Free Access
Issue
A&A
Volume 600, April 2017
Article Number A15
Number of page(s) 7
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201629066
Published online 21 March 2017

© ESO, 2017

1. Introduction

Molecular lines play an essential role in our understanding of star-formation activity and galaxy evolution. With molecular lines of different species and their different transitions, not only can the chemical composition of the interstellar medium be investigated, but other important physical parameters, such as temperature, pressure, density, and non-collisional pumping mechanisms can be derived as well (e.g., Henkel et al. 1991; Evans 1999; Fukui & Kawamura 2010; Meier & Turner 2012; Meier et al. 2014). New facilities providing wide band and highly sensitive instruments are making weak line surveys and multi-species analyses feasible, and the detections and measurements of a variety of species are helping us to reveal the gas components of galaxies, and how their abundances, densities, and ratios reflect their radiative properties. Multi-species, multi-transition molecular lines can be combined to identify the evolution stage of galaxies (Baan et al. 2014), because different species are sensitive to different physical environments, such as photo dissociation regions (PDRs) dominated by young massive stars, X-rays dominated regions (XDRs) induced by active galactic nuclei (AGNs), and shock waves by cloud-cloud collisions (Aladro et al. 2011; Greve et al. 2009; Costagliola et al. 2011; Viti et al. 2014).

Table 1

The source list of the 21 galaxies observed for HC3N emission using the Effelsberg 100m and the SMT telescope.

One of the interstellar species that benefits from the upgraded facilities is cyanoacetylene (HC3N). HC3N was first detected in 1971 at 9.0977 GHz (J = 2−1) in the Galactic star-forming region Sgr B2 (Turner 1971). The critical density of HC3N is comparable to the widely-used dense gas tracer HCN and can also trace dense molecular gas around star-forming sites. HC3N has been detected in many star formation regions in the Milky Way with several transitions from centimeter to sub-millimeter (e.g., Suzuki et al. 1992). Due to the small rotational constant (~1/13 of CO), there are many closely spaced rotational transitions of HC3N (separated by only 9.1 GHz) at centimeter and millimeter wavelengths, and its levels are very sensitive to changes in excitation (Meier & Turner 2012). This makes it easier to conduct multi-transition observations of HC3N lines than of other dense molecular gas tracers, and can help better understand the excitation conditions of star-forming regions. In contrast, the high-J lines of other dense molecular gas tracers such as HCN and HCO+ are at very high frequencies, and it is therefore difficult to observe them with ground-based telescopes. Another advantage of using HC3N lines is that HC3N is very likely optically thin even in low-J transitions, due to the relatively low abundance (Irvine et al. 1987; Lindberg et al. 2011); low opacity is important for accurate estimate of dense molecular gas mass for the study of the relationship between dense molecular gas and star formation (Gao & Solomon 2004a,b; Wang et al. 2011; Zhang et al. 2014).

There have been efforts to detect HC3N in nearby galaxies, mainly in millimeter band. Observations suggest that HC3N is related to the warm, dense, star-forming gas, and is easily dissociated by UV radiation (Henkel et al. 1988; Costagliola et al. 2011; Lindberg et al. 2011; Aladro et al. 2011, 2015). HC3N was found to be unusually luminous in NGC 4418, and it is attributed to its high abundance (10-7) as well as the intense radiation field in the dense and warm gas at the center of NGC 4418 (Aalto et al. 2007; Costagliola & Aalto 2010). Meier & Turner (2005, 2012), Meier et al. (2011, 2014) presented high-resolution observations of HC3N (J = 5−4, 109, 1211 and 1615) of a few very nearby galaxies, and gave detailed analysis of the galactic structures and morphology traced by HC3N and other dense gas tracers (HNC, HCN, CS, etc.). However, these results are still limited by their sample size, and the chemical process of HC3N (both formation and destruction) is still unclear. Larger samples are still necessary for analyzing the properties of HC3N and how it relates to other galactic parameters. In this paper, we present the first systematic survey of HC3N (J = 2−1) and HC3N (J = 24−23) in a relatively large sample of nearby galaxies, and the results are compared with the observations of HC3N in other transitions. The critical densities of HC3N J = 2−1 and HC3N J = 24−23 are approximately 3 × 103 cm-3 and 4 × 106 cm-3, respectively, and the upper state energies (Eu) of the two transitions are 1.3 K and 131 K, respectively (Costagliola & Aalto 2010).

