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A&A
Volume 661, May 2022
Article Number A125
Number of page(s) 21
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/202142854
Published online 16 May 2022

© ESO 2022

1. Introduction

OH megamasers (OHMs) are luminous 18 cm masers found in (ultra-)luminous infrared galaxies ([U]LIRGs) produced predominantly by major galaxy mergers (Roberts et al. 2021). Generally, OH molecules are believed to be pumped by far-IR radiation (Lockett & Elitzur 2008; Huang et al. 2018) and triggered by dense molecular gas (Darling 2007). Both star formation and OHM activity are consequences of tidal density enhancements accompanying galaxy interactions (Darling 2007). Theoretical studies show that major galaxy mergers are the dominant processes leading to supermassive black hole (SMBH) growth at high masses (≥108 M⊙), which can destabilize large quantities of gas, driving massive inflows towards the nuclear region of galaxies and triggering bursts of star formation (Storchi-Bergmann & Schnorr-Müller 2019).

The dominant energy source in the central regions of OHM galaxies usually presents the features of a starburst and an active galactic nucleus (AGN), and in turn the radio continuum emission is produced from these activities. Based on multi-band observations, Hekatelyne et al. (2020) present a hypothesis that OHM galaxies harbor a recently triggered AGN. High-resolution observations might be essential to determine whether OHM galaxies are hosting an AGN or compact starburst, and their connections with merging stages and other environmental parameters (see Peng et al. 2020, and references therein).

IIZw 096 is classified as a LIRG (Inami et al. 2010), and is one of the most luminous known OHM galaxies (Bottinelli et al. 1986). This particular source is the second system to host formal megamasers involving both OH and H2O species (Wagner 2013; Wiggins et al. 2016). Optically, IIZw 096 shows complex morphology (Inami et al. 2010). It contains four main regions, denoted A, B, C, and D (see Fig. 1); sources A and B are possibly two spiral galaxies. Near-IR imaging and spectroscopic observations show that source D is a powerful starburst not associated with the primary nuclei (sources A and B), which could be a starburst in the disturbed disk of source A, or even the nucleus of a third galaxy (Goldader et al. 1997; Inami et al. 2010).

thumbnail Fig. 1.

Particular spots, regions, and contours superimposed on an HST image of IIZw 096. Panel I: HST-ACS F814W image (grayscale) for IIZw 096. The green crosses indicate the bright spots in this optical image. Panel II: VLA (A configuration) contour map at 33 GHz (white line) overlaid on the HST image. The contour levels are 0.0000441 × (1, 2, 4, 8) Jy beam−1 and the beam FWHM is 82.4 × 59.5 (mas) at −69.1°. The red circles show the regions around the comp D1, and the radius is about 0.1 arcsec. The red contour stands for the OH megamaser emission (red) from EVN archival data (project ES064B) (see details of the image parameters of the OH emission in Fig. A.2). Panel III: Zoomed-in map of the D1 region from panel II. The yellow ellipses are the two regions where we extracted the integrated OH emission lines.

The results from Multi-Element Radio-Linked Interferometer Network (MERLIN) observations show that the OH megamaser emission originated from component (comp) D1 and it is distributed in the form of an elongated structure (∼300 pc) with a velocity range of 200 km s−1 (Migenes et al. 2011). The estimated lower limit for the enclosed mass is ∼109 M⊙, which is consistent with a massive black hole, and an AGN could also be in this merging system (Migenes et al. 2011). High-resolution EVN observations (Cooprider 2010) found that the OH emission originated from two regions (OH1 and OH2 in Fig. 1) and indicated that a new epoch of VLBI observations could confirm the assumed structure of the OHM emission and potentially determine the proper motions of the two components.

Generally, the OH megamasers are produced through significant galaxy merging; however, the environment that facilitates such a phenomenon is still not completely understood, primarily because OHM originating from a central AGN or represents a transition stage between a starburst and AGN (Hekatelyne et al. 2018). IIZw 096 is one of the few bright OHM galaxies in OH 1667 MHz line emission, potentially observable for a detailed study of compact megamasers with high-resolution spectral-line VLBI observations. This particular source also contains rich merging components from the infrared and optical observations, as noted in the literature (e.g., Goldader et al. 1997; Inami et al. 2010; Migenes et al. 2011). It is likely to be a rare nearby example, potentially appropriate for studying the environment around the OH megamaser emission regions. The main aims of this paper are to further study the properties of the high-resolution structure of the OHM emission (Cooprider 2010) and find its possible connections with the environmental conditions, including the existence of dense gas, compact radio continuum emission, and also to study the possible merging status of the pertinent galaxies. The details about radio data collection, reduction, and associated analyses are presented in Sect. 2. The results and discussion are presented in Sects. 3 and 4, respectively. In Sect. 5 we provide a summary of the primary results and conclusions of this paper.

2. Data collection, reduction, and analysis

2.1. Archival radio data

We collected the archival radio data from EVN, VLBA, VLA, and ALMA. The detailed information about the spectral line projects and supporting multi-frequency continuum data are presented in Tables 1 and A.1, respectively.

Table 1.

Parameters of the high-resolution spectral line observations.

2.2. Data reduction

The VLBI data (EVN and VLBA) were calibrated using the NRAO Astronomical Image Processing System (AIPS) package. The main procedures of the VLBI data reduction include ionospheric correction, amplitude calibration, editing, bandpass calibration, instrumental phase corrections, antenna-based fringe-fitting of the phase calibrator, and subsequently applying the solutions to the target source. The EVLA data were calibrated using the pipeline of the Common Astronomy Software Application package (CASA McMullin et al. 2007). The calibration of historical VLA data was done in AIPS following standard procedures. To accurately determine the velocities of the line emission, we also corrected for the effects of the Earth’s rotation and its motion within the Solar System.

