2010).

thumbnail Fig. 11.

Half-light radius as a function of FIR luminosity (as in Eq. (1)) for galaxies within the CGII (black) and CGI (red) sample. Sources without an AGN are plotted as open squares, while sources with an AGN are plotted as filled squares.

Open with DEXTER

Figure 12 shows a comparison between and the IR radius at 70 μm taken from Díaz-Santos et al. (2017) for all C-GOALS sources. Most sources in the sample are placed close to the line. CGI sources tend to have a larger X-ray half-light radius for a given characteristic IR radius than CGII sources.

thumbnail Fig. 12.

Soft X-ray half-light radius as a function of 70 μm FIR radius, taken from Díaz-Santos et al. (2017). CGI data are shown in red, and CGII data in black. X-ray selected AGN and MIR selected AGN (as specified in Table 5) are plotted separately. The dashed line shows , and is not a fit to the data. Eleven systems within the whole C-GOALS sample are resolved into individual galaxies in X-rays but not at 70 μm, and are thus not plotted.

Open with DEXTER

Outliers with very compact soft X-ray emission but a large IR radius are X-ray selected AGN ESO 343−IG013 (N) and IR selected AGN NGC 7592 (W). Both sources show clear strong hard X-ray peaks in the nucleus (see images in Appendix B). Another extreme outlier is IRAS F12112+0305, with but much more compact IR emission.

4.7. Luminosity surface density

Using the luminosities listed in Table A.1 and the in Table 8, we derived luminosity surface densities for all sources in the C-GOALS sample, as . ΣIR is derived using 70 μm radii from Díaz-Santos et al. (2017).

Figure 13 shows ΣSX as a function of ΣIR. The X-ray surface density tends to increase with IR surface density, although the correlation is broader than the one existing between luminosities. The left plot highlights AGN as filled symbols and separates the CGI and CGII samples. CGI sources, brighter in the IR, tend to have lower ΣSX for a given ΣIR. Within a given sample, sources with AGN tend to have a larger ΣSX, which is to be expected because they are both brighter in X-rays and also more compact. Figure 13 also shows, on the right, the same figure but highlighting the merger stage of the sources in the sample. Information on the merger stage is taken from Stierwalt et al. (2013), derived from visual inspection of IRAC 3.6 μm (Channel 1) images. Classification is non-merger, pre-merger (galaxy pairs prior to a first encounter), early-stage merger (post first-encounter with galaxy disks still symmetric and intact, but with signs of tidal tails), mid-stage merger (showing amorphous disks, tidal tails, and other signs of merger activity), and late-stage merger (two nuclei in a common envelope). Sources in late-stage mergers tend to have higher ΣIR, but lower ΣSX for the same ΣIR.

thumbnail Fig. 13.

Plots of X-ray luminosity surface density vs. IR luminosity surface density. Left panel: red data correspond to CGI sources, and black data corresponds to CGII sources. AGN are highlighted as filled symbols. Right panel: sources in the full C-GOALS sample are plotted as different symbols according to merger stage, as derived by Stierwalt et al. (2013). 11 systems within the whole C-GOALS sample are resolved into individual galaxies in X-rays but not at 70 μm, and are thus not plotted.

Open with DEXTER

The percentages of merger stages in the sample are listed in Table 9. Many CGI sources are late mergers, hence their higher IR luminosity. This is also visible in Fig. 13, as CGI and late mergers have the same behavior with respect to the rest of the sources in the sample.

Table 9.

Merger stage.

5. Discussion

5.1. X-ray to IR luminosity relation

The X-ray to IR luminosity (or SFR) correlation has been studied in numerous previous works (e.g., Fabbiano & Trinchieri 1985; Fabbiano et al. 1988; Fabbiano 1989; Bauer et al. 2002; Grimm et al. 2003; Ranalli et al. 2003; Gilfanov et al. 2004; Persic & Rephaeli 2007; Mineo et al. 2014). For soft X-rays, it originates in starburst-wind shock-heated gas. For hard X-rays (2−10 keV), the relation is thought to originate in high mass X-ray binaries (HMXB), which are end products of star formation. At low SFRs, that is, for local starburst galaxies with LIR ≪ 1011 L, low-mass X-ray binaries (LMXBs) can significantly contribute to the X-ray luminosity. The luminosity from LMXBs correlates with galaxy stellar mass (M), and this dependence must be considered, along with the contribution of SFR (e.g., Colbert et al. 2004; Gilfanov 2004; Lehmer et al. 2008; Lehmer et al. 2010).

Figure 9 shows a comparison between C-GOALS data and the correlation derived by Ranalli et al. (2003) for a sample of nearby starbursting galaxies. Works that include the LMXB contribution at low luminosities show a slight decrease in the slope at the high-luminosity end. Therefore, their correlation can be used as a point-of-reference against which to plot the LIRG and ULIRG data, although a rigorous comparison would require including all previously mentioned works and is beyond the scope of this work.

It is clear that at higher IR luminosities the correlation breaks down in an apparent deficit of X-ray flux, more extreme in the 0.5−2 keV band. This has been observed since the inclusion of the C-GOALS ULIRGs and high-luminosity LIRGs into the described correlations (e.g., Iwasawa et al. 2009, 2011; Lehmer et al. 2010). The inclusion of our CGII data provides more information on the transition between low-IR-luminosity galaxies and ULIRGs.

This underluminosity, or X-ray quietness, is explained in many works as an effect of obscuration. LIRGs and ULIRGs have extremely high concentrations of gas and dust in their inner regions, resulting in compact starbursts. High gas column densities can easily absorb soft X-rays, and in the most extreme cases, even hard X-rays.

Galaxies in the CGII sample, which are less IR-luminous than those in the CGI sample, are generally found in less-advanced mergers (see Table 9). The concentrations of gas and dust in the inner regions of the galaxies are higher in the more advanced mergers (e.g., Ricci et al. 2017), implying that the contribution of obscuration is stronger at higher IR luminosities. From an IR point of view, Díaz-Santos et al. (2010) observed that late-stage mergers are much more compact, which also indicates larger column densities.

5.1.1. Soft X-ray faintness

As shown in Fig. 9, the obscuring column densities necessary to dim the soft X-ray emission in most of the sources are compatible with those derived from the two-component model, plotted in Fig. 6 and listed in Table A.2. The derived values of NH are lower limits, as any gas phase with higher obscuration contributes less significantly to the X-ray emission, or is even completely absorbed, and therefore cannot be fit.

Our spectral model is based on the existence of two distinct phases in the galaxy interstellar medium (ISM). Emission is likely to come from a complex phase-continuum of gas, and thus individual estimates of properties based on the spectral fitting should be taken with caution (see, e.g., Strickland & Stevens 2000, for a discussion). However, the simple two-phase model is the most complex we can fit given our data, and it shows that the column densities can at least explain the data.

Figure 13 shows lower ΣSX for CGI galaxies, which reflects both their X-ray faintness and larger soft X-ray sizes (Fig. 12). We defined the size of the emission as the half-light radius, meaning that larger sizes indicate a less compact source. This implies that the faintness most likely originates in the center of the source. As CGI galaxies are in more advanced merger stages and should have higher column densities, this is likely to be an effect of obscuration. Another likely contribution to the larger soft X-ray sizes are the strongest starbursts in CGI galaxies, which generate larger soft X-ray nebulae.

We note that the Chandra resolution is much better than that of Spitzer, which should be taken into account in any direct comparison between characteristic sizes or luminosity surface densities. Higher resolution should imply a tendency toward deriving higher compactness, while Fig. 12 shows the opposite: X-ray sizes are generally similar, or even larger than IR sizes. However, we do not know what the IR emission would look like at similar resolutions. This difference between the datasets could explain the presence of outliers below the line, and add to the dispersion of the data. Future observations with the James Webb Space Telescope (JWST) would allow for better comparison.

In conclusion, the soft X-ray faintness, and therefore the quadratic best-fit curve given by Eq. (4), can be explained through obscuration, as the necessary column densities are present in the galaxies within the C-GOALS sample.

5.1.2. Hard X-ray faintness

Attributing to extinction the observed faintness in hard X-rays requires much higher gas column densities. While it is clear that more IR-luminous sources (i.e., late-stage mergers) are generally more heavily obscured (e.g., Ricci et al. 2017, for a GOALS subsample), the most extreme sources are ∼1−2 dex below the correlation shown in Fig. 9. This implies that between ∼90−99% of the central starburst region must be covered in medium that is dense enough to suppress even hard X-rays. To obscure the emission in the 2−8 keV band in which Chandra is sensitive, the necessary column densities would be of the order of ∼1024 cm−2. Sources in the sample that are undetected by NuSTAR (Ricci et al. 2017) would require even higher column densities, of the order of ∼1025 cm−2.

In order to explain the observed faintness, regions of sizes of the order of the listed in Table 8 would need to be covered in the high NH we described. A column density of 1025 cm−2 could imply H2 masses of the order of ∼1010 − 1011 M for a nuclear star-forming region of 500 pc of radius. It is unclear if such high gas masses are truly concentrated in the inner regions of ULIRGs in the GOALS sample, and thus if this “self-absorbed starburst” scenario is feasible.

To distinguish the origin of the faintness, Iwasawa et al. (2009) stacked spectra of non-AGN sources in CGI, recovering a high-ionization Fe K feature. This feature can be explained by the presence of an internally shocked, hot bubble that is produced by thermalizing the energy of supernovae and stellar winds (e.g., Chevalier & Clegg 1985), which, in contrast to the SNe and HMXB emission, could be visible through the obscuring material. With high SFRs, the luminosity and the spectra with strong Fe XXV line can be reproduced (e.g., Iwasawa et al. 2005, for Arp 220). This high-ionization line could also originate from low-density gas that is photoionized by a hidden AGN (e.g., Antonucci & Miller 1985; Krolik & Kallman 1987), and it has been observed as the dominant Fe K feature in some Compton-thick AGN (e.g., Nandra & Iwasawa 2007; Braito et al. 2009).