2. Observations and data reduction

We selected nearby infrared bright galaxies (Sanders et al. 2003) with IRAS 60 μm flux greater than 30 Jy and declination greater than −21° to carry out this survey. It is not a complete sample, but is representative of infrared bright galaxies. The sample consists of 21 galaxies. We note that, due to the different beam size of the two telescopes we used, the merger Arp 299 (IC 694 and NGC 3690) was observed as a single pointing by Effelsberg 100-m, while the two galaxies were observed separately by the SMT 10-m.

thumbnail Fig. 1

Spectra of the detected HC3N(J = 2−1) in NGC 660, M 66, and IC 342 by the Effelsberg. The bottom row also shows the stacked spectra and the spectra of Mrk 231 from Aladro et al. (2015). Blue dashed lines are the Gaussian fit of the HC3N 2-1 lines. The temperature scale is Tmb in mK.

2.1. HC3N 21 observations with the Effelsberg 100-m

HC3N (J = 2−1) (νrest = 18.196 GHz) of 20 galaxies was observed with the Effelsberg 100-m telescope in 2010. The half power beam width (HPBW) is 46.5′′ at 18 GHz for the 100-m telescope. We used the 1.9 cm band receiver (500 MHz bandwidth with 16384 channels correlator setup) that provided ~8300 km s-1 velocity coverage and ~0.5 km s-1 velocity resolution during the observations. Position-switching mode with beam-throws of approximately ± 2′ was used. Pointing and focus were checked approximately every two hours. The typical system temperature of the Effelsberg observations was approximately 46 K. The on-source time for each galaxy is approximately 1447 min. The weather during the observations is not ideal, and the baselines of many sources are affected and induced artificial features that are difficult to remove.

2.2. HC3N 2423 observations with the SMT 10-m

HC3N (J = 24−23) (νrest = 218.324 GHz) of nine galaxies was observed in 2009 with the SMT 10-m telescope. The HPBW is approximately 33′′ at ~218 GHz for SMT, and a single pointing was used for each galaxy toward their central positions. We used the ALMA Sideband Separating Receiver and the Acousto-Optical-Spectrometers (AOS), which have dual polarization, 970 MHz (~1300 km s-1) bandwidth and 934 kHz channel spacing. Observations were carried out with the beam-switching mode with a chop throw of 2′ in azimuth (AZ) and a chopping frequency of 2.2 Hz. Pointing and focus were checked approximately every two hours by measuring nearby QSOs with strong millimeter continuum emission. The typical system temperature at 218 GHz was less than 300 K, and the on-source time for each galaxy was ~60168 min.

2.3. Data reduction

The basic parameters of our sample galaxies are listed in Table1. The data were reduced with the CLASS program of the GILDAS1 package. First, we checked each spectrum and discarded the spectra with unstable baselines. Most of the Effelsberg spectra do not have flat baselines, but over several hundred km s-1 near the line, the baselines can still be fixed. In the SMT spectra, the image signal of strong CO 21 in the upper sideband affects the baseline of the lower sideband and for M 82 and Arp 220, the HC3N 2423 is contaminated. However, for other galaxies, the image CO line does not affect the HC3N line. Then, we combined spectra with both polarizations of the same source into one spectrum. Depending on the quality of the spectral baselines, a first-order or second-order fitting was used to subtract baselines from all averaged spectra. The identifications of the transition frequencies of HC3N have made use of the NIST database Recommended Rest Frequencies for Observed Interstellar Molecular Microwave Transitions2.