We imported all the calibrated data into the DIFMAP package (Shepherd 1997) to obtain the continuum and spectral line (OH and H I) channel images. The EVN data at epoch 2005 Jun. 08 (see Table 1) show a high signal-to-noise ratio for the peak line channel image (S/N > 60). We cleaned the brightest region in this peak line channel image (V ∼ 10 886 km s−1) and used these clean models to calibrate the data (phase-only self-calibration) prior to making all the channel images, which is similar to the self-calibration procedure performed by Cooprider (2010) for this project. We did not conduct self-calibration for other continuum and line emission projects. The multi-band VLA observations have different resolutions and show extended structures (see Fig. A.1). We remade these images with the same cell size and restored the beam to be 2″ × 2″ (the beam size of the VLA-A L-band map, see Vardoulaki et al. 2015). Subsequently, we measured the total flux densities and uncertainties of comps D and A (see Table A.1) by fitting with a single component using the task “imfit” in the CASA package.

3. Results

3.1. Two-epoch EVN results of OH megamaser emission

Cooprider (2010) presented the results from one-epoch EVN observations (project EK020, see Table 1) and found that the high-resolution OH emission is distributed in two regions as OH1 and OH2 from comp D1 (see Fig. 1). Based on the two-epoch spectral-line EVN observations (EK020 and ES064, see Table 1), we investigated the integrated OH 1667 line spectrum by averaging the 3σ signals from each channel image over two elliptical regions for OH1 and OH2 (see Fig. 2). We can see that the detected OH line profiles are partly resolved and consistent with the Arecibo observations.

thumbnail Fig. 2.

Integrated OH line profiles of IIZw 096. The red and blue spectra were obtained by integrating the signals above 3σ over two OH emission regions (OH1 and OH2, see Fig. 1) from each channel image of the two EVN line observations listed in Table 1. The black spectrum is the OH profile from Arecibo observations by Baan et al. (1989). The two arrows represent the velocity of the OH 1667 MHz (left) and 1665 MHz (right) lines based on the optical redshift.

The uncertainties of integrated 3σ flux density from each channel image is related to the area of the 3σ pixels and the VLBI calibration error (∼10% of the measured flux): , where N represents the number of beams for the region of the 3σ pixels and Speak is the peak flux density. The estimated uncertainties of the 3σ profiles of OH1 and OH2 regions are about 4–5 mJy for the brightest channels and less than 1 mJy for the weak channels. Based on these uncertainties, the OH profiles from the two epochs are consistent with each other and show no evident variabilities.

We also averaged the channels of OH 1667 MHz line emission from 10 750 to 10 950 km s−1 (see Fig. 2) and imaged the OH1 and OH2 regions (see Fig. A.2). We found that the peak positions of OH1 and OH2 show no evident changes between the two-epoch EVN line observations (see Table 2). Because the OH 1665 MHz line emission is resolved with high-resolution observations, we averaged the channels for this line emission (V ∼ 11 080 − 11 320 km s−1, see Fig. 2) and combined the calibrated data from the two EVN projects (using the task “DBCON” in AIPS). As a result, the OH 1665 MHz line emission is detected at the 6σ level with a peak of about 0.42 mJy beam−1 distributed at the OH1 region (see Fig. A.2).

Table 2.

Parameters of the emission lines detected in IIZw 096.

3.2. High-resolution CO(3-2) and HCO+(4-3) line emission of this galaxy

The Hubble Space Telescope (HST) I-band image (see Fig. 1) shows that the OH emission region coincides with D1, surrounded by nuclei A and B, together with some other bright regions. We selected 12 bright spots from the I-band image, named A1–A3, B1–B3, D1–D3, C1–C3, and we then extracted the CO(3-2) and HCO+(4-3) spectra at these spots from the primary beam (PB) corrected image files available online for the ALMA project 2012.1.01022.S (see Table 1).

We found that D1 is associated with the brightest CO(3–2) line emission (∼153 mJy beam−1) among all the selected regions (see Figs. 3 and A.3), and at least ten times brighter than other spots in the peak CO(3–2) emission (e.g., A0 ∼ 6.35 mJy beam−1, B0 ∼ 13.80 mJy beam−1, see Tables 2 and A.2). We further extracted the CO spectra from six regions around D1 (see Table A.2 and Fig. A.3), and noted that all the selected areas showed CO spectra with peak velocities higher than comp D1. The CO(3–2) emission in regions 1–3 is much brighter than regions 4–6, which means that the southeast region might have higher CO densities at high velocities (∼ from 10 935.5 km s−1 to 11 023.9 km s−1) compared to the northwest region. We further present the velocity structure of the CO emission around comp D1 (see Fig. 4), and confirmed that some regions show high-velocity clouds, while other areas show clear double peak emission lines (DP regions, DP1–DP6). The CO spectra in these regions are presented in Fig. A.3.

thumbnail Fig. 3.

CO emission lines extracted from regions or components in IIZw 096. The CO line profiles were fitted with one or two Gaussian components. The blue and red lines are the fitted Gaussian components and the sum of these components, respectively.

thumbnail Fig. 4.

Velocity structure of CO (left panel) and HCO+ (right panel) line emission (> 10 mJy beam−1) around the D1 region. The two black crosses are the OH emission regions OH1 and OH2, as shown in Fig. 1. The green and blue contours stand for the CO and HCO+ emissions, respectively. The contour levels are present in the caption of Fig. 9. The blue spots indicate where the extracted spectrum shows separated double peaks, roughly distributed in six regions (DP1–DP6).

We find that the HCO+ line can only be detected in the D1 region (see Fig. 5), and the velocity map shows that some pixels at the edge exhibit slightly higher peak velocities (see Table 2 and Fig. 4). We find that the CO and HCO+ line profiles extracted at the central region around D1 (with a size of ∼400 mas) all show broad full width at half maximum (FWHM) features (> 100 km s−1), which are much wider than in other regions around this merging system.

thumbnail Fig. 5.

