A survey of HC₃N in extragalactic sources

Is HC₃N a tracer of activity in ULIRGs?^⋆

¹ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K, Denmark
e-mail: jlindberg@snm.ku.dk
² Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Santa Cruz de Tenerife, Spain
³ Department of Earth and