Therefore, another explanation for the X-ray faintness could be the presence of a completely obscured AGN in the nucleus of these galaxies. This AGN would contribute to the IR emission, but would escape X-ray detection. While the column densities needed to cover the AGN are as high as those needed to self-absorb a starburst, the obscured region would be much smaller. This would imply much lower masses, that are easily found in the nuclei of GOALS galaxies. The scenario of hidden AGN versus extremely compact starburst has been previously discussed for some of the C-GOALS sources that show a higher X-ray faintness. Cases such as Arp 220 (e.g., Scoville et al. 2017; Barcos-Muñoz et al. 2018) or NGC 4418 (e.g., Costagliola et al. 2013, 2015) are compatible with both scenarios. However, it is worth noting that the hidden-AGN scenario requires the presence of an AGN with significant IR emission in order to explain the X-ray faintness, which means that it is probably unlikely that MIR determinations would systematically fail to pick up their signature.

Díaz-Santos et al. (2017) suggested from interpretation of Herschel FIR data on the full GOALS sample that the fraction of young, massive stars per star-forming region in ULIRGs might be higher than expected. This does not imply a change in the initial mass function, but the presence of very young star-forming regions, in which most massive stars still have not disappeared (age younger than a few Myr; Inami et al. 2013). In such a case, the massive stars can contribute to the IR emission, but the number of HMXB and SNe associated with the region will be low, as they are end-products of the star formation. This would result in a lower-than-expected X-ray luminosity for a given IR luminosity. Furthermore, in such a scenario, the winds of very massive stars could generate the hot gas that explains the Fe XXV line, without the need of invoking extreme obscuration over a large population of HMXB.

In order to truly understand the origin of the X-ray faintness, further observations are needed that provide information on the obscuration within the sources (e.g., ALMA or NuSTAR observations), or on the unobscured SFRs (e.g., through radio observations).

5.2. AGN and double AGN fraction

In Sect. 4.3 we have shown that (38 ± 6)% of the systems (24 of 63) within CGII contain an AGN, (31 ± 5)% (26 of 84) of individual galaxies being classified as AGN, according to MIR and/or X-ray criteria. This fraction can be compared to the (50 ± 7)% of the systems, or (38 ± 7)% (21 of 55) of the analyzed individual galaxies classified by Iwasawa et al. (2011) when analyzing the more IR-luminous objects in the CGI sample. This result may indicate a slight increase of AGN presence with IR luminosity, although the fractions in the two samples are compatible within the statistical errors. Although the increase of the AGN fraction as a function of luminosity found here is not statistically significant, it is consistent with previous findings in optical and IR spectroscopy (e.g., Veilleux et al. 1995; Kim et al. 1995; Yuan et al. 2010; Stierwalt et al. 2013).

Double AGN are detected in two interacting systems, NGC 5256 and ESO 432-IG006, of the 30 multiple systems analyzed here (7 ± 4)%. In the C-GOALS sample, one double AGN system, NGC 6240, was detected out of 24 multiple systems that were analyzed (4 ± 4)%.

Theoretical estimates derived from merger simulations performed by Capelo et al. (2017), which took into account observational effects (e.g., observation angle, distance dimming of X-ray luminosity, and obscuration of gas surrounding central BH), have concluded that in a sample of major mergers hosting at least one AGN, the fraction of dual AGN should be ∼20−30%. Koss et al. (2012) studied a sample of 167 nearby (z < 0.05) X-ray selected AGN and found a fraction of dual AGN in multiple systems of 19.5%. When separated into major pairs (mass ratio ≥ 0.25) and minor pairs, they found 37.1% and 4.8% respectively. Other studies (e.g., Ellison et al. 2011; Satyapal et al. 2017) found a statistical excess of dual AGN that decreased with galaxy separation. Therefore, dual AGN activation is more likely in advanced merger stages.

Within CGI and CGII, the fraction of double AGN in systems that host at least one AGN is (11 ± 10)% (1 of 9) and (29 ± 14)% (2 of 7) respectively. The fraction found in CGII falls well within the ranges found in the two previously described works, while the dual AGN fraction in CGI is just barely compatible within the errors. Moreover, CGI galaxies are generally found in more advanced merger stages (see Table 9), meaning that according to predictions, their dual AGN fraction should be closer to the mentioned 37.1%, and not to the lower 20%.

The lack of dual AGN in the CGI sample could be explained with heavy obscuration, which is expected to be important for these sources, as discussed in the previous section. Compton-thick (NH > 1.5 × 1024 cm−1) AGN may be completely obscured in our Chandra data (e.g., Mrk 273, Iwasawa 2018), and their scattered continuum or Fe Kα lines too faint to be detected . MIR criteria can be effective in such cases, even though they may also miss the most heavily buried AGN (e.g., Snyder et al. 2013). Recent simulations by Blecha et al. (2018) found that much of the AGN lifetime is still undetected with common MIR selection criteria, even in the late stages of gas-rich major mergers. This effect is incremented for AGN that do not contribute significantly to the bolometric luminosity, especially when considering that the presence of a strong starburst can help dilute the AGN signature. Figure 5 shows up to 19 of the 32 X-ray selected AGN in C-GOALS that contribute less than 20% to the bolometric luminosity, most of which are missed by MIR selection criteria (see Table 5).

Another likely contribution to the low fraction of dual AGN found in CGI comes from the inability to resolve individual nuclei in a late-stage merger. Many CGI galaxies are found in such a stage. While Chandra has a high spatial resolution (∼0.5″), very closely interacting nuclei, with separations of the order of ≲200−300 pc, would remain unresolved in our sample. Spitzer data, also used in this work for the MIR AGN selection, have a much lower resolution, and would not resolve double AGN with even further separations.

However, as our sample sizes are small and therefore the statistical errors large, we cannot make any strong statements regarding a decreasing trend of the double AGN presence with IR luminosity.

6. Conclusions

We analyzed Chandra-ACIS data for a sample of 63 LIRGs and ULIRGs, composed of 84 individual galaxies (CGII). These galaxies are a low-IR-luminosity subsample of GOALS, a complete flux-limited sample of the 60 μm selected, bright galaxies in the local Universe (z < 0.08). Arcsecond-resolution images, spectra, and radial surface brightness profiles were presented. We compared the observations with Spitzer and Herschel data to contrast their X-ray and IR properties. We also compared our results to those found by Iwasawa et al. (2011) for the high-IR-luminosity subsample of GOALS (CGI). We summarize our main findings below.

  • Objects with an AGN signature represent (31 ± 5)% of the CGII sample, compared to the (38 ± 7)% reported for the CGI sample. Double AGN are detected in two interacting systems, implying that the fraction of double AGN in systems that host at least one AGN is (29 ± 14)%, in contrast to the (11 ± 10)% found for the CGI sample.

  • 19 of 32 of the X-ray selected AGN in the full C-GOALS sample (CGI+CGII) are not energetically significant, contributing less than 20% to the bolometric luminosity of the galaxy, according to MIR determinations.

  • The brightest LIRGs, at LFIR > 8 × 1010 L, show a hard X-ray faintness with respect to the luminosities predicted by correlations found for nearby star-forming galaxies. This behavior is accentuated for the CGI ULIRGs. Possible explanations for the sources with most extreme deviations include a self-absorbed starburst, an obscured AGN, or the presence of extremely young star-forming regions.

  • The extended soft X-ray emission shows a spectrum that is consistent with thermal emission from a two-phase gas, with an inner, hotter and more heavily obscured component, and an outer, colder and unobscured component.

  • According to our modeling, an obscuration of the inner component in the range of NH = 1−5 × 1022 cm−2 can explain the soft X-ray faintness for the vast majority of the sources.

  • Most sources within CGII have a compact soft X-ray morphology. (50 ± 8)% of the sources generate half of the emission within the inner ∼1 kpc. This behavior is accentuated for AGN, with (83 ± 7)% of the sources with a half-light radius below ∼1 kpc.

  • CGI sources are, in comparison, less compact, which is most likely an effect of obscuration in the inner regions.

  • Most sources in CGII have similar soft X-ray and MIR sizes, although there is important dispersion in this relation.

Acknowledgments

We thank the anonymous referee for helpful comments and suggestions. We acknowledge support by the Spanish Ministerio de Economía y Competitividad (MINECO/FEDER, UE) under grants AYA2013-47447-C3-1-P and MDM-2014-0369 of ICCUB (Unidad de Excelencia “María de Maeztu”). N.T-A. acknowledges support from MINECO through FPU14/04887 grant. T.D.-S. acknowledges support from ALMA-CONICYT project 31130005 and FONDECYT regular project 1151239. G.C.P. acknowledges support from the University of Florida. This work was conducted in part at the Aspen Center for Physics, which is supported by NSF grant PHY-1607611; V.U., G.C.P, D.B.S., A.M.M., T.D-S. and A.S.E. thank the Center for its hospitality during the Astrophysics of Massive Black Holes Mergers workshop in June and July 2018. The scientific results reported in this article are based on observations made by Chandra X-ray Observatory, and has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology under contract with NASA. We acknowledge the use of the software packages CIAO and HEASoft.

References

Appendix A: Notes on individual objects

Table A.1.

X-ray spectral properties for the sample.

Table A.2.

X-ray spectral parameters for the sample.

[45] UGC 08387. This source meets our [Ne v] 14.32 μm line selection criterion, and is thus classified as an AGN, although there is no hint of its presence in the X-ray Chandra data. As described in Iwasawa et al. (2011), a soft X-ray nebulae extends perpendicular to the plane of the galaxy; this is most likely associated with a galactic-scale outflow.

Previous evidence of the AGN has come from detection of compact radio sources at mili-arcsecond resolution (e.g., Lonsdale et al. 1993; Parra et al. 2010), which Romero-Cañizales et al. (2012) attributed to various supernovae in coexistence with a low-luminosity AGN. Using VLBI data, Romero-Cañizales et al. (2017) provided evidence for a parsec-scale radio jet.

[47] CGCG 436−030. This galaxy shows three bright X-ray peaks in the soft band; only the central peak corresponds to a hard-band peak. The other two peaks, placed following the spiral arm structure in the optical images, most likely correspond to star-forming regions.

