A survey of HC₃N in extragalactic sources. Is HC₃N a tracer of activity in ULIRGs?

Context. HC₃N is a molecule that is mainly associated with Galactic star-forming regions, but it has also been detected in extragalactic environments. Aims: To present the first extragalactic survey of HC₃N, when combining earlier data from the literature with six new single-dish detections, and to compare HC₃N with other molecular tracers (HCN, HNC), as well as other properties (silicate absorption strength, IR flux density ratios, C ii flux, and megamaser activity). Methods: We present mm IRAM 30 m, OSO 20 m, and SEST observations of HC₃N rotational lines (mainly the J = 10-9 transition) and of the J = 1-0 transitions of HCN and HNC. Our combined HC₃N data account for 13 galaxies (excluding the upper limits reported for the non-detections), while we have HCN and HNC data for more than 20 galaxies. Results: A preliminary definition ''HC₃N-luminous galaxy'' is made based upon the HC₃N/HCN ratio. Most (\~{}80%) HC₃N-luminous galaxies seem to be deeply obscured galaxies and (U)LIRGs. A majority (\~{}60% or more) of the HC₃N-luminous galaxies in the sample present OH mega- or strong kilomaser activity. A possible explanation is that both HC₃N and OH megamasers need warm dust for their excitation. Alternatively, the dust that excites the OH megamaser offers protection against UV destruction of HC₃N. A high silicate absorption strength is also found in several of the HC₃N-luminous objects, which may help the HC₃N to survive. Finally, we find that a high HC₃N/HCN ratio is related to a high dust temperature and a low C ii flux. Appendix is only available in electronic form at http://www.aanda.org