A&A 600, A15 (2017) Astronomy DOI: 10.1051/0004-6361/201629066 & c ESO 2017 Astrophysics HC3N observations of nearby galaxies Xue-Jian Jiang (蒋êe)1; 5, Jun-Zhi Wang (王Gz)2, Yu Gao (高\)1; 5, and Qiu-Sheng Gu (~Ë生)3; 4; 5 1 Purple Mountain Observatory & Key Laboratory for Radio Astronomy, Chinese Academy of Sciences, 2 West Beijing Road, 210008 Nanjing, PR China e-mail: [email protected] 2 Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, 200030 Shanghai, PR China 3 School of Astronomy and Space Sciences, Nanjing University, 210093 Nanjing, PR China 4 Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, 210093 Nanjing, PR China 5 Collaborative Innovation Center of Modern Astronomy and Space Exploration, 210093 Nanjing, PR China Received 7 June 2016 / Accepted 22 December 2016 ABSTRACT Aims. We aim to systematically study the properties of the different transitions of the dense molecular gas tracer HC3N in galaxies. Methods. We have conducted single-dish observations of HC3N emission lines towards a sample of nearby gas-rich galaxies. HC3N(J = 2−1) was observed in 20 galaxies with the Effelsberg 100-m telescope. HC3N(J = 24−23) was observed in nine galaxies with the 10-m Submillimeter Telescope (SMT). Results. HC3N 2−1 is detected in three galaxies: IC 342, M 66, and NGC 660 (>3σ). HC3N 24−23 is detected in three galaxies: IC 342, NGC 1068, and IC 694. These are the first measurements of HC3N 2−1 in a relatively large sample of external galaxies, although the detection rate is low. For the HC3N 2−1 non-detections, upper limits (2σ) are derived for each galaxy, and stacking the non-detections is attempted to recover the weak signal of HC3N. The stacked spectrum, however, does not show any significant signs of HC3N 2−1 emission. The results are also compared with other transitions of HC3N observed in galaxies. Conclusions. The low detection rate of both transitions suggests low abundance of HC3N in galaxies, which is consistent with other + observational studies. The comparison between HC3N and HCN or HCO shows a large diversity in the ratios between HC3N and + HCN or HCO . More observations are needed to interpret the behavior of HC3N in different types of galaxies. Key words. galaxies: active – galaxies: ISM – galaxies: evolution – ISM: molecules 1. Introduction comparable to the widely-used dense gas tracer HCN and can also trace dense molecular gas around star-forming sites. HC3N Molecular lines play an essential role in our understanding has been detected in many star formation regions in the Milky of star-formation activity and galaxy evolution. With molecu- Way with several transitions from centimeter to sub-millimeter lar lines of different species and their different transitions, not (e.g., Suzuki et al. 1992). Due to the small rotational constant only can the chemical composition of the interstellar medium (∼1/13 of CO), there are many closely spaced rotational tran- be investigated, but other important physical parameters, such sitions of HC3N (separated by only 9.1 GHz) at centimeter as temperature, pressure, density, and non-collisional pumping and millimeter wavelengths, and its levels are very sensitive to mechanisms can be derived as well (e.g., Henkel et al. 1991; changes in excitation (Meier & Turner 2012). This makes it eas- Evans 1999; Fukui & Kawamura 2010; Meier & Turner 2012; ier to conduct multi-transition observations of HC3N lines than Meier et al. 2014). New facilities providing wide band and of other dense molecular gas tracers, and can help better under- highly sensitive instruments are making weak line surveys and stand the excitation conditions of star-forming regions. In con- multi-species analyses feasible, and the detections and mea- trast, the high-J lines of other dense molecular gas tracers such surements of a variety of species are helping us to reveal the as HCN and HCO+ are at very high frequencies, and it is there- gas components of galaxies, and how their abundances, densi- fore difficult to observe them with ground-based telescopes. An- ties, and ratios reflect their radiative properties. Multi-species, other advantage of using HC3N lines is that HC3N is very likely multi-transition molecular lines can be combined to identify the