The DSS image faintly shows a bridge of material between the galaxy and a fainter galaxy ∼1′ to the east, with which it seem to be interacting (Mirabel & Sanders 1988; Zink et al. 2000). This other galaxy is not detected in the Chandra X-ray or the MIPS data, and is only visible in near-infrared observations such as the longest-wavelength IRAC channels. Therefore, we have not considered any contribution to the IRAS flux originating from this nearby companion.

[49] NGC 0695. This source has a rather flat spectrum in the 0.4−7 keV range. There is considerable extended emission in the soft band, and both bands present a very intense emission from the central region.

[51] MCG+07−23−019. This ring galaxy is composed of an elongated main body with double components separated by ∼5″(∼4 kpc) and an oval ring with a diameter of 16″ to the west of the main body (Hattori et al. 2004). As has been suggested by JHKL-band mapping, the nucleus of the galaxy lies between the two optical components and is heavily obscured in optical images (Joy & Harvey 1987). Chandra data show clear emission coming from the elongated disk of the galaxy. The X-ray emission is more intense in the center, and unobscured in the hard and soft bands. Extended soft X-ray emission around the nucleus partly follows the oval ring, most likely tracing star formation.

[52] NGC 6670. This closely interacting merger is composed of two sources, NGC 6670A (or East) and NGC 6670B (or West), separated ∼0.5′. Both galaxies contribute to the IRAS flux (Chu et al. 2017), with the western component being slightly brighter.

X-ray emission from the eastern component is mostly observed in the soft band and is concentrated around the nucleus. The western source, however, shows extended diffuse emission, particularly along the plane of the galaxy. The emission near the center is more intense in the hard and soft band at both sides of the nucleus. This morphology suggests high absorption in the innermost region. The spectrum is also suggestive of a hard excess, and a simple power-law fit results in a photon index of Γ = 0.5 ± 0.5. Fitting an absorbed power-law of fixed photon index Γ = 1.8 results in a moderate absorbing column density of NH ∼ 4 × 1022 cm−2, and no significant improvement on the fit. The X-ray luminosity of the source is LX ∼ 1041 cm−2. The excess at ∼6.4 keV, if interpreted as a possible Fe Kα line, is not significant to the 1σ level.

XMM-Newton data for both sources, resolved, in this double system were analyzed by Mudd et al. (2014), with no hint of an AGN presence detected. However, their short exposure implied a detection of ∼100 cts per source, which lower than the counts detected in our Chandra data.

We consider that even though we cannot rule out the possibility of the western source containing an AGN, we have no strong evidence to claim its presence.

[53] UGC 02369. This double system, separated by ∼0.4′, is clearly dominated in X-rays by the southern component, which, as shown in Table 3, is also responsible for ∼98% of the IR emission. Because the contribution to the IRAS flux originating in the northern galaxy is negligible, we do not present any results for this component. An X-ray analysis would not be possible either, as only ∼5 cts are detected for this source.

The southern source is compact in X-rays, with emission coming both from the nucleus and from a star-forming region in the spiral arm, in the south in the soft and hard bands.

[54] NGC 1614. This source has been classified as a possible obscured AGN through X-ray spectroscopy (Risaliti et al. 2000), although VLBI studies with a sensitivity limit of 0.9 mJy do not detect a compact radio core in it (Hill et al. 2001). Recent studies in subarcsecond MIR observations do not completely rule out a possible (weak) AGN scenario, but they constrain the nuclear luminosity to < 5% of the overall bolometric luminosity of the galaxy (Pereira-Santaella et al. 2015b). ALMA observations do not detect the nucleus in either the CO (6-5) line emission or in the 435 μm continuum, ruling out a Compton-thick AGN with relatively high confidence (Xu et al. 2015).

This source also does not meet any of our AGN selection criteria either, and we do not see any signs of AGN presence in the Chandra spectrum.

Emission in the hard and soft band is peaked in the nucleus, and the soft-band emission also shows elongated extension in the E–W direction, as opposed to the optical edge-on disk, which is elongated toward the N–S direction. Intense star formation, very compact in the nucleus, is the most likely origin of the X-ray emission.

[56] NGC 5331. Both galaxies in this system, separated by ∼0.4′, contribute to the IRAS flux, although the southern component is responsible for ∼80% of the emission, as shown in Table 3. However, since their X-ray luminosity is similar, the northern galaxy has a much higher logarithmic ratio (HX/IR)= − 3.84 (as defined in Table A.1). This value is close to the expected ratio given the correlation derived by Ranalli et al. (2003), but higher than the characteristic X-ray faintness of the GOALS sample.

[57] IRAS F06076−2139. This closely interacting merger is clearly dominated by the northern source in IR and X-rays. However, with only ∼10 cts, the southern source meets our HR criterion for AGN selection (HR = −0.03 ± 0.17). The spectrum also shows an increase in flux toward higher energies, despite the significant error bars. Only the hardness ratio is computed as part of the analysis of this source, because of the low number of counts. For the same reason, a radial profile is not provided for this component.

The northern source comes close to meeting the same AGN selection criteria, with HR = −0.34 ± 0.08. The spectrum might indicate an excess in the hard band, although an absorbed power-law fit with a fixed photon index of 1.8 yields a column density of only NH ∼ 1.9 × 1022 cm−2. With a full-band X-ray luminosity of LX ∼ 1041 erg s−1, and fitting statistics also favoring a non-absorbed power-law, we opt not to consider this source an AGN.

[60] IC 2810. Both galaxies in this system, separated by ∼1.1′, contribute to the IRAS flux as shown in Table 3, with the northwestern source contributing ∼70% of the IR luminosity.

[63] IRAS 18090+0130. Both galaxies in this system, separated by ∼1.3′, contribute to the IRAS flux, with the eastern component being responsible for ∼80% of the emission. The western component has a low X-ray flux and not enough counts to provide reliable data for any analysis further than computing a hardness ratio.

[64] III Zw 035. This closely interacting double system is completely unresolved in the Herschel and MIPS images we used to derive the contribution of each galaxy into the IRAS source, as shown in Table 3. However, Chapman et al. (1990) reported that the majority of the radio continuum (and also probably FIR) emission originates in the northern galaxy. High angular resolution radio continuum observations from (Barcos-Muñoz et al. 2017) indicate that the northern component is the most compact source of the brightest and closest ULIRGs from the GOALS sample, while the authors did not detect the southern component at 33 GHz. The IRAC channel 1–4 images (at 3.6, 4.5, 5.8 and 8.0 μm) show that the northern source clearly dominates and the southern source fades with increasing wavelength. Thus, we assign a contribution to the IRAS flux of 100% to the northern source.

Because of the lack of IRAS flux originating in the southern component, we do not present results for its X-ray analysis in this work, and we did not consider it a source within our sample. The total X-ray counts for this galaxy are ∼25 cts in the full 0.5−7 keV range, which does not allow for a detailed X-ray analysis either, although a simple power-law fit gives an estimated LX ∼ 2 × 1040 erg s−1. However, while the soft-band X-ray flux is dominated by the northern source, the hard-band X-ray flux is very similar for both, and the southern source is optically classified as a Seyfert 2 (Yuan et al. 2010). It is then possible that the northern source is responsible for the high IRAS flux, most likely having a burst of star formation, while the NIR and MIR contribution from the southern source might be due to an AGN.

[65] NGC 3256. This source is assumed to be in an advanced merger stage, with a northern brighter component (the central peak) and a southern component at about ∼10″, elongated in the E−W direction. The possibility of this being a merging obscured companion galaxy was first suggested by Moorwood & Oliva (1994), and radio observations by Norris & Forbes (1995) supported this theory by detecting two equally bright knots. However, high-resolution MIR imaging shows that the northern peak is ∼20 times brighter than the southern region, suggesting that most of the star formation in the galaxy originates there (Lira et al. 2002). The authors also found the northern peak to be brighter in X-rays in Chandra data. The images shown in this work mark the two hard-band peaks reported by them, the southern clearly falling in an obscured region, with dimmer soft-band emission.

The very advanced merger stage of this source makes it hard to determine how much of the surrounding extended emission was initially associated with any of the cores. Therefore, we analyzed it as a single source in order to avoid introducing errors into the determination of its IR and X-ray emission.

[67] IRAS F16399−0937. This closely interacting pair is unresolved in Herschel and MIPS data, although Haan et al. (2011) derived that most of the MIR emission (> 90%) comes from the northern source. We used this value to correct for the fraction contributed to the IRAS flux by each galaxy.

The northern source shows two intense hard X-ray peaks, both corresponding to soft X-ray emitting regions, the more southern of which is the nucleus. The other, as well as the less-intense knots seen in both sources, probably correspond to star-forming regions. The spectrum of this source shows an excess at > 4 keV, with a few hard counts coming from the nuclear region. Fitting an absorbed power-law with a photon index of 1.8 yields an absorbing column density of NH ∼ 2 × 1023 cm−2, although an unabsorbed power-law of photon index ∼1.6 is an equally good fit. With a net count number of ∼23 cts, we cannot confidently classify this source as an AGN.

Sales et al. (2015) also considered the possibility that the northern source might contain an embedded AGN, fitting the 0.435−500 μm SED with a model that includes an AGN torus component. The fit suggests an AGN with bolometric luminosity Lbol ∼ 1044 erg s−1, although the spectrum is also consistent with shocks (v ∼ 100−200 km s−1). This bolometric luminosity would imply a fraction erg s−1, much higher than the LX = 1041 erg s−1 detected in the Chandra data.

The southern source does not have a clear center in X-rays or MIR and FIR, and so the center for the radial profile was determined using the brightest region in the optical HST image. Because the source is clearly elongated, annuli centered on the eastern edge will include photons from the northern source, and to avoid interference, we removed it from the computation of radial profiles. This component meets our HR AGN selection criterion, although the image in Appendix B shows that no strong hard X-ray peak comes from the nucleus of the source; the origin of the hard counts is concentrated in two point-sources west of the nucleus. Therefore, we did not classify this source as an AGN.

