2017A&A...600A..15J

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.