To reduce the noise level, the spectra were smoothed to velocity resolutions ~ 20−40 km s-1. The velocity-integrated intensities of the HC3N line were derived from the Gaussian fit of the spectra, or integrated over a defined window if the line profiles significantly deviated from a Gaussian. The intensities were calculated using , where Tmb is the main beam brightness temperature. Molecular line intensity in antenna temperature () was converted to main beam temperature Tmb via , with the main beam efficiency MBE = 53% at 18 GHz for Effelsberg telescope, and 70% at 218 GHz for SMT during the observations. The flux density was then derived from Tmb, using S/Tmb = 0.59 Jy/K for the Effelsberg telescope, and 24.6 Jy/K for the SMT.

thumbnail Fig. 2

Spectra of detected HC3N(J = 24−23) in NGC 1068, IC 342 and IC 694 by the SMT. At the bottom left, the spectra of NGC 1068 from Aladro et al. (2015) are shown for comparison. Blue dashed line is the Gaussian fit. We note that, in IC 694, it is difficult to distinguish the HC3N emission from the possibly blended H2CO lines. The temperature scale is Tmb in mK.

3. Results and discussion

The spectral measurements and estimated intensities of the HC3N lines, including RMS noise and on-source time, are listed in Table 2 (HC3N 21) and Table 3 (HC3N 2423).

3.1. HC3N 21

Among the 20 galaxies observed by Effelsberg 100-m telescope, HC3N 21 is detected in three galaxies: IC 342, NGC 660, and M 66 (See Fig. 1). This is the first report of HC3N 21 detection in external galaxies, although limited by the signal to noise ratio (SNR), the detection rate is low.

IC 342:

IC 342 has the strongest peak intensity (Tmb ~ 14 mK) of HC3N 21 in the sample, which is approximately twice the strength of the HC3N(98) line of IC 342 detected by the IRAM 30-m telescope (Aladro et al. 2011), while the line width (full width at half medium (FWHM) ~ 60 km s-1) is similar to their result.

NGC 660:

the detected HC3N 21 in NGC 660 has a similar line width (FWHM ~ 294.7 km s-1) to CO 10 (~280 km s-1). While the HC3N survey by Lindberg et al. (2011) did not observe NGC 660, its 109 and 1211 transitions were not detected by Costagliola et al. (2011). This difference in the detection of HC3N lines may imply that there is little warm and dense gas content in NGC 660, thus the high-J HC3N lines can not be excited.

M 66:

in M 66, HC3N 21 is only detected on the approximately 2σ level, but this is the first tentative detection of HC3N in M 66. It was not observed by Costagliola et al. (2011) or Lindberg et al. (2011).

Non-detections:

due to the poor quality (and probably insufficient integration time) of the HC3N 21 data, 16 out of 19 galaxies were not detected. Assuming their line width is approximate to CO 10 line width (FWHM, from Young et al. 1995), we derive upper limits of the integrated intensity for each galaxy (2σ, where ) and show them in Table 2. Note that the line width of HC3N is likely narrower than that of CO, and such an assumption might overestimate the upper limits of integrated intensity and is therefore only an approximate estimate. The upper limits are in the range of ~0.31.2 K km s-1. For those non-detection galaxies, we also stack their spectra together, weighted by the RMS level of each galaxy, to examine whether or not a cumulated signal can be obtained (see Fig. 1). Although the RMS of the the stacking HC3N 21 spectrum is reduced down to 0.66 mK, we do not see any signs of emission (at a resolution of 30 km s-1). Since these galaxies have similar line widths (100400 km s-1), we can estimate the stacked upper limit assuming a line width of 200 km s-1 based on the RMS (0.66 mK) of the stacked spectra. Thus, the 2σ upper limit of these galaxies is approximately 0.26 K km s-1. To eliminate the possible effect induced by different line widths of galaxies, we also tried to group the non-detection galaxies based on their CO line width. Galaxies with CO FWHM (Table 1) wider than 200 km s-1 are stacked as one group, and other galaxies are stacked as another group. Neither group shows any signs of emission.

Table 2

HC3N 2-1 spectral measurements.

Table 3

HC3N 2423 spectral measurements.