[68] IRAS F16164–0746. This source meets two of our AGN selection criteria, the HR and the [Ne v] line, and is also classified as an optical Seyfert 2 in Yuan et al. (2010). The X-ray source is elongated in the soft band, in the direction perpendicular to the disk of the galaxy, which could be interpreted as an outflow. There is also a secondary point source ∼3″ from the nucleus, in the soft and hard band, without any obvious overlap with a star-forming knot. With an associated X-ray luminosity in the 2−10 keV range of ∼3 × 1040 erg s−1, it could be classified as a ULX.

[69] IC 4686/7. This source is part of a triple merger system, formed by IC 4687 in the north, which closely interacts with the central galaxy, IC 4686, at ∼0.5′, and IC 4689 ∼1′ south of IC 4686 (West 1976). All three galaxies contribute more than 10% to the IRAS flux (Chu et al. 2017), and were therefore analyzed here.

[71] NGC 2623. With a spectrum that clearly raises toward higher energies, giving a hardness ratio of HR = −0.11 ± 0.02, and also meeting the [Ne v] line criterion, this source is classified as an AGN. This source has been classified as an AGN previously in radio (Lonsdale et al. 1993) and X-rays (Maiolino et al. 2003).

Optical HST images show extended tidal tails, approximately 20−25 kpc in length, with a southern region rich in bright star clusters (Evans et al. 2008), although no X-ray emission is detected with Chandra in the region.

[72] IC 5298. This source is a clear absorbed AGN. It is visible in the Chandra spectrum presented here, and through XMM-Newton data analysis. When a photon index of 1.8 is assumed, a column density of NH ∼ 4 × 1023 is obtained when data from both telescopes are used. A faint line at 6.4 keV is visible in the Chandra data, with a significance lower than 1σ. The line can be confirmed with a significance of ∼2σ from XMM-Newton EPIC data, with a fit that is also consistent with the derived absorbing column density. The AGN diagnostics is also confirmed through the [Ne v] line and the optical S2 classification (Veilleux et al. 1995).

[73] IRAS 20351+2521. This galaxy shows strong central emission in X-rays, originating in the nucleus, with extended emission and point-sources along the spiral arms that trace star-forming knots.

[75] NGC 6090. This closely interacting system is completely unresolved in Herschel and MIPS data. Therefore, we resorted to the analysis performed by Hattori et al. (2004) to derive the contribution of each component to the IRAS flux listed in Table 3.

The northeastern source shows hard-band diffuse emission corresponding to the optical central region of the galaxy, and a peak ∼3″ north of the center. It corresponds to a particularly bright region in the optical and IR in one of the spiral arms.

The sources interact so closely that the radial profiles interfere with each other past 4−5″ from each nucleus, and have therefore been limited to this radius.

[79] NGC 5256. This closely interacting system is surrounded by diffuse soft-band emission in X-rays, part of which extends toward the northern direction, following a blue tidal stream seen in optical images. Between the two sources, a slightly curved excess is visible, which can be interpreted as a shock between colliding winds from both galaxies (see Mazzarella et al. 2012). This excess is the reason that the radial profile in Appendix B for the NE source shows an increase in soft-band surface brightness with distance at 5−6″.

This source has been detected in the [Ne v] line at kiloparsec scales, meeting our AGN selection criteria. However, because the two nuclei are located ver close to each other, it is not possible to know which (or if both) is responsible for this emission. The two optical classifications we used (Veilleux et al. 1995; Yuan et al. 2010) mark the NE source as a Seyfert 2, and the SW source as LINER or composite. However, Mazzarella et al. (2012) reported the opposite optical classification for the sources: the NE source as LINER and the SW source as a Seyfert 2 galaxy.

Based on the X-ray spectra, the NE source can be best fit with an absorbed AGN model, fixing a spectral index of 1.8 and obtaining a column density of NH ∼ 8 × 1022cm−2, which is interpreted as a mildly absorbed AGN.

The SW source shows an excess that can be fit as an iron 6.4 keV line with a confidence of ∼2.1σ, which meets one of our X-ray AGN selection criteria.

As reported by Mazzarella et al. (2012), XMM-Newton EPIC data only marginally resolve the two sources, and the spectrum is presented for the whole system. However, given the spectra resolved by Chandra, the iron line seen in the EPIC data most likely originates in the southwestern source. Their combined analysis also results in a Compton-thick classification of the south-western source.

[80] IRAS F03359+1523. Only one of the two sources in this system is observed in X-rays, the eastern source, with an elongated morphology that corresponds to the length of the edge-on disk in the optical data. The sources are unresolved in the Herschel data, and only one source is visible in the MIPS data, which is centered at the position of the eastern source. It is not possible to confirm whether this is due to lack of resolution, or if the western source does not contribute to the MIPS flux. However, from observing the IRAC images from channel 1 through 4 (at 3.6, 4.5, 5.8 and 8.0 μm), it is possible to see that the eastern source clearly dominates and the western source fades with increasing wavelength. Goldader et al. (1997) also described that only one source (believed to be the eastern source) is prominent at radio wavelengths. This, together with the complete lack of X-ray emission originating in this companion source, leads us to believe that the western galaxy does not contribute to the IRAS flux.

Another source, prominent in radio wavelengths, lies ∼1.5′ to the south of IRAS F03359+1523. Clemens et al. (2008) used NVSS radio data to extrapolate that this nearby galaxy could be responsible for about ∼50% of the IRAS flux. However, images at 8 and 24 microns show a weak source that fades completely at 70 micron, leading us to believe that its contribution to the FIR luminosity is most likely negligible.

[81] ESO 550–IG025. Both sources in this system, with a separation of ∼0.3′, contribute to the IRAS flux. The southern source has a rather flat spectrum, which is partly due to the contribution of the hard X-ray peak placed at about ∼4″ west of the nucleus. This source cannot be easily interpreted as the X-ray counterpart to any star-forming regions in the galaxy. If it is associated with this galaxy, its X-ray luminosity in the 2−10 keV range is of ∼3 × 1040 erg s−1, implying it could be classified as a ULX.

[82] NGC 0034. This source, optically classified as a Seyfert 2 (e.g., Veilleux et al. 1999; Yuan et al. 2010), has an X-ray spectrum that shows a hard band excess. Fitting an absorbed AGN with a fixed photon index of 1.8 gives an absorbing column density of NH ∼ 1 × 1023 cm−2. Previous analyses of XMM-Newton data confirmed an AGN, either through marginal detection of the Fe Kα line or by modeling an absorption or reflection component (e.g., Shu et al. 2007; Brightman & Nandra 2011).

Ricci et al. (2017) used joint data from Chandra, XMM-Newton, and NuSTAR and found a clear Fe Kα feature at . Their spectral analysis shows a heavily obscured AGN with a column density of NH = 5.3 ± 1.1 × 1023 cm−2. Their results certainly confirm the AGN, and their derived column density differs from the one derived with only Chandra, most likely because Chandra has much lower sensitivity at high energies.

[83] MCG+12−02−001. We consider this system to be composed of three individual sources: a northern component and a main pair, separated by ∼0.3′. The western source in the pair is considered an individual galaxy in close interaction with the eastern source, although it may also be an extended star-forming region. The X-ray peak at its center together with its X-ray luminosities of LSX = 1.3 × 1040 erg s−1 and LHX = 2.4 × 1040 erg s−1, which are comparable to those of the eastern galaxy, mean that this whole system likely is a triple.

The northern source does not contribute to the IRAS flux (Díaz-Santos et al. 2010), as specified in Table 3, and therefore was not analyzed. It is detected with Chandra, with ∼9 cts in the full 0.5−7 keV range.

[85] IRAS F17138−1017. This source has a rather flat spectrum in X-rays, with a flux that slightly increases toward higher energies. It meets the HR criterion for AGN selection, although no [Ne v] line is observed. Fitting with an absorbed AGN model, fixing a spectral index of 1.8, a low column density of NH ∼ 2 × 1022cm−2 is obtained.

Morphologically, Chandra data show a soft X-ray deficit at the optical center of the galaxy, that could be caused by absorption. The hard X-ray image does not show a clear emission peak, but a rather homogeneous flux around a larger circular region. X-ray contours in the HST image show very prominent dust lanes close to the nucleus of the galaxy, to which the obtained column density could belong. These dust lanes are most likely absorbing an important part of the soft-band X-ray emission, and might be responsible for the hardness of the spectrum. Based on this and the clear lack of observation of a hard-band peak in the nucleus, we opt not to classify this source as an AGN.

Ricci et al. (2017) fit a combined Chandra and NuSTAR spectrum with a simple power-law model, obtaining a photon index of ∼1.1, which is harder than the typical X-ray emission expected of a star-forming region, but still consistent with this hypothesis.

The hard-band X-ray luminosity of LHX ∼ 1.8 × 1041 erg s−1 we derive is high, but not incompatible with being caused by a strong starburst, as this source falls within the uncertainties of the correlation derived by Ranalli et al. (2003).

[95] ESO 440−IG058. Both galaxies, with an angular separation of ∼2′, contribute to the IRAS flux in this source, although the southern component dominates at almost ∼90% (Díaz-Santos et al. 2010). Soft X-ray emission from the southern component is extended. This is most likely an outflow with its origin in a starburst wind.

[100] NGC 7130. This galaxy shows clear extended emission in soft X-rays around a strong peak that follows the disk of the face-on optical galaxy, tracing the spiral arms. The spectrum shows a hard excess due to absorption and an iron 6.4 keV line at high energies, which could be due to absorption in the soft band, or due to reflection. A reflection component fitting, using a pexrav model (Magdziarz & Zdziarski 1995) results in an iron line with an equivalent width of ∼0.6 keV, which is too low for a reflection-originated line. Therefore, our data favor an absorption model, which, when a photon index of 1.8 is imposed, results in a column density of NH = 3 × 1023 cm−2 and an iron line equivalent width of 0.8 keV, that is detected with a significance of ∼2.5σ.

Based only on the Chandra data, we find it difficult to distinguish between this scenario and a Compton-thick source with an imposed photon index of Γ = 0.0, as modeled by Levenson et al. (2005). Ricci et al. (2017) confirmed the Compton-thick AGN using a combined analysis with NuSTAR data.