HCO+ emission line profiles of IIZw 096. Left panel: integrated HCO+ spectrum extracted in a region of size 0.45′×0.45′ centered at D1. Right panel: HCO+ emission line extracted at D1. The line profiles were fitted with one Gaussian component.

3.3. The H I emission from VLA project AG0613

Based on the VLA H I spectral line observations (project AG0613, see Table 1), we have produced the H I channel image of IIZw 096 (at a velocity of about 10849.3 km s−1) which is presented in Fig. 6. We see that there are two bright H I emission regions: one is IIZw 096, the other is a new galaxy centered at RA: 20 57 39.307 and Dec: +17 01 53.762. The optical counterpart is likely to be SDSS J205738.81+170151.0, based on their celestial coordinates, as there is no optical redshift of this galaxy. The H I profiles of the two galaxies and spots in IIZw 096 are presented in Figs. 7 and A.4. The majority of the H I emission (3σ) for IIZw 096 is distributed in a region of about 70″ × 70″ in size (see Fig. 6), and the integrated H I spectrum in this region (see Fig. 7) agrees well with the H I spectrum from Arecibo observations by Courtois & Tully (2015). The H I spectra from VLA and Arecibo observations show that the galaxy contains both H I emission and absorption. The H I gas in absorption is detected only corresponding to comp D1 (see Fig. 7), and this result is consistent with the GMRT observations made by Dutta et al. (2019). The H I absorption feature is at the highest end of the H I spectrum velocity ranges. We find that the H I spectra from spots around IIZw 096 show similar characteristics (see Table A.3 and Fig. A.4) and exhibit no evident orbiting velocity structures caused by the circular motions of the gas.

thumbnail Fig. 6.

H I emission channel image (V ∼ 10 849.3 km s−1) of IIZw 096. Left panel: H I emission of IIZw 096 mainly distributed in a region of size 70″ × 70″ centered at D1 for IIZw 096. The H I emission for the new galaxy is centered at RA: 20 57 39.307, Dec: +17 01 53.762 with a region of size 44″ × 44″. Right panel: zoomed-in image of IIZw 096. The plus signs indicate the spots where we extracted the H I spectrum.

thumbnail Fig. 7.

H I emission profiles of IIZw 096. The black solid line profile is the detected H I spectrum, the blue and red lines are the fitted Gaussian components and the sum of these components, respectively. The dashed line from the left panel is the H I line profile from Arecibo observations made by Courtois & Tully (2015).

3.4. High-resolution radio continuum emission

We collected one epoch of VLBA data (see Table A.1) in order to obtain the high-resolution images of the continuum emission. The results show that no significant continuum emission detection occurs at a noise level of about 14.8–23.2 μJy beam−1 (see Fig. A.6).

The VLA projects of this source and the measured radio flux densities of D and A are listed in Table A.1, with the radio maps overlaid on the HST image of this source presented in Fig. A.1. The data reduction and flux densities measurement methods are presented in Sect. 2.2. Figures 8 and A.5 show the multi-band radio spectra of D and A from the total and peak flux densities (see Table A.1). We used two models; one employs a single power law and the other uses a mixed equation of a single power law and free–free emission (Sth × ν−0.1 + Snth × ν−α) to fit the radio spectra. We see that the radio spectra of D cannot be fitted well with a single power-law model; it is fitted with the mixed equation, which indicates that the source D might contain free-free emission and steep synchrotron emission.

thumbnail Fig. 8.

Multi-band integrated flux densities of D and A from VLA projects listed in Table A.1. The dashed and solid lines stand for fitting results from the equations a × ν−α and Sth × ν−0.1 + Snth × ν−α, respectively, where Sth and Snth stand for non-thermal (synchrotron) and thermal (free–free) flux densities.

4. Discussion

As the detected OH line emission mainly originated from the comp D1 of this merging system, we further investigated the high-resolution CO, HCO+, and H I line emission and radio continuum emission of D1 and some other regions of this source (see Sect. 3). A combination of these properties might be helpful in analyzing the possible scheme associated with the OH emission in this source.

4.1. Total mass of D1 estimated from CO and HI observations

The mass contained in a region can be an indicator of whether an AGN might be present. The D1 region has been investigated in detail and here we give present our mass estimates of D1 and those published in the literature.

The first estimate is based on the H-band optical image and solar metallicity. Inami et al. (2010) estimated that the mass of D1 is approximately 1 − 4 × 109 M⊙.

The second estimate is based on the OH line velocities and emission region from the MERLIN observations. Migenes et al. (2011) obtained a lower limit for the enclosed mass of about 3 × 109 M⊙.

For the third estimate, the H2 masses from the CO(1–0) fluxes detected by the interferometer can be derived with the following equation (Russell et al. 2017; Planesas et al. 1991):

(1)

Here XCO is the CO-to-H2 conversion factor in units of , z is the redshift, SJy is the CO(1–0) flux in Jy, and dMpc is the luminosity distance of this object in Mpc. The CO(3–2)/CO(1–0) line ratio is about 0.51 for this galaxy (Leech et al. 2010), and likely the LIRGs, the nearby star-forming galaxies, and AGN all show similar line ratios (see Leech et al. 2010; Lamperti et al. 2020). We adopted the quantity XCO  =  0.4 × 1020 cm−2 (K km−1)−1 for starburst galaxies and ULIRGs used in the literature (e.g., Russell et al. 2017; Downes & Solomon 1998), which is about five times lower than the Galactic value. For this source the value of dMpc is about 148 Mpc, ∫SJy dυ is about 29175.5 mJy km s−1 beam−1 (see Table 2), and the derived M(H2) is about 2.5 × 109 M⊙ in one beam with size of 0.2″ × 0.16″ (∼134 pc × 107 pc). Since the CO line emission of source D1 is much brighter than B0 and A0 (see Table 2), the central mass in one beam for B0 and A0 are about 1.6 × 108 and 2.0 × 107 M⊙, respectively, which means that D1 possibly contains a much more massive central mass than the other two apparent nuclei.