3.2. HC3N 2423

Among the nine galaxies observed by SMT, HC3N(J = 24−23) is detected in three galaxies: IC 342, NGC 1068 and IC 694 (Fig. 2).

IC 342:

HC3N 2423 of IC 342 was previously detected and measured by Aladro et al. (2011), and our observation obtains consistent results, although by comparison to their observation, we do not detect H2CO simultaneously. In our observations, IC 342 is the only galaxy detected in both 21 and 2423 transitions. The line center and width of the two transitions are similar, considering observational uncertainties. This might imply that the two transitions have similar emitting areas. Furthermore, the ratio between the integrated intensities of HC3N 2423 and HC3N 21 is approximately 0.6.

NGC 1068:

in NGC 1068, the integrated intensity of HC3N 2423 is approximately 2.0 K km s-1 (in Tmb), which is stronger than that of HC3N 109 (~1.1 K km s-1) reported by Costagliola et al. (2011). This may imply that there is sufficient warm and dense gas, which is able to excite the high transition HC3N 2423 line. Besides, it could also be affected by the strong AGN signature of this galaxy (Wang et al. 2014; Tsai et al. 2012).

IC 694:

previous observations only obtained upper limits of HC3N 1211 for IC 694 (Lindberg et al. 2011). In our observations, a tentative detection in IC 694 (> 2σ) is obtained. The line profile of IC 694 obviously deviates from a Gaussian, so we derive the HC3N intensity by integrating the line within a window of 400 km s-1 width (Table 3).

We note that, in NGC 1068 and IC 694, HC3N 2423 is possibly blended with H2CO 3(0, 3)2(0, 2) emission (fν = 218.22219 GHz). The upper state energy of this para-H2CO line is approximately 10.5 K, which is likely to be excited in these cases. The H2CO line is shifted by 141.1 km s-1 or −102 MHz from the HC3N 2423 line, and it is unclear how much intensity of HC3N 2423 in NGC 1068 and IC 694 is contributed by H2CO (see Fig. 2). We still lack sufficient data to disentangle this issue, and can only compare with other observations. For example, in the observation of M 82 by Ginard et al. (2015), they showed that near the frequency of 145 GHz, H2CO 2(0, 1)1(0, 1) is as strong as HC3N 1615. H2CO is not detected in M 82 in the 3 mm band (Aladro et al. 2015). In the observations toward NGC 4418 by Aalto et al. (2007), they showed that HC3N 1615 is blended with H2CO, and H2CO may contribute 20% of the total integrated line intensity.

Non-detections:

the spectra of M 82 and Arp 220 are seriously contaminated by the image signal of CO 21 from the upper side-band (ν = 230 GHz), which is strong and wide and therefore difficult to remove. As a consequence, we could not extract the spectrum of HC3N properly. We treat the HC3N 2423 in M 82 and Arp 220 as non-detections, and their 2σ upper limits are also only indicative. Although not contaminated by adjacent CO image signal, HC3N 2423 was not detected in NGC 2146, NGC 6946, NGC 3690, or NGC 6240. For these non-detections, we present a 2σ upper limit of the integrated intensity of HC3N 2423 in Table 3. Only four galaxies are not contaminated by CO image signal, thus no stacking is implemented for their HC3N 2423 spectra.

3.3. Discussion: HC3N in galaxies

The HPBW of SMT and Effelsberg observations are 33′′ and 46′′, respectively, which should be able to cover the bulk of the sample galaxies, especially the galaxy center. Thus, our observations should be able to cover the region where the majority of dense gas resides. However, with single-dish observations, we cannot constrain the emission size of either HC3N 21 or HC3N 2423, and cannot easily estimate the filling factors. Along with the large uncertainty of the emission intensity measurements, it is difficult to estimate the brightness temperature of the sample.

To better understand the excitation environment of HC3N, the effect of free-free and synchrotron emission near 18 GHz should also be taken into account, as they are more prominent than that in the millimeter band that is dominated by dust thermal emission. We detect HC3N 2-1 lines in emission and not in absorption, and this may be due to the fact that the beam filling factor of the HC3N gas is higher than the radio continuum. In the high-resolution radio observations toward a few nearby galaxies (Tsai et al. 2006), it is found that compact radio sources contribute 2030% of the total 2 cm (15 GHz) emission from the central kiloparsec of these galaxies. In contrast, the distribution of gas with moderate critical density such as HC3N 21 is likely more diffuse.