[104] NGC 7771. This galaxy is part of an interacting quartet of galaxies, along with close companion NGC 7770 at an angular distance of ∼1.1′, NGC 7771A at ∼2.8′, and NGC 7769 at ∼5.4′ (e.g., Yeghiazaryan et al. 2016).

About 90% of the IRAS flux originates in NGC 7771, with NGC 7770 being responsible for the remaining ∼10% and NGC 7769 being resolved as a separate source by IRAS. NGC 7771A is faint in the IR, remaining undetected at 8 μm and above. There is no detection for this small component in the Chandra data either.

Of the many point sources seen along the disk of NGC 7771, Luangtip et al. (2015) classify as ULXs.

[105] NGC 7592. This source is a closely interacting triple system, formed by two main IR and X-ray sources (East and West) and a smaller southern source. This third source, seen in the optical SDSS images, is undetected in X-rays, and does not contribute to the IRAS flux either.

There is unresolved detection of the [Ne v] line for this triple source that meets our AGN selection criterion. However, as the western source is classified as an optical Seyfert 2, it is likely that it is the origin of the IR line. The spectrum of the western source shows an excess between 6−7 keV, originating in the nucleus, that can be fitted as a Gaussian line with an energy of . The significance of this line is at the 1σ level, and thus we did not use this hint as a selection criterion. Given the uncertainties, however, a 6.4 keV line cannot be ruled out completely, especially when combined with the continuum.

The western source presents very compact X-ray emission, as derived from its radial profile, compared to its extended IR emission (as plotted in Fig. 12).

[106] NGC 6286. This source interacts with NGC 6285, ∼1.5′ to the northwest, showing very extended tidal disruption features. Both sources contribute to the IRAS flux (Chu et al. 2017).

The X-ray spectrum of NGC 6286 shows hard excess emission above 5 keV. With fewer than 20 cts in the 5−8 keV range, the excess is difficult to fit as an absorbed AGN using only Chandra data. MIR studies find possible hints of an AGN (e.g., Vega et al. 2008; Dudik et al. 2009), which are confirmed by hard X-ray NuSTAR observations. Ricci et al. (2016) found compelling evidence of a Compton-thick, low-luminosity AGN (NH ⋍ (0.95−1.32)×1024 cm−2). We thus classify this source as an AGN.

This galaxy shows a very extended soft X-ray emission, spreading perpendicular to the optical edge-on disk up to a distance of ∼5−7 kpc, depending on the direction. A super-wind outflow generated by a strong starburst has been suggested by Shalyapina et al. (2004) through detection of an increase of [NII]λ6583/Hα ratios, and an emission nebula extending up to ∼9 kpc from the galactic plane.

[107] NGC 4922. This system contains two galaxies, separated by distance of ∼0.4′; the northern source is brighter in both X-rays and IR (Díaz-Santos et al. 2010). The southern source contributes ∼1% of the IRAS flux, and therefore its analysis is not included in this work. With only ∼40 cts, all in the 0.4−2 keV range, it is also a weak X-ray source. Another source (2MASX J13012200+2920231) lies ∼1.7′ to the north, which is undetected at 8 μm and above, and most likely does not contribute to the IRAS flux.

The northern source is selected as an AGN through the [Ne v] line, and the pair (unresolved) is also classified as a Seyfert 2 in Yuan et al. (2010). Our X-ray analysis also classifies it as an absorbed AGN, with a column density of NH ∼ 3 × 1023 cm−2 when the photon index is fixed to 1.8. An iron 6.4 keV line is faintly visible, although only at a significance of about 1σ.

Ricci et al. (2017) analyzed NuSTAR observations and based on their similar Chandra results concluded that the source detected at high energies must correspond to NGC 4922 (N), because the companion is not detected in the 2−7 keV range. They detected a prominent Fe Kα line at keV, and found that the source is Compton-thick, with NH ≥ 4.27 × 1024 cm−2; this is more than one order of magnitude higher than our best Chandra fit.

[110] NGC 3110. This source interacts with nearby galaxy MCG−01−26−013 at its southwest, separated by ∼1.8′, which is not detected in X-rays in the Chandra data. Both sources contribute to the IRAS flux, although ∼90% of the IR emission has its origin in NGC 3110 (Díaz-Santos et al. 2010). The companion galaxy is not analyzed because it has no significant X-ray emission and low IR luminosity, although the IRAS flux associated with NGC 3110 is corrected for the pair’s contribution.

This source has diffuse soft X-ray emission along the spiral arms, which also contain strong hard X-ray peaks that are most likely associated with star-forming knots. The nucleus of the galaxy, however, does not show peaked emission in the 2−7 keV band. Because of this particular morphology, HST optical and IRAC channel 1 images were used to center the derived radial profiles.

[114] NGC 0232. This source is paired with NGC 0235, at a distance of ∼2′, which is a known Seyfert 2 galaxy. Despite having previously been classified as non-interacting, a faint tidal bridge has been observed to connect the two galaxies (Dopita et al. 2002). NGC 0235 has two nuclei and is classified as a minor interaction (Larson et al. 2016). However, as this companion galaxy is resolved by IRAS (Surace et al. 2004) as an individual source, we did not include it in our analysis.

[117] MCG+08−18−013. This galaxy is paired with MCG+08−18−012 at ∼1′ to its west. MCG+08−18−013 is clearly dominant in the IR and the origin of the IRAS flux (Chu et al. 2017). This component’s X-ray emission originates from two point sources close to the nucleus of the galaxy, one of which is bright in soft-band X-rays, and could be associated with a star-forming region, and the other in hard-band X-rays. We consider this hard-band peak to originate from the nucleus of the galaxy, and used it to center the radial profiles.

MCG+08−18−013 is undetected in X-rays and therefore not included in the analysis.

[120] CGCG 049−057. This source, despite only having a total of ∼30 cts in the 0.5−7 keV band of the available Chandra observation, has a hardness ratio of HR = −0.04 ± 0.09, and so meets one of the X-ray AGN selection criteria. The spectrum of the source shows, despite the large error bars, a tendency to rising flux toward higher energies. The X-ray image in the 2−7 keV band shows about ∼5 cts originating from the innermost region of the source, and thus we classified it as an AGN.

Baan & Klöckner (2006) also classified it as an AGN based on radio observations. Although optical and MIR observations (e.g., Veilleux et al. 1995; Stierwalt et al. 2013; Meléndez et al. 2014) classified it as a starburst, Herschel spectroscopic data analyzed by Falstad et al. (2015) showed very high column densities in the nucleus (NH = 0.3−1.0 × 1025 cm−2), meaning that a Compton-thick AGN could be present. This would explain the X-ray weakness we observe, which has previously been reported by Lehmer et al. (2010).

[121] NGC 1068. This well-known AGN meets our selection criteria in all bands: Seyfert 2 in both of the used optical classifications, presence of the [Ne v] line in IR, and clear detection of the Fe Kα line at 6.4 keV in X-rays with a significance of ∼3.6σ. The equivalent width of the 6.2 μm PAH feature is not presented in Stierwalt et al. (2013) because the spectrograph was saturated. Howell et al. (2007) analyzed PAH and warm dust emission in NGC 1068 in detail. Their 6.2 μm images are saturated within the inner r ∼ 500 pc, although they measure an equivalent width of the PAH feature immediately outside the region of saturation of ∼0.1. This suggests that the value of the equivalent width might drop below 0.1 farther in.

Diffuse X-ray emission is clearly observed in this source, following the optical spiral arms and star-forming regions. In order to outline all features, X-ray contours to a low enough level were necessary, which also resulted in the clear contours around the saturated feature that diagonally crosses the image.

Individual point sources can be seen spread throughout the galaxy disk in the soft and hard bands; they most likely correspond to X-ray binaries. We note that we did not mark them as hard X-ray peaks in the image in Appendix B because they are numerous and clearly do not originate from a region near the nucleus of the galaxy. None of these point sources were removed in order to derive radial profiles. Luangtip et al. (2015) classified three of them as ULXs.

[123] UGC 02238. This source presents a rather diffuse emission, showing three main X-ray peaks near the nucleus, only one of which (the westernmost) is also peaked in the 2−7 keV band. However, as shown by the contours over the IRAC channel 1 image, this region is outside of the galaxy nucleus. We consider this emission to most likely originate from different intense starburst regions because no clear hard-band central peak is visible. Optical and IR imaging data show a highly disturbed disk and tidal tails, and classify this galaxy as a post-merger stage (e.g., Smith et al. 1996; Larson et al. 2016), which is consistent with the described X-ray morphology.

We fit the overall X-ray 0.5−7 keV emission of this galaxy with a single power-law. Attempts to fit a one or two component mekal model produced unsatisfactory results. Two strong point sources can be seen in the 2−7 keV band image presented in Appendix B, which, if associated with the galaxy, would be classified as ULXs. The point-source at the easternmost edge of the disk of the galaxy would have an estimated luminosity of ∼4 × 1040 erg s−1, and the strong point-source immediately south of the nucleus of the galaxy would be emitting ∼1 × 1040 erg s−1 in the 2−10 keV range. However, the very low number of counts means that these are very rough estimates.

[127] MCG-03-34-064. This source has a northeastern companion at ∼1.8′, MCG−03−34−063, which is responsible for about ∼25% of the IRAS flux (Chu et al. 2017). The analysis of this companion source is not included in this work because it is undetected in the Chandra data. A correction to the IR luminosity for the contribution of MCG−02−34−063 has been considered for this source.

MCG-03-34-064 presents a very peaked central emission in all X-ray bands and is a clear absorbed AGN, as seen from the spectrum. Fitting with a fixed photon index of 1.8 yields an absorbing column density of NH ∼ 5 × 1023 cm−2. The iron Kα line at 6.4 keV is detected, with a ∼3σ significance. With an HR = −0.32 ± 0.01 our other X-ray AGN selection criterion is almost also met.