The fourth estimate is for the optically thin case. The H I mass (MHI) can be derived from the integrated line flux with the equation , where SJy is the line profile in Jy, integrated over the Doppler velocity V in km s−1. The ∫SJy dυ for the H I spectrum of D1 is about 300.58 mJy km s−1 beam−1 (see Table 2), whereby the H I mass are about 1.6 × 109 M⊙ in one beam with a size of 19.7″ × 17.9″ (∼13.2 kpc × 12.0 kpc). Similarly, the estimated H I mas from the total H I spectrum (see Table 2) is about 1.2 × 1010 M⊙ for the total H I mass in IIZw 096.

All four of the masses given above, estimated through different methods, indicate that there is a mass of about 109 M⊙ concentrated in the central region of D1. The direct proof of the existence of a SMBH requires very high angular resolution to probe close to the Schwarzschild radius, which has been difficult to establish (see Lo 2005). Alternatively, the large concentrations of molecular gas might result in high gas surface densities, which may signify that a luminosity source other than star formation, for example an AGN (Bryant & Scoville 1999).

Based on the derived M(H2) and the physical size of one beam from the CO(3-2) line observation (see the third estimation), the surface density is estimated to be about 2 × 105 M⊙ pc−2, which is consistent with the value of Arp 220 determined by Barcos-Muñoz et al. (2015). The high gas surface density value resembles the maximum stellar surface density of ∼105 M⊙ pc−2 (Hopkins et al. 2010). It is believed that the most massive nuclear star clusters can reach mass surface densities of ∼ 106 M⊙ pc−2 or more (see Neumayer et al. 2020). It should be noted that the high surface gas density found in this source still appears below the highest mass surface densities in nuclear star clusters. Meanwhile, the excessively high gas surface density is also consistent with the view that there is a possibility it also hosts an obscured AGN, which might be responsible for the formation of an H2O megamaser in this galaxy (see Wiggins et al. 2016; Migenes et al. 2011).

4.2. Comparison of OH and other line emissions

4.2.1. Merging and inflow of gas to the central region

The merging or interaction between two or more galaxies can reduce the angular momentum of the circumnuclear material and enhance the inflow of material from galactic scales into the close environments of an AGN (Ricci et al. 2021; Di Matteo et al. 2005). We found that the H I profiles extracted from locations away from the center of this merging system (regions 1, 2, 4, 5, 7, and A0; see Fig. A.4 and Table A.3) show similar H I profiles. They may contain two components, with central velocities of about 10 760 km s−1 and 10 840 km s−1, respectively. The optical redshift of this source shows a system velocity of about 10 770 km s−1 (e.g., Dutta et al. 2019; Kim et al. 1995), which is consistent with the low-velocity H I component. The similar components for H I emission profiles on a large scale at various regions indicate that this galaxy is in a stage of ongoing merging. It suggests that the H I gas clouds in this system might be distributed in a common face-on envelope and that the orbiting velocity cannot be measured through this observation, which is also observational evidence for intermediate merger (e.g., Ricci et al. 2021; Haan et al. 2011). The high-velocity line component might also be related to the gas clouds moving towards the center caused by the merging process.

The regions HI6, D1, and B0 show two components with slightly higher velocities (about 10 780 km s−1 and 10 880 km s−1) than other regions (see Figs. 7 and A.4, and Table A.3). Since these regions are close to the central region of this merging system, which contains massive nuclei (B0) and a possible supermassive region (D1) (see Sect. 4.1), the velocity offset with other regions might be caused by the velocities directed toward the center of mass. The H I emission is completely resolved with GMRT observations with a resolution of about 2″ (Dutta et al. 2019). The H I absorption profile shows higher velocities than the H I emission lines (see Fig. 7). The detected H I gas clouds in absorption were detected towards D1, which indicates that they may be closer to the central D1 than the large-scale H I gas in emission. The velocity of the H I absorption profiles is consistent with the high-velocity clouds seen from the velocity structure of CO emission (see Sect. 3.2). The high-velocity H I and CO clouds around D1 might be related to the inflows towards the central region of this source.

4.2.2. Formation of OH line emission

Two-epoch EVN observations of the OH 1667 MHz line have confirmed the results from Cooprider (2010) that the high-resolution OH megamaser emission is detected from two regions (OH1 and OH2 as shown in Fig. 1). The OH1 emission is about two times brighter than OH2 (see Table 2). We further found no evident variation in the OH properties from the two-epoch observations (see Sect. 3.1). Baan et al. (2008) show that the high-density tracers represent the molecular medium in the regions where star formation is taking place, which indicates a strong relationship to the far-IR luminosity. In particular, the CO(3–2) transition is commonly used to trace the warmer, denser components of the ISM associated with star formation (Leech et al. 2010). The HCO+ is also believed to be a good dense gas tracer (Farhan et al. 2020).

We presented the results from the high-resolution observational data of OH 1667 MHz, CO(3–2), HCO+, and HI emission. We find that the two OH emission regions (OH1 and OH2) reside in a dense gas environment (see Fig. 4). The central velocity of OH2 (∼10 804 km s−1) is consistent with the peak velocity of the total H I emission spectra (see Table 2 and Fig. 7). The central velocity of OH1 (∼10 886 km s−1) is consistent with the dense gas tracer CO(3–2) and HCO+ of comp D1. The OHM emerges when mergers experience a tidally driven density enhancement (Darling 2007). Since the environment from CO(3–2) emission around D1 shows three regions with different velocity structures (see Sect. 3.2), one possible scheme is that this region is in a merging stage. The OH1 and OH2 also emerge from the merging process and possibly originate from two or more systems. Our results show that OH is indeed associated with the densest molecular gas regions found in the IIZw 096 system, agreeing with a scenario where star formation is crucial for the formation of OHMs (Lo 2005).