Comparing to other dense molecular gas tracers such as the popular HCN and HCO+, HC3N is generally optically thin in galaxies owing to its relatively low abundance, which makes it an ideal dense gas tracer for calculating the column density and/or mass of molecular hydrogen content of galaxies. In the observations by Lindberg et al. (2011) and Costagliola et al. (2011), a low detection rate of HC3N was reported and was explained as the intrinsically faint emission of HC3N, and our stacked result also implies that the HC3N is relatively weak in the non-detected galaxies (2σ upper limit = 0.14 K km s-1), which is also in favor of this explanation. The non-detection in M 82 is consistent with the low abundance of HC3N in M 82 suggested by Aladro et al. (2011); HC3N traces a nascent starburst of galaxy, and can easily be destroyed by the UV radiation in PDRs, which is ubiquitous in active galaxies.

In very recent line surveys of a small number of local active galaxies (AGN and/or starbursts, Aladro et al. 2015; Costagliola et al. 2015), several HC3N transitions in 3 mm band (HC3N J = 10−9, J = 11−10 and J = 12−11) were detected. The ALMA observations by Costagliola et al. (2015) even reported the HC3N J = 32−31 rotational transition, and some of the vibrationally excited HC3N lines. The latest high-resolution line surveys in a number of very nearby galaxies (Meier & Turner 2005, 2012) and Meier et al. (2014, 2015) show that the derived HC3N abundances (on ~100 pc, approximately GMC scales) are approximately several 10-10 (relative to H2), which is approximately an order of magnitude lower than the abundance of HCN and some other molecules.

The results in Aladro et al. (2015) show that, the HC3N fractional abundance is generally several times lower than that of HCN, HCO+, and CS, and comparing to other AGN or starburst galaxies in their sample, HC3N abundance is significantly higher in the two ultra luminous infrared galaxies (ULIRGs) Arp 220 and Mrk 231, implying that it is well-suited for studying the activity of ULIRGs. Besides, there was no obvious evidence of AGN having an effect on the intensity of HC3N. Four galaxies in our sample (NGC 1068, M 82, Mrk 231 and Arp 220) were also studied in Aladro et al. (2015). We compared our data with their results, and the HC3N spectra of Mrk 231 and NGC 1068 from Aladro et al. (2015) are shown in Figs. 1 and 2, to be compared with the non-detection of HC3N 21 in Mrk 231, and the detection of HC3N 2423 in NGC 1068, respectively. Their results show that, in 3mm band, the intensities between the three transitions of HC3N (109, 1110 and 1211) differ very little, and the peak temperature (Tmb) of HC3N is ~4 mK for NGC 1068, ~11 mK for M 82, ~1.11.7 mK for Mrk 231, and ~10 mK for Arp 220. In our results, the detection of HC3N 2423 in NGC 1068 shows a peak Tmb ~ 7 mK, while the non-detections of HC3N 21 in Mrk 231 and Arp 220 show that, the RMS we have (~46 mK) might not be low enough to detect the HC3N lines. Here, we conclude that, besides the low abundance of HC3N, insufficient integration time and non-ideal observing conditions are the main causes of the low detection rate of HC3N.

It would be interesting to compare the intensity ratios between HC3N and other dense gas tracers, such as HCN and HCO+. In the HC3N survey by Lindberg et al. (2011), ratios such as HC3N/HCN were used to compare HC3N between galaxies. Based on that ratio, IC 342 and M 82 were classified as HC3N-luminous galaxies. In our observation, we detect both HC3N 21 and HC3N 24-23 in IC 342, but neither HC3N transition is detected in M 82. On the other hand, we obtained HC3N 2423 detections in NGC 1068, which was classified as a HC3N-poor galaxy in Lindberg et al. (2011). In the sample of some nearby galaxies observed by Aladro et al. (2015), the ratio between the peak temperature (Tmb) of HC3N/HCN or HC3N/HCO+also showed large variation. In NGC 253 and M 82, HC3N 109 is only ~1/20 the strength of HCO+ 10, while in Arp 220, HC3N 109 is nearly as strong as HCO+ 10. In our results, such line ratios also show large diversity. It is not yet clear how to interpret the ratio between HC3N and other molecular lines, and more data of HC3N in different transitions would be helpful to disentangle its properties in different types of galaxies.

