HC₃N observations of nearby galaxies

¹ Purple Mountain Observatory & Key Laboratory for Radio Astronomy, Chinese Academy of Sciences, 2 West Beijing Road, 210008 Nanjing, PR China
e-mail: xjjiang@pmo.ac.cn
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.