4.2.3. Comparison with a general picture of OH megamaser emission

The Ka-band VLA-A observations of this source (see Fig. 1) present the highest resolution radio continuum emission. We note that the CO(3–2), HCO+, and Ka-band VLA-A radio continuum emission are roughly coincident with each other, while the two OH emission regions (OH1 and OH2) arise from a location about 50–76 mas to the center of the 33 GHz emission (see Fig. 9).

thumbnail Fig. 9.

High-resolution radio contour images of the continuum and molecular line emission. The two black crosses are the OH emission regions OH1 and OH2 shown in Fig. 1, and the colored crosses correspond to the following center coordinate positions: The blue line is the HCO+ emission contour averaged channels with velocity in the range 10763.3–11031.8 km s−1, with the contour level: 0.00075 × (1, 2, 4, 8, 16) Jy beam−1; the magenta contour represents the continuum emission from the 33 GHz VLA-A observation with contour level: 0.0000441 × (1, 2, 4, 8) Jy beam−1; the green contour is the CO line emission at the channel with a peak velocity about 10 887 km s−1, with contour: 0.0063 × (1, 2, 4, 8, 16) Jy beam−1.

Baan (2009) showed that a far-IR radiation field from the dust could pump the OH molecules in an environment with n(H2) in the range 103 to 2 × 104 cm−3. The maximum density of an OH maser emitting gas is on the order of n(H2) = 105 cm−3, while higher densities thermalize the energy levels and quench the maser emission (Parra et al. 2005). We calculated the n(H2) using the following assumptions: is about 4 × 104 cm−3, where M(H2) is about 2.5 × 109 M⊙ and V is the volume of the central region in one beam with size of about 130 pc in diameter (see Sect. 4.1). Since the accuracies of these contour maps are all less than 3.3 mas (estimated from , where beam stands for the beam FWHM and S/N stands for the signal-to-noise ratio), the offset might be related with the high n(H2) in the central region.

Generally, high-resolution observations of the OH megamaser emission will find a velocity gradient across the region with two or more OH emission components, for example Arp 220 (Ulvestad 2009), III Zw 35 (Trotter et al. 1997), IRAS 17208-0014 (Momjian et al. 2006), and IRAS 12032+1707 (Pihlström et al. 2005), which is a sign of a circular rotating disk or torus. Based on these high-resolution imaging of the 1667 MHz emission, Pihlström (2007) presented a general picture where most of the maser emission arises in thick circumnuclear structures. Parra et al. (2005) and Lockett & Elitzur (2008) showed that a clumpy maser model could provide a phenomenological explanation for both compact and diffuse OH emissions. Each maser cloud produces a low-gain unsaturated emission, while compact emission would be observed when the line of sight intersects many maser clouds. This particular model explains several OHM sources, for example III Zw 35, IRAS 17208−0014, Mrk 231, and NGC 6240 (Lockett & Elitzur 2008).

MERLIN observations of IIZw 096 found weak evidence for a velocity gradient along the right ascension direction (Migenes et al. 2011). However, the EVN observations showed that such a velocity gradient was a sign of double structure, and in this case no velocity gradient could be detected (see Cooprider 2010). We also found that IIZw 096 showed no clear presence of a velocity gradient from CO and HCO+ velocity distributions around D1 (see Fig. 4). Since the velocity fields of OHMs might only become ordered during the final stage of merging (Peng et al. 2020), this specific source might still be dominated by a phase of intense merging, as seen from the optical images, rather than an ordered circumnuclear disk or torus. Such a scheme agrees very well with the view provided by Goldader et al. (1997) that this source contains very young starbursts seen prior to the final major-merger stage.

Generally, the merger classification scheme is based on the morphology obtained from the high-resolution optical or infrared images. IIZw 096 was classified at the intermediate stage of the merging process based on the reported morphological characteristics (e.g., Goldader et al. 1997; Haan et al. 2011; Ricci et al. 2021). Wiggins et al. (2016) show that galaxies with coexisting OH and water megamasers might be a distinct population at a brief phase along the merger sequence due to the independent natures of the two types of maser emission. Although extensive effort has been made to detect OH+water megamasers, IIZw 096 and Arp 299 are the only two galaxies that are confirmed with such a dual-megamaser (see Wiggins et al. 2016, and references therein). The similarities between IIZw 096 and Arp 299 have shown that they are both luminous intermediate stage starburst (see Goldader et al. 1997; Inami et al. 2010; Wiggins et al. 2016). The evidence of inflow found in the two galaxies (see Falstad et al. 2017, and Sect. 4.2.1) might also be a similarity related to their analogous merging stage. Since strong HCN and OH emissions are both detected in the source IC 694 of Arp 299 (Casoli et al. 1999; Klöckner & Baan 2002), it further confirms that the OH megamaser emission is associated with the dense gas environment in the two galaxies. One significant difference between the two galaxies is in their velocity structure: a regular velocity gradient is seen around IC 694 from H I, CO, and OH line emission (Polatidis & Aalto 2000, 2001; Casoli et al. 1999), while we found no velocity gradient in IIZw 096. An explanation given by Sargent & Scoville (1991) is that Arp 299 is at a more advanced state of merging, and IC 694 is well on its way to becoming the ultimate core of the merger. Although the two galaxies might be both at the intermediate stage of merging, the absence of a velocity gradient means that IIZw 096 might be experiencing a slightly earlier stage of merger compared to Arp 299.