Our observations and other works have presented detection of HC3N emission lines from nearly 18 GHz up to ~292 GHz. The newly commissioned Tianma 65 m telescope in Shanghai, China, is able to observe low transition HC3N emission, and has great potential for further HC3N 21 surveys for large samples of galaxies.

4. Summary

We carried out single-dish observations toward a sample of nearby gas-rich galaxies with the Effelsberg telescope and the Submillimeter Telescope. These are the first measurements of HC3N 2-1 in a relatively large sample of external galaxies.

HC3N(J = 2−1) (ν = 18.196 GHz) was observed with the 100-m telescope in 20 galaxies and only three galaxies are detected (> 3σ): IC 342, M 66 and NGC 660. These are the first measurements of HC3N 21 reported in external galaxies, and the first HC3N detections in M 66. We stack the spectra of those non-detections, yet there is still no sign of HC3N emission. The 2σ upper limit of HC3N intensity from the stacked spectrum is approximately 0.12 K km s-1.

HC3N(J = 24−23) (ν = 218.324 GHz) was observed in nine galaxies with the SMT, and was detected in three galaxies: IC 342, IC 694, and NGC 1068.

IC 342 is the only galaxy detected in both HC3N 21 and HC3N 2423 transitions in our observations, and the two transitions have similar line centers and widths, suggesting a similar emitting area. The ratio of integrated intensity of HC3N 2423/HC3N 21 is approximately 0.82. Due to the contamination of CO 21 image signal in the upper sideband, M 82 and Arp 220 are treated as non-detections of HC3N 2423.

The ratios between HC3N and HCN, HCO+ show large variation among the galaxies with HC3N detections, implying different behavior of the molecular lines in galaxies. More samples are needed to better understand the relationship between HC3N and other molecules.


Acknowledgments

Based on observations with the 100-m telescope of the MPIfR (Max-Planck-Institut für Radioastronomie) at Effelsberg, and the Submillimeter Telescope (SMT). The SMT is operated by the Arizona Radio Observatory (ARO), Steward Observatory, University of Arizona. We thank the staff of the Effelsberg telescope and the SMT for their kind help and support during our observations. This project is funded by China Postdoctoral Science Foundation (grant 2015M580438), National Natural Science Foundation of China (grant 11420101002, 11311130491, 11590783 and 11603075), and the CAS Key Research Program of Frontier Sciences. This research has made use of NASA’s Astrophysics Data System, and the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

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All Tables

Table 1

The source list of the 21 galaxies observed for HC3N emission using the Effelsberg 100m and the SMT telescope.

Table 2

HC3N 2-1 spectral measurements.

Table 3

HC3N 2423 spectral measurements.

All Figures

thumbnail Fig. 1

Spectra of the detected HC3N(J = 2−1) in NGC 660, M 66, and IC 342 by the Effelsberg. The bottom row also shows the stacked spectra and the spectra of Mrk 231 from Aladro et al. (2015). Blue dashed lines are the Gaussian fit of the HC3N 2-1 lines. The temperature scale is Tmb in mK.

In the text
thumbnail Fig. 2

Spectra of detected HC3N(J = 24−23) in NGC 1068, IC 342 and IC 694 by the SMT. At the bottom left, the spectra of NGC 1068 from Aladro et al. (2015) are shown for comparison. Blue dashed line is the Gaussian fit. We note that, in IC 694, it is difficult to distinguish the HC3N emission from the possibly blended H2CO lines. The temperature scale is Tmb in mK.

In the text

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