Ricci et al. (2017) performed a NuSTAR analysis of this source, combined with XMM-Newton EPIC data, and derived an absorbing column density of cm−2, which is compatible with our derived result within the errors. They also detected the Fe Kα line and a Gaussian line at keV with EW ∼ 0.2 keV, that is not detected in the Chandra data.

This source also meets the [Ne v] line and 6.2 μm PAH feature AGN selection criteria. Yuan et al. (2010) classified it as a star-forming galaxy, although other works classified it as a Seyfert galaxy (e.g. Lipovetsky et al. 1988; Corbett et al. 2002).

[134] ESO 350−IG038. This galaxy presents three main star-forming condensations (Kunth et al. 2003; Atek et al. 2008). Only two of these knots, the eastern and western, are clearly resolved as X-ray sources in the Chandra data, both presenting peaked emission in the soft and hard bands. The region is surrounded by diffuse, soft X-ray emission. These knots, separated by ∼4″, are analyzed together as the X-ray source corresponding to the IRAS source, and not separated as two individual galaxies, as there is no clear evidence of them being individual galaxy nuclei in a state of a closely interacting merger.

[136] MCG−01−60−022. This source is near galaxies MCG−01−60−021 and Mrk 0399, at ∼4.4′ interacting with the former (e.g., Dopita et al. 2002), connected through thin and long tidal bridges. The two nearby galaxies are undetected in the Chandra data, and are detected together as another IRAS source, resolved from MCG−01−60−022 (Díaz-Santos et al. 2010).

This source presents diffuse soft-band emission that surrounds the central X-ray peak, which has its origin in an absorbed AGN. This sources meets the HR X-ray criterion, and spectral fitting of an absorbed power-law with a fixed photon index of 1.8 yields a an absorbing column density of NH ∼ 1 × 1023 cm−2.

[141] IC 0563/4. This source is a double system, composed of IC 0564 in the north and IC 0563 in the south, separated by ∼1.6′. Both contribute similarly to the IRAS flux and to the overall X-ray luminosity. Morphologically, the two galaxies have faint emission that originates in the nucleus and various point-sources spread throughout the spiral disks.

IC 0563 has a hardness ratio of −0.34 ± 0.05, which exceeds our AGN selection threshold. However, the origin of the hardness (also seen as an excess at 3−5 keV in the spectrum shown in Fig. C.1) is not the nucleus of the galaxy, but a point-source located north of it. If the source is associated with the galaxy, with a roughly estimated luminosity of ∼3 × 1040 erg s−1, it could be classified as a ULX. Interestingly, the point-source spectrum shows a faint line at ∼1.50 ± 0.03 keV, with a significance of ∼2σ. If this source is a ULX within the galaxy, this excess cannot be easily explained as an emission line. If this source is a background quasar for which we detect a redshifted 6.4 keV iron line, a high z ∼ 3.3 would be necessary. If it were an object at z = 3.3, the X-ray spectrum would suggest that its origin is in reflected light form a Compton-thick AGN. This scenario, however, leads to an unreasonably luminous quasar.

Similarly, IC 0564 shows a spectrum with a high flux at high energies, although the error bars are significant, which is also emitted by a northern point-source. With a roughly estimated luminosity of ∼3 × 1040 erg s−1, it might also be classified as a ULX if it is associated with the galaxy.

Both point-sources are marked with green crosses in the images shown in Appendix B.

[142] NGC 5135. This galaxy is classified as an optical Seyfert 2 (e.g. Yuan et al. 2010), and meets our IR [Ne v] line criterion for AGN selection (Petric et al. 2011).

It shows an excess in hard X-rays, with a Fe Kα line at 6.4 keV with a ∼2.9σ significance, which could be the result of either absorption or a reflection component. Fitting an absorbed power-law with fixed photon index of 1.8 yields an absorbing column density of NH ∼ 4 × 1023 cm−2, which is not large enough to produce the equivalent width of the iron line of EW ∼ 0.9 keV obtained through the same model. Fitting a pexrav model (Magdziarz & Zdziarski 1995) with a fixed photon index of 2.0 yields a plausible EW ∼ 1.1 keV, which means that our data favor a reflection-dominated AGN. Suzaku observations extending up to 50 keV allow a better estimate of the absorbing column density, ∼2.5 × 1024 cm−2, classifying this source as Compton-thick and providing a good estimate of the strength of the reflection component (Singh et al. 2012).

Morphologically, this source presents a very extended soft-band emission, with two central X-ray peaks that are visible only when the smoothing in the image is set to 0.5″ or less. The northern peak is responsible for the iron emission line, which indicates that it is associated with the nucleus of the galaxy. The southern peak is brighter in the 0.5−2 keV band, and most likely associated with a star-formation region. Of the many point-sources seen in the full band image, up to are classified as ULXs (Luangtip et al. 2015).

[144] IC 0860. With only ∼25 cts, no X-ray analysis beyond the calculation of the HR and the extraction of the radial profile has been performed for this source. However, despite the low count-rate, this galaxy is classified as an AGN with a value of HR = −0.29 ± 0.17. The spectrum we obtained also shows a rising tendency toward higher energies. However, because of the small number of counts detected, the classification of this X-ray source remains ambiguous.

[147] IC 5179. This galaxy shows dim soft-band extended emission near the nucleus and many X-ray point-sources spread throughout the optical disk, which most likely correspond to X-ray binaries, of which are classified as ULXs (Luangtip et al. 2015). These sources cause the radial profile to seem rather irregular, especially in hard band (see Appendix B).

[148] CGCG 465−012. This galaxy is paired with UGC 02894 at its northwest, at a distance of ∼4.2′, which is resolved as a separate source by IRAS.

CGCG 465−012 shows diffuse soft X-ray emission throughout its optical disk, concentrated in the nucleus and in a northeastern region ∼5″ from it, most likely a star-forming region. The hard-band emission is very dim and not peaked.

[157] MCG−02−33−098/9. This system is composed of two very closely interacting galaxies (separated by ∼14″), with the western one contributing ∼70% of the IRAS flux (Díaz-Santos et al. 2010). Two nearby galaxies, at ∼0.7′ northwest and ∼2′ southeast, most likely do not contribute to the IRAS flux, as they are not detected in MIR wavelengths.

Terashima et al. (2015), using XMM-Newton EPIC-PN data, reported the detection of a faint iron line at 6.97 keV. The Chandra spectrum of MCG−02−33−098 shows a slight increase at ∼7 keV, which is not significant enough to claim the presence of an excess.

[163] NGC 4418. This galaxy is paired with MCG+00−32−013 at its southeast, at a distance of ∼3′. Using IR photometry, it can be determined that more than 99% of the IRAS flux originates in NGC 4418 (Chu et al. 2017), therefore this nearby galaxy was not considered in the analysis. It also meets our PAH EW selection criteria (Stierwalt et al. 2013), and thus we classified it as AGN, despite the current debate regarding its nature.

Only two central peaks can be seen in the Chandra data; the eastern peak is brighter in the hard band, and thus was used to center the radial profiles. However, this source is known as a possible candidate for containing a heavily obscured AGN, and it is possible that we resolved the non-absorbed emission at either side of the nucleus. While some studies in radio and IR seemed to favor a compact starburst as a central source (e.g., Roussel et al. 2003; Lahuis et al. 2007), it is at least clear that the nucleus is extremely Compton-thick and could host either an AGN or a starburst of obscuration as extreme as the one in Arp 220 (see Costagliola et al. 2013; Costagliola et al. 2015, and references therein).

[169] ESO 343−IG013. Both galaxies in this closely interacting merger, separated by only ∼0.2′, contribute to the IRAS flux, with the northern component dominant in both IR (Díaz-Santos et al. 2010) and X-rays.

The X-ray emission is diffuse in the southern source and between sources. The northern source shows a bright X-ray peak in the soft and hard bands, which meets one of our AGN selection criteria, having HR = 0.01 ± 0.05. The X-ray spectrum of this component also shows an increase in flux toward higher energies.

The radial profile of the southern source has been centered using the brightest peak in optical and IR images, which corresponds to the dimmer X-ray peak of the 0.5−7 keV image shown in Appendix B. The northern source presents very compact X-ray emission, as derived from its radial profile, compared to its extended IR emission (as plotted in Fig. 12).

[170] NGC 2146. This galaxy, most likely a post-merger object (Hutchings et al. 1990), shows very extended soft-band X-ray emission in the direction perpendicular to the plane of the optical disk, originating in a super-wind driven by the central starburst (see Kreckel et al. 2014). The hard-band emission is limited to the region encompassed by the galaxy disk, which presents a lack of soft-band emission. This lack is most likely a result of absorption in the plane of the galaxy. The radial profiles have been centered using the brightest peak in the NIR IRAC channel 1 image.

A detailed Chandra analysis of point sources in the galaxy, including seven ULXs, and the extended emission, can be found in Inui et al. (2005).

[174] NGC 5653. This source presents a diffuse X-ray emission along the spiral arms seen in the optical images, with a bright X-ray knot at a distance of ∼15″ from the nucleus. This knot falls on one of the spiral arms and appears very blue in optical images; it also is the strongest X-ray and IR source in the galaxy (Díaz-Santos et al. 2010). It could be argued that this source is a second galaxy, that merges with the larger NGC 5653. This source has been classified as a lopsided galaxy (Rudnick et al. 2000), which is usually assumed to be an indicator of weak tidal interaction. However, as we have no clear evidence of this and do not see a central point-source in hard X-rays, we opt to consider it a particularly active star-forming region. Luangtip et al. (2015) found 1 ± 1 ULXs in this source.

The NIR IRAC channel 1 image was used to centre the radial profile, due to the difficulty of finding a clear nucleus in the X-ray data.

[178] NGC 4194. This source, commonly known as the Medusa, is the result of a merger with very particular tidal features, as seen in optical images. The X-ray morphology of the source is also particular, showing a very extended emission in soft X-rays, especially toward the northwest, most likely indicative of a strong starburst-driven wind. Luangtip et al. (2015) find 1 ± 1 ULXs in this source, while a detailed study of all X-ray point sources was performed by Kaaret & Alonso-Herrero (2008).