optically thin even in low-J transitions, due to the relatively low evolution stage of galaxies (Baan et al. 2014), because differ- abundance (Irvine et al. 1987; Lindberg et al. 2011); low opacity ent species are sensitive to different physical environments, such is important for accurate estimate of dense molecular gas mass as photo dissociation regions (PDRs) dominated by young mas- for the study of the relationship between dense molecular gas sive stars, X-rays dominated regions (XDRs) induced by active and star formation (Gao & Solomon 2004a,b; Wang et al. 2011; galactic nuclei (AGNs), and shock waves by cloud-cloud col- Zhang et al. 2014). lisions (Aladro et al. 2011; Greve et al. 2009; Costagliola et al. There have been efforts to detect HC N in nearby galaxies, 2011; Viti et al. 2014). 3 mainly in millimeter band. Observations suggest that HC3N is One of the interstellar species that benefits from the upgraded related to the warm, dense, star-forming gas, and is easily dis- facilities is cyanoacetylene (HC3N). HC3N was first detected in sociated by UV radiation (Henkel et al. 1988; Costagliola et al. 1971 at 9.0977 GHz (J = 2−1) in the Galactic star-forming 2011; Lindberg et al. 2011; Aladro et al. 2011, 2015). HC3N region Sgr B2 (Turner 1971). The critical density of HC3N is was found to be unusually luminous in NGC 4418, and it Article published by EDP Sciences A15, page 1 of7 A&A 600, A15 (2017) Table 1. The source list of the 21 galaxies observed for HC3N emission using the Effelsberg 100m and the SMT telescope. Galaxy RA (J2000) Dec VHelio Distance log LIR ∆VCO Telescope ◦ 0 00 −1 −1 h m s (km s ) (Mpc) (L ) (km s ) (1) (2) (3) (4) (5) (6) (7) (8) NGC 520 01 24 34.9 +03 47 30.0 2281 30.22 10.91 270 Effelsberg NGC 660 01 43 02.4 +13 38 42.0 850 12.33 10.49 280 Effelsberg NGC 891 02 22 33.4 +42 20 57.0 528 8.57 10.27 110 Effelsberg NGC 972 02 34 13.4 +29 18 41.0 1543 20.65 10.67 220 Effelsberg NGC 1068 02 42 41.4 −00 00 45.0 1137 13.7 11.27 280 Effelsberg & SMT IC 342 03 46 48.5 +68 05 46.0 31 4.60 10.17 72.8a Effelsberg & SMT UGC 2855 03 48 20.7 +70 07 58.0 1200 19.46 10.75 . Effelsberg UGC 2866 03 50 14.9 +70 05 40.9 1232 20.06 10.68 . Effelsberg NGC 1569 04 30 49.0 +64 50 53.0 −104 4.60 9.49 90 Effelsberg NGC 2146 06 18 39.8 +78 21 25.0 882 16.47 11.07 320 Effelsberg & SMT NGC 2403 07 36 51.3 +65 36 29.9 161 3.22 9.19 90 Effelsberg M 82 09 55 53.1 +69 40 41.0 187 3.63 10.77 150 Effelsberg & SMT NGC 3079 10 01 57.8 +55 40 47.0 1116 18.19 10.73 380 Effelsberg NGC 3310 10 38 45.9 +53 30 12.0 993 19.81 10.61 140 Effelsberg M 66 11 20 15.0 +12 59 30.0 727 10.04 10.38 180 Effelsberg IC 694 11 28 33.8 +58 33 45.0 3120 47.74 11.63 250 Effelsbergb & SMT NGC 3690 11 28 30.8 +58 33 43.0 3120 47.74 11.32 260 Effelsbergb & SMT Mrk 231 12 56 14.2 +56 52 25.0 12600 171.84 12.51 167 Effelsberg Arp 220 15 34 57.1 +23 30 10.0 5352 79.90 12.21 360 Effelsberg & SMT NGC 6240 16 52 58.9 +02 24 03.0 7160 103.86 11.85 420 SMT NGC 6946 20 34 52.6 +60 09 12.0 53 5.32 10.16 130 Effelsberg & SMT Notes. The columns are: (1) galaxy name; (2) and (3) coordinates; (4) heliocentric velocities; (5) distances; (6) total infrared luminosities (from Sanders et al. 2003); (7) CO 1−0 line width (FWHM) of the galaxies; and (8) the telescope these galaxies were observed with. (a) For IC 342, the ∆V is of CO 2−1 from Gao & Solomon(2004a); for other galaxies ∆V are of CO 1−0 from Young et al.(1995). (b) The Arp 299 system (IC 694 and NGC 3690) was observed as a single pointing by the Effelsberg. is attributed to its high abundance (10−7) as well as the in- 100-m, while the two galaxies were observed separately by the tense radiation field in the dense and warm gas at the cen- SMT 10-m. ter of NGC 4418 (Aalto et al. 2007; Costagliola & Aalto 2010). Meier & Turner(2005, 2012), Meier et al.(2011, 2014) pre- 2.1. HC N 2−1 observations with the Effelsberg 100-m sented high-resolution observations of HC3N(J = 5−4, 10−9, 3 12−11 and 16−15) of a few very nearby galaxies, and gave de- J − tailed analysis of the galactic structures and morphology traced HC3N( = 2 1) (νrest = 18:196 GHz) of 20 galaxies was observed with the Effelsberg 100-m telescope in 2010. The by HC3N and other dense gas tracers (HNC, HCN, CS, etc.).