4.3. Radio continuum

The OHM galaxies could either represent a transition stage between a starburst and the emergence of an AGN through the merging process (Peng et al. 2020) or harbor a recently triggered AGN (Hekatelyne et al. 2020). IIZw 096 is the second object to co-host both water and OH megamasers, and these dual-megamasers might be only emerging during a brief phase of the galaxy evolution (e.g., from starburst nucleus to an AGN, see Wiggins et al. 2016). Migenes et al. (2011) first proposed the possibility that IIZw 096 might host an obscured AGN. However, multi-band observations reported in the literature show no clear evidence for an obscured AGN: the X-ray spectrum can be well-fit using the star formation mode (Ricci et al. 2021; Iwasawa et al. 2011), and the Spitzer mid-IR spectra indicate no high-ionization lines from a buried AGN (Inami et al. 2010).

Baan & Klöckner (2006) classified this source as a starburst galaxy, based on the radio brightness temperature (Tb) from VLA observations, the radio spectral index, and the ratio of far-IR and radio flux. Since the high-resolution radio structure and Tb are extremely significant ways to distinguish AGN from SB galaxies (Condon et al. 1991), we investigated the 33 GHz (Ka-band) VLA-A observation of this source (project 14A-471, see Table 1) and model-fitted the visibility data. Assuming the flux density of the fitted component can also be obtained at 1.4 GHz with the same component size, the estimated temperature Tb of the radio continuum emission is 1.21 × 105 K. The estimated upper limit Tb from the peak flux densities of VLBA images (see Fig. A.6) is about 106 K. Therefore, the detected radio continuum emission from comp D of IIZw 096 shows Tb to be within the range 105–106 K, which is also consistent with the starburst origin radio emission of other OHM galaxies reported in the literature (e.g., IRAS 12032+1707, IRAS 02524+2046 in Parra et al. 2005; Peng et al. 2020).

Clemens et al. (2008) showed that very few LIRGs/ULIRGs have a straight power-law slope, where the overall radio SED begins to flatten at higher frequencies as the contribution of thermal emission increases. We fitted the multi-band radio continuum of comps D and A as presented in Sect. 3.4. The radio SED of A can be well fitted with the power-law equation, while comp D might contain contributions from the free-free emission, which might indicate the existence of H II regions related to the massive stars (Linden et al. 2019). Vardoulaki et al. (2015) show that if the starburst is situated in a HII region, the value of α is hardly steeper than 1.1 from the α-map when adopting a 2σ uncertainty, which is an effective way to classify the LIRGs as radio-AGN, radio-SB, and AGN/SB (a mixture).

Following the methods descripted in Vardoulaki et al. (2015), we constructed the α-maps (see Fig. 10). First, we restored the images (at 1.4 GHz, 3 GHz, and 9 GHz) to a beam size of 2″ (comparable to the default beam of VLA-A observations at the L band) and constructed the α-maps. We found no regions with a steep spectral index higher than 1.1. Second, we restored the images at 3 and 9 GHz to a beam size of 1″ (comparable to the default beam of VLA-B observations at 9 GHz, see Fig. A.1) and re-constructed the α-maps, whereby we found that there are some pixels with α steeper than 1.1. Although this might be a sign of the possible existence of radio AGN in the central region, the number of pixels with α > 1.1 is scarce based on the histograms as shown in Fig. A.1. By combining properties of the radio continuum emission from brightness temperature and multi-band fitting, along with the properties of X-ray and infrared emission (Ricci et al. 2021; Iwasawa et al. 2011; Inami et al. 2010), we conclude that the dominated radio continuum emission from D1 might be of starburst origin, contributing to both synchrotron and free-free emission.

thumbnail Fig. 10.

Radio spectral index (α) maps of IIZw 096. The color maps in each row represent α (left panels) and its uncertainties dα (middle panels) derived from each pixel value. The color bar on the right of each figure shows the α values from low to high. The right panel presents the histograms of the α-maps: the α, α + dα, and α − dα (from top to bottom). The uncertainties of α were derived from the following equation: , where the rms corresponds to the noise of the radio image, and the Sυ is the flux densities at each pixel. The top row is the result obtained from images at 1.4 GHz and 9 GHz, with restored beam 2″ × 2×. The middle row is the result obtained from images at 3 GHz and 9 GHz, with restored beam 2″ × 2″. The bottom row is obtained from images at 3 GHz and 9 GHz, with restored beam 1″ × 1″. The contour levels are magenta (1.4 GHz): 0.000335 × 1, 2, 4 mJy beam−1, blue (9 GHz): 0.000069 × 1, 2, 4, 8, 16, 32 mJy beam−1. green (3.0 GHz): 0.000119 × 1, 2, 4, 8, 16, 32 mJy beam−1.

5. Summary

IIZw 096 is likely to be a rare nearby example, potentially for the study of merging environments and OH megamaser emissions. We analyzed two-epoch EVN archival data of the OH 1667 MHz line emission of IIZw 096 and confirmed that this source’s OH 1667 MHz line emission is mainly from two regions. We found no significant variations of the OH 1667 MHz line emission from the two areas, including the integrated flux densities and peak positions. The OH 1665 MHz line emission is detected at the 6σ level, with a peak of about 0.42 mJy beam−1 from the OH1 region. The OH emission regions reside in comp D1, which shows the brightest CO and HCO+ emission. The molecular mass in the central part (∼130 pc) is about 2.5 × 109, which is consistent with the view that there is a high mass concentrated in the central region (Migenes et al. 2011; Inami et al. 2010).