[179] NGC 7591. This galaxy interacts with PGC 214933, at ∼1.8′ southwest; and a dimmer galaxy lying ∼3.6′ to the east is detected in HI (Kuo et al. 2008). PGC 214933 contributes ∼6% of the IRAS flux (Chu et al. 2017), and is undetected in the Chandra data.

With only ∼26 cts in the 0.5−7 keV band, no X-ray analysis beyond the calculation of the HR and the extraction of the radial profile has been performed for NGC 7591.

NGC 7591 is the only source in our sample that is optically classified as a Seyfert 2 galaxy (Yuan et al. 2010), while being classified as a LINER by Veilleux et al. (1999), and does not meet any of our AGN selection criteria. The contribution of an AGN component to the bolometric luminosity of the galaxy, as estimated by (Díaz-Santos et al. 2017), is low: AGNbol = 0.09 ± 0.02. With a flux estimation using the CIAO tool srcflux, the obtained luminosity of this source in the 0.5−7 keV band is erg s−1. This X-ray luminosity is low for an AGN, and only possible for this type of source if is Compton-thick. Even though we cannot rule out this possibility, we consider that a single optical classification as Seyfert 2 is not a strong enough criterion to classify this source as an AGN.

[182] NGC 0023. This source is paired with NGC 0026, at a distance of ∼9.2′ (Hattori et al. 2004), which is far enough to guarantee no contribution to the IRAS flux.

This source shows central extended emission surrounding the nucleus, which is not a strongly peaked hard X-ray source. The source spectrum shows a lack of emission at > 3 keV and a small excess at higher energies, which is not significant enough indicate an AGN. Luangtip et al. (2015) classified 2 ± 2 of the galaxy’s point sources as ULXs.

[188] NGC 7552. This source shows extended soft-band X-ray emission in the inner region of the galaxy, surrounded by numerous point-sources, which most likely correspond to X-ray binaries. Of these, are classified as ULXs (Luangtip et al. 2015).

[191] ESO 420-G013. This source meets our IR [Ne v] line criterion for AGN selection, and shows a slight excess at around 6.4 keV, which is not significant enough in the Chandra data to confirm the presence of an X-ray AGN. Although Yuan et al. (2010) classified is as HII dominated, other optical classifications have previously pointed toward a Seyfert nature (e.g., Maia et al. 1996)

[194] ESO 432-IG006. The two galaxies in this system, separated by ∼0.5′, contribute similarly to the IRAS flux, and have significant X-ray emission. They also present signs of an absorbed AGN. Fitting such a model, with a fixed photon index of 1.8, on the northeastern source yields an absorbing column density of NH ∼ 4 × 1023 cm−2; and on the south-western source it yields NH ∼ 1 × 1023 cm−2.

[195] NGC 1961. Most of the X-ray emission of this source is concentrated on the nucleus, with a few point sources spread throughout the spiral arms. Another emission peak is found in a region about ∼20″ west of the nucleus. It is unclear whether this source truly overlaps with emission regions in IR images (IRAC channels 1−4, MIPS24/70), or has no counterpart.

The spectrum presented in Fig. C.1 includes the full region, and the hard X-ray excess originates in this outer source. Therefore, we see no spectral traces of the presence of an AGN.

[196] NGC 7752/3. This double system is composed of NGC 7752 to the southwest and NGC 7753 ∼2′ to the northeast, the latter being the dominant source in the IRAS flux (Chu et al. 2017).

X-ray emission in the northeastern source is point-like in the nucleus, with a few other point sources spread throughout the galaxy spiral arms. The X-ray best spectral fit for this source was performed using a single mekal component for the full spectrum, but no fit is truly satisfactory.

The southwestern source presents much more diffuse emission, with no clear central point source. The most concentrated emission in the soft band comes from a region just west of the IR nucleus of the galaxy, which is the one we used to center the radial profile. This region seems to correspond to a slight increase in IR emission in the IRAC channel 1 image, which means that it is most likely a star-forming region. (Kewley et al. 2001, 2006) used optical diagnostic diagrams to classify this source either as an AGN or the composite of AGN and starburst (Zhou et al. 2014; Pereira-Santaella et al. 2015a), although we see no X-ray signs of activity.

[198] NGC 1365. This very bright source meets all three of our X-ray AGN selection criteria, with HR = −0.19 ± 0.00, an absorbing column density of NH ∼ 3 × 1023 cm−2 when fitting an absorbed AGN model with fixed photon index of 1.8, and a 6.4 keV iron Kα line at a significance of ∼2.9σ. This galaxy hosts a very well-known AGN, with frequent dramatic spectral variability (e.g., Risaliti et al. 2005, 2007) that is attributable to variations in the column density along the line of sight.

The Chandra data show bright and extended diffuse emission in soft band, spreading along the central region of the galaxy, and a strong central point source (see, e.g., Wang et al. 2009). Luangtip et al. (2015) found ULXs in this galaxy. We have chosen to show only the central emission in the galaxy in Appendix B and not the open spiral arms around it, in order to better distinguish the X-ray morphology of the region of interest.

[199] NGC 3221. This source consists mostly of point-like sources scattered along the optical edge-on disk of the galaxy, of which are classified as ULXs (Luangtip et al. 2015). The hard-band spectrum can be fit with a power law of index Γ = 0.3 ± 0.6, with the nucleus being a stronger hard X-ray source than the other point sources. Even though such a flat spectrum could be indicative of an obscured AGN, fitting an absorbed power-law with a fixed photon index of 1.8 yields an absorbing column density of only NH ∼ 5 × 1022 cm−2. We conclude that we cannot confirm the presence of an absorbed AGN, even though we cannot rule out the possibility either.

[201] NGC 0838. This galaxy is in a complex system. NGC 0839 is placed ∼2.4′ to the south–east, and the center of the closely interacting system formed by NGC 0833 and NGC 0835 is found at ∼4′ to its west. IRAS resolves three of these four sources; NGC 0833/0835 interact too closely to derive their fluxes separately. However, because NGC 0838 is resolved, we did not include the rest of the components of the complex system in this work.

Oda et al. (2018) analyzed 3−50 keV NuSTAR data of this compact group. NGC 0838 is not detected above 8 keV, showing no evidence of an obscured AGN. The conclusion that NGC 0838 is a starburst-dominated galaxy is also reached in the detailed works by O’Sullivan et al. (2014); Turner et al. (2001).

This source is very bright in X-rays and has a complex morphology of diffuse soft-band emission surrounding the nucleus, a clear example of a strong starburst wind. Of the point sources, are classified as ULXs (Luangtip et al. 2015).

Appendix B: Observations

thumbnail Fig. B.1.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.2.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.3.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.4.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.5.

Overlay on HST-ACS F814W. Contours: Interval 3.

Open with DEXTER

thumbnail Fig. B.6.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.7.

Overlay on HST-ACS F814W. Contours: Interval 4.

Open with DEXTER

thumbnail Fig. B.8.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.9.

Overlay on HST-ACS F814W. Contours: Interval 2.

Open with DEXTER

thumbnail Fig. B.10.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.11.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.12.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.13.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.14.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.15.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.16.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.17.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.18.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.19.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.20.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.21.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.22.

Overlay on HST-ACS F814W. Contours: Interval 2.

Open with DEXTER

thumbnail Fig. B.23.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.24.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.25.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.26.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.27.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.28.

Overlay on IRAC channel 1. Contours: Interval 2.

Open with DEXTER

thumbnail Fig. B.29.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.30.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.31.

Overlay on SDSS DR-12 i-band. Contours: Interval 2.

Open with DEXTER

thumbnail Fig. B.32.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.33.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.34.

Overlay on IRAC channel 1. Contours: Interval 2.

Open with DEXTER

thumbnail Fig. B.35.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.36.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.37.

Overlay on SDSS DR-12 i-band. Contours: Custom.

Open with DEXTER

thumbnail Fig. B.38.

Overlay on SDSS DR-12 i-band. Contours: Custom.

Open with DEXTER

thumbnail Fig. B.39.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.40.

Overlay on IRAC channel 1. Contours: Interval 4.

Open with DEXTER

thumbnail Fig. B.41.

Overlay on IRAC channel 1. Contours: Interval 2.

Open with DEXTER

thumbnail Fig. B.42.

Overlay on SDSS DR-12 i-band. Contours: Interval 2.

Open with DEXTER

thumbnail Fig. B.43.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.44.

Overlay on IRAC channel 1. Contours: Custom.

Open with DEXTER

thumbnail Fig. B.45.

Overlay on SDSS DR-12 i-band. Contours: Interval 2.

Open with DEXTER

thumbnail Fig. B.46.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.47.

Overlay on SDSS DR-12 i-band. Contours: Interval 2.

Open with DEXTER

thumbnail Fig. B.48.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.49.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.50.

Overlay on IRAC channel 1. Contours: Interval 2.

Open with DEXTER

thumbnail Fig. B.51.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.52.

Overlay on SDSS DR-12 i-band. Contours: Interval 2.

Open with DEXTER

thumbnail Fig. B.53.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.54.

Overlay on SDSS DR-12 i-band. Contours: Interval 4.

Open with DEXTER

thumbnail Fig. B.55.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.56.

Overlay on IRAC channel 1. Contours: Interval 4.

Open with DEXTER

thumbnail Fig. B.57.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.58.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.59.

Overlay on IRAC channel 1. Contours: Interval 3

Open with DEXTER

thumbnail Fig. B.60.

Overlay on SDSS DR-12 i-band. Contours: Interval 4.

Open with DEXTER

thumbnail Fig. B.61.

Overlay on IRAC channel 1. Contours: Custom.

Open with DEXTER

thumbnail Fig. B.62.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER

thumbnail Fig. B.63.

Overlay on SDSS DR-12 i-band. Contours: Custom.

Open with DEXTER

Appendix C: X-ray spectra

thumbnail Fig. C.1.

X-ray flux density spectra for the 84 individual galaxies of CGII, obtained from the Chandra ACIS. Flux density in units of 10−14 erg s−1 cm−2 keV−1.

Open with DEXTER

All Tables

Table 1.

Basic parameters of the objects in the C-GOALS II sample.

Table 2.

Chandra observation log for the objects in the CGII sample.

Table 3.

IR fractions.

Table 4.

X-ray contour ranges.

Table 5.

Sources with an AGN signature in IR or X-rays.

Table 6.

Fe Kα line fits.

Table 7.

Statistical X-ray properties of the sample.

Table 8.

Half-light radius

Table 9.

Merger stage.

Table A.1.

X-ray spectral properties for the sample.

Table A.2.

X-ray spectral parameters for the sample.

All Figures

thumbnail Fig. 1.

Distribution of luminosity distance (left panel) and IR luminosity LIR(8−1000 μm) (right panel) for the 44 objects of C-GOALS I (Iwasawa et al. 2011), the 63 objects of C-GOALS II, and the 201 systems of the full GOALS sample (Armus et al. 2009). The vertical dashed line represents LIR = 1012 L, the boundary between LIRGs and ULIRGs.

Open with DEXTER
In the text
thumbnail Fig. 2.

X-ray images and surface brightness profiles for NGC 2146. North is up and east to the left. Similar figures for all 59 objects in the CGII sample are presented in Appendix B. Upper left: X-ray (0.4−7 keV) brightness contours (magenta) with marked hard X-ray peaks (green crosses) overlaid on optical/IR images. Upper right: Radial surface brightness profiles in the 0.5−2 keV band (open squares) and the 2−7 keV band (filled squares). Profiles have been centered using the brightness peak in the hard X-ray band, when clearly originating in the nucleus. We refer to Appendix A for ambiguous objects. Bottom: From left to right, unsmoothed and smoothed images in the 0.4−7 keV band, and smoothed images in the soft (0.5−2 keV) and hard (2−7 keV) bands. The pixel size is ∼0.5″ × 0.5″. The scale bar in the bottom left image represents 5″.

Open with DEXTER
In the text
thumbnail Fig. 3.

X-ray flux density spectra for MCG-03-34-064, obtained from the Chandra ACIS. Flux density in units of 10−14 erg s−1 cm−2 keV−1.

Open with DEXTER
In the text
thumbnail Fig. 4.

Hardness ratio as a function of the 2−7 keV luminosity for all sources in the CGII sample. All AGN from Table 5 are plotted as filled squares, and those in which absorption features are fit (labeled A in the table) are marked with an open circle. The dashed line shows the −0.3 boundary, above which sources are selected as AGN (unless evidence points toward a lack of AGN presence, see Appendix A).

Open with DEXTER
In the text
thumbnail Fig. 5.

Hardness ratio as a function of the fractional contribution of the AGN to the bolometric luminosity (as derived from MIR data, Díaz-Santos et al. 2017) of the source, in red for CGI sources and black for CGII. X-ray selected AGN from Table 5 are plotted as filled squares.The horizontal dashed line shows the HR = −0.3 threshold. The vertical dashed line shows the value above which the AGN is energetically significant.

Open with DEXTER
In the text
thumbnail Fig. 6.

Left panel: distribution of mekal model temperatures, where T1 is the temperature of the external, colder gas component and T2 is the temperature of the internal, hotter gas component, for the CGII sample. Right panel: distribution of absorbing column densities associated with the inner, hotter gas component, for the CGII sample.

Open with DEXTER
In the text
thumbnail Fig. 7.

Distributions of soft-band X-ray luminosity, 0.5−2 keV (left panel), and hard-band X-ray luminosity, 2−10 keV (right panel), for the individual galaxies of CGI and CGII.

Open with DEXTER
In the text
thumbnail Fig. 8.

Plots of soft (left panel, 0.5−2 keV) and hard (right panel, 2−10 keV) X-ray luminosity vs. IR luminosity, where the X-ray luminosity is corrected only for Galactic absorption. X-ray selected AGN, shown in Table 5, are shown in black. When multiple objects are present in a source, their IR luminosity is divided, as shown in Table 3.

Open with DEXTER
In the text
thumbnail Fig. 9.

Plots of soft (left panel, 0.5−2 keV) and hard (right panel, 2−10 keV) X-ray luminosity versus FIR luminosity derived as in Eq. (1), where the X-ray luminosity is corrected only for Galactic absorption. Data used by Ranalli et al. (2003), along with their derived correlation, are shown in blue. CGI and CGII data (for galaxies without an AGN) are plotted in red and black squares respectively. When multiple objects are present in a source, their IR luminosity is divided as shown in Table 3. All sources containing AGN, as listed in Table 5 or classified as AGN by Iwasawa et al. (2011) have been removed both from the plot and from the fits. The red, dashed line shows our best quadratic fit for the C-GOALS + Ranalli et al. (2003) data. Grey, dashed lines (left panel) show theoretical lines of obscuration for NH = 0.5, 1.0, 2.0, 5.0 × 1022 cm22, as described in Sect. 4.5.

Open with DEXTER
In the text
thumbnail Fig. 10.

Histogram of half-light radii for the sources in the CGII sample.

Open with DEXTER
In the text
thumbnail Fig. 11.

Half-light radius as a function of FIR luminosity (as in Eq. (1)) for galaxies within the CGII (black) and CGI (red) sample. Sources without an AGN are plotted as open squares, while sources with an AGN are plotted as filled squares.

Open with DEXTER
In the text
thumbnail Fig. 12.

Soft X-ray half-light radius as a function of 70 μm FIR radius, taken from Díaz-Santos et al. (2017). CGI data are shown in red, and CGII data in black. X-ray selected AGN and MIR selected AGN (as specified in Table 5) are plotted separately. The dashed line shows , and is not a fit to the data. Eleven systems within the whole C-GOALS sample are resolved into individual galaxies in X-rays but not at 70 μm, and are thus not plotted.

Open with DEXTER
In the text
thumbnail Fig. 13.

Plots of X-ray luminosity surface density vs. IR luminosity surface density. Left panel: red data correspond to CGI sources, and black data corresponds to CGII sources. AGN are highlighted as filled symbols. Right panel: sources in the full C-GOALS sample are plotted as different symbols according to merger stage, as derived by Stierwalt et al. (2013). 11 systems within the whole C-GOALS sample are resolved into individual galaxies in X-rays but not at 70 μm, and are thus not plotted.

Open with DEXTER
In the text
thumbnail Fig. B.1.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.2.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.3.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.4.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.5.

Overlay on HST-ACS F814W. Contours: Interval 3.

Open with DEXTER
In the text
thumbnail Fig. B.6.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.7.

Overlay on HST-ACS F814W. Contours: Interval 4.

Open with DEXTER
In the text
thumbnail Fig. B.8.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.9.

Overlay on HST-ACS F814W. Contours: Interval 2.

Open with DEXTER
In the text
thumbnail Fig. B.10.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.11.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.12.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.13.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.14.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.15.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.16.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.17.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.18.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.19.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.20.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.21.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.22.

Overlay on HST-ACS F814W. Contours: Interval 2.

Open with DEXTER
In the text
thumbnail Fig. B.23.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.24.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.25.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.26.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.27.

Overlay on HST-ACS F814W. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.28.

Overlay on IRAC channel 1. Contours: Interval 2.

Open with DEXTER
In the text
thumbnail Fig. B.29.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.30.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.31.

Overlay on SDSS DR-12 i-band. Contours: Interval 2.

Open with DEXTER
In the text
thumbnail Fig. B.32.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.33.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.34.

Overlay on IRAC channel 1. Contours: Interval 2.

Open with DEXTER
In the text
thumbnail Fig. B.35.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.36.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.37.

Overlay on SDSS DR-12 i-band. Contours: Custom.

Open with DEXTER
In the text
thumbnail Fig. B.38.

Overlay on SDSS DR-12 i-band. Contours: Custom.

Open with DEXTER
In the text
thumbnail Fig. B.39.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.40.

Overlay on IRAC channel 1. Contours: Interval 4.

Open with DEXTER
In the text
thumbnail Fig. B.41.

Overlay on IRAC channel 1. Contours: Interval 2.

Open with DEXTER
In the text
thumbnail Fig. B.42.

Overlay on SDSS DR-12 i-band. Contours: Interval 2.

Open with DEXTER
In the text
thumbnail Fig. B.43.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.44.

Overlay on IRAC channel 1. Contours: Custom.

Open with DEXTER
In the text
thumbnail Fig. B.45.

Overlay on SDSS DR-12 i-band. Contours: Interval 2.

Open with DEXTER
In the text
thumbnail Fig. B.46.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.47.

Overlay on SDSS DR-12 i-band. Contours: Interval 2.

Open with DEXTER
In the text
thumbnail Fig. B.48.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.49.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.50.

Overlay on IRAC channel 1. Contours: Interval 2.

Open with DEXTER
In the text
thumbnail Fig. B.51.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.52.

Overlay on SDSS DR-12 i-band. Contours: Interval 2.

Open with DEXTER
In the text
thumbnail Fig. B.53.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.54.

Overlay on SDSS DR-12 i-band. Contours: Interval 4.

Open with DEXTER
In the text
thumbnail Fig. B.55.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.56.

Overlay on IRAC channel 1. Contours: Interval 4.

Open with DEXTER
In the text
thumbnail Fig. B.57.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.58.

Overlay on IRAC channel 1. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.59.

Overlay on IRAC channel 1. Contours: Interval 3

Open with DEXTER
In the text
thumbnail Fig. B.60.

Overlay on SDSS DR-12 i-band. Contours: Interval 4.

Open with DEXTER
In the text
thumbnail Fig. B.61.

Overlay on IRAC channel 1. Contours: Custom.

Open with DEXTER
In the text
thumbnail Fig. B.62.

Overlay on SDSS DR-12 i-band. Contours: Interval 1.

Open with DEXTER
In the text
thumbnail Fig. B.63.

Overlay on SDSS DR-12 i-band. Contours: Custom.

Open with DEXTER
In the text
thumbnail Fig. C.1.

X-ray flux density spectra for the 84 individual galaxies of CGII, obtained from the Chandra ACIS. Flux density in units of 10−14 erg s−1 cm−2 keV−1.

Open with DEXTER
In the text

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.