The H I emission from the VLA data shows that the H I gas at large scales may be distributed in a common face-on envelope, which is consistent with a stage of intermediate merger. The CO velocity structures show three velocity structures around D1: (1) a broad line profile region, which is the central region around D1 where CO emission shows broad line profiles; (2) a double-peak region, which has several small areas surrounding the central region where the CO line profile showed double peaks; (3) a high-velocity cloud region, beyond the double-peak regions, where the CO spectrum reveals high velocities around 11 000 km s−1. One possible explanation is that this source is in a stage of ongoing merging of two or more systems. The velocity structure around D1 shows no evidence of circular motion, making it different from most other OHMs reported in the literature, which might be caused by an effect resulting from the merger stage. The CO, HCO+ line emission, and the K-band VLA-A continuum emission are roughly aligned with the brightest center, while the two OH emission regions show an offset of about 50–75 mas to the central region and tend to the direction of double peak region. Therefore, the two OH emission regions might also be related to the merging process and may originate from more than one system.

We found that there is no significant continuum emission from the VLBA archival data. The multi-band radio continuum emission shows that the radio SED of comp A can be well fitted with the power-law equation, while comp D might contain contributions from the free-free emission. The α-map shows regions steeper than 1.1, which might be a sign of the possible existence of radio AGN in the central part as reported by Vardoulaki et al. (2015). However, the pixels steeper than 1.1 are very rare, and it is likely that the dominated radio continuum emission has a starburst origin mixed with synchrotron and free–free emission.

Acknowledgments

We thank the referee for the constructive comments and suggestions, which helped improve this paper. The study was funded by RFBR and NSFC, project number 21-52-53035 “The Radio Properties and Structure of OH Megamaser Galaxies”. This work has been supported by the grants of NSFC (Grant No. U1931203, 12111530009, 11763002) and the Kazan Federal University Strategic Academic Leadership Program (“PRIORITY-2030”). The European VLBI Network is a joint facility of European, Chinese, and other radio astronomy institutes funded by their national research councils. The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2012.1.01022.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.

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Appendix A: Online materials

Table A.1.

Parameters of the high-resolution radio continuum observations.

Table A.2.

CO line spectrum of the components and regions in IIZw 096.

Table A.3.

Parameters of the HI emission line regions in IIZw 096

thumbnail Fig. A.1.

Multi-band radio contour maps of IIZw 096 from VLA archival data overlaid on the HST F814W(I) image.

thumbnail Fig. A.1.

continued.

thumbnail Fig. A.2.

High-resolution OH 1667/1665 MHz line emission from EVN observations of IIZw 096. Top panels: Images of two-epoch OH 1667 MHz line emission (V ∼ 10750-10950 km s−1), which have been restored to the same beam size. Bottom panel: Dirty image of the OH-1665 line emission (V ∼ 11080-11320 km s−1) obtained by combining the two-epoch EVN data.

thumbnail Fig. A.3.

Extracted CO emission lines at various spots or regions of IIZw 096. The CO line profiles are fitted with one or two Gaussian components. The blue and red lines are the fitted Gaussian components and the sum of these components, respectively

thumbnail Fig. A.3.

continued.

thumbnail Fig. A.3.

continued.

thumbnail Fig. A.4.

Extracted H Iemission lines at various spots or regions of IIZw 096. The blue and red lines are the fitted Gaussian components and the sum of these components, respectively

thumbnail Fig. A.4.

continued.

thumbnail Fig. A.5.

Multi-band peak flux densities of D and A from VLA projects listed in Table A.1. The dashed and solid lines respectively illustrate the fitted results from the two equations, a×ν−α and Sth × ν−0.1+Snth × ν−α, where Sth and Snth stand for non-thermal (synchrotron) and thermal (free-free) flux densities, respectively.

thumbnail Fig. A.6.

Dirty maps of the continuum emission from the VLBA project BS0233. Map center of the two images: RA: 20 57 24.377, Dec: +17 07 39.144. Right panel: Cell size of about 8 mas and estimated dirty beam of about 22.28 mas × 36.8 mas at 38.0 degree; the 1σ noise is about 23.2 μJy/beam. Left panel: Cell size of about 1 mas and the estimated dirty beam of about 4.83 × 11.5 at 2.1 degree; the 1σ noise is about 14.8 μJy/beam.

All Tables

Table 1.

Parameters of the high-resolution spectral line observations.

In the text
Table 2.

Parameters of the emission lines detected in IIZw 096.

In the text
Table A.1.

Parameters of the high-resolution radio continuum observations.

In the text
Table A.2.

CO line spectrum of the components and regions in IIZw 096.

In the text
Table A.3.

Parameters of the HI emission line regions in IIZw 096

In the text

All Figures

thumbnail Fig. 1.

Particular spots, regions, and contours superimposed on an HST image of IIZw 096. Panel I: HST-ACS F814W image (grayscale) for IIZw 096. The green crosses indicate the bright spots in this optical image. Panel II: VLA (A configuration) contour map at 33 GHz (white line) overlaid on the HST image. The contour levels are 0.0000441 × (1, 2, 4, 8) Jy beam−1 and the beam FWHM is 82.4 × 59.5 (mas) at −69.1°. The red circles show the regions around the comp D1, and the radius is about 0.1 arcsec. The red contour stands for the OH megamaser emission (red) from EVN archival data (project ES064B) (see details of the image parameters of the OH emission in Fig. A.2). Panel III: Zoomed-in map of the D1 region from panel II. The yellow ellipses are the two regions where we extracted the integrated OH emission lines.
In the text
thumbnail Fig. 2.

Integrated OH line profiles of IIZw 096. The red and blue spectra were obtained by integrating the signals above 3σ over two OH emission regions (OH1 and OH2, see Fig. 1) from each channel image of the two EVN line observations listed in Table 1. The black spectrum is the OH profile from Arecibo observations by Baan et al. (1989). The two arrows represent the velocity of the OH 1667 MHz (left) and 1665 MHz (right) lines based on the optical redshift.
In the text
thumbnail Fig. 3.

CO emission lines extracted from regions or components in IIZw 096. The CO line profiles were fitted with one or two Gaussian components. The blue and red lines are the fitted Gaussian components and the sum of these components, respectively.
In the text
thumbnail Fig. 4.

Velocity structure of CO (left panel) and HCO+ (right panel) line emission (> 10 mJy beam−1) around the D1 region. The two black crosses are the OH emission regions OH1 and OH2, as shown in Fig. 1. The green and blue contours stand for the CO and HCO+ emissions, respectively. The contour levels are present in the caption of Fig. 9. The blue spots indicate where the extracted spectrum shows separated double peaks, roughly distributed in six regions (DP1–DP6).
In the text
thumbnail Fig. 5.

HCO+ emission line profiles of IIZw 096. Left panel: integrated HCO+ spectrum extracted in a region of size 0.45′×0.45′ centered at D1. Right panel: HCO+ emission line extracted at D1. The line profiles were fitted with one Gaussian component.
In the text
thumbnail Fig. 6.

H I emission channel image (V ∼ 10 849.3 km s−1) of IIZw 096. Left panel: H I emission of IIZw 096 mainly distributed in a region of size 70″ × 70″ centered at D1 for IIZw 096. The H I emission for the new galaxy is centered at RA: 20 57 39.307, Dec: +17 01 53.762 with a region of size 44″ × 44″. Right panel: zoomed-in image of IIZw 096. The plus signs indicate the spots where we extracted the H I spectrum.
In the text
thumbnail Fig. 7.

H I emission profiles of IIZw 096. The black solid line profile is the detected H I spectrum, the blue and red lines are the fitted Gaussian components and the sum of these components, respectively. The dashed line from the left panel is the H I line profile from Arecibo observations made by Courtois & Tully (2015).
In the text
thumbnail Fig. 8.

Multi-band integrated flux densities of D and A from VLA projects listed in Table A.1. The dashed and solid lines stand for fitting results from the equations a × ν−α and Sth × ν−0.1 + Snth × ν−α, respectively, where Sth and Snth stand for non-thermal (synchrotron) and thermal (free–free) flux densities.
In the text
thumbnail Fig. 9.

High-resolution radio contour images of the continuum and molecular line emission. The two black crosses are the OH emission regions OH1 and OH2 shown in Fig. 1, and the colored crosses correspond to the following center coordinate positions: The blue line is the HCO+ emission contour averaged channels with velocity in the range 10763.3–11031.8 km s−1, with the contour level: 0.00075 × (1, 2, 4, 8, 16) Jy beam−1; the magenta contour represents the continuum emission from the 33 GHz VLA-A observation with contour level: 0.0000441 × (1, 2, 4, 8) Jy beam−1; the green contour is the CO line emission at the channel with a peak velocity about 10 887 km s−1, with contour: 0.0063 × (1, 2, 4, 8, 16) Jy beam−1.
In the text
thumbnail Fig. 10.

Radio spectral index (α) maps of IIZw 096. The color maps in each row represent α (left panels) and its uncertainties dα (middle panels) derived from each pixel value. The color bar on the right of each figure shows the α values from low to high. The right panel presents the histograms of the α-maps: the α, α + dα, and α − dα (from top to bottom). The uncertainties of α were derived from the following equation: , where the rms corresponds to the noise of the radio image, and the Sυ is the flux densities at each pixel. The top row is the result obtained from images at 1.4 GHz and 9 GHz, with restored beam 2″ × 2×. The middle row is the result obtained from images at 3 GHz and 9 GHz, with restored beam 2″ × 2″. The bottom row is obtained from images at 3 GHz and 9 GHz, with restored beam 1″ × 1″. The contour levels are magenta (1.4 GHz): 0.000335 × 1, 2, 4 mJy beam−1, blue (9 GHz): 0.000069 × 1, 2, 4, 8, 16, 32 mJy beam−1. green (3.0 GHz): 0.000119 × 1, 2, 4, 8, 16, 32 mJy beam−1.
In the text
thumbnail Fig. A.1.

Multi-band radio contour maps of IIZw 096 from VLA archival data overlaid on the HST F814W(I) image.
In the text
thumbnail Fig. A.1.

continued.
In the text
thumbnail Fig. A.2.

High-resolution OH 1667/1665 MHz line emission from EVN observations of IIZw 096. Top panels: Images of two-epoch OH 1667 MHz line emission (V ∼ 10750-10950 km s−1), which have been restored to the same beam size. Bottom panel: Dirty image of the OH-1665 line emission (V ∼ 11080-11320 km s−1) obtained by combining the two-epoch EVN data.
In the text
thumbnail Fig. A.3.

Extracted CO emission lines at various spots or regions of IIZw 096. The CO line profiles are fitted with one or two Gaussian components. The blue and red lines are the fitted Gaussian components and the sum of these components, respectively
In the text
thumbnail Fig. A.3.

continued.
In the text
thumbnail Fig. A.3.

continued.
In the text
thumbnail Fig. A.4.

Extracted H Iemission lines at various spots or regions of IIZw 096. The blue and red lines are the fitted Gaussian components and the sum of these components, respectively
In the text
thumbnail Fig. A.4.

continued.
In the text
thumbnail Fig. A.5.

Multi-band peak flux densities of D and A from VLA projects listed in Table A.1. The dashed and solid lines respectively illustrate the fitted results from the two equations, a×ν−α and Sth × ν−0.1+Snth × ν−α, where Sth and Snth stand for non-thermal (synchrotron) and thermal (free-free) flux densities, respectively.
In the text
thumbnail Fig. A.6.

Dirty maps of the continuum emission from the VLBA project BS0233. Map center of the two images: RA: 20 57 24.377, Dec: +17 07 39.144. Right panel: Cell size of about 8 mas and estimated dirty beam of about 22.28 mas × 36.8 mas at 38.0 degree; the 1σ noise is about 23.2 μJy/beam. Left panel: Cell size of about 1 mas and the estimated dirty beam of about 4.83 × 11.5 at 2.1 degree; the 1σ noise is about 14.8 μJy/beam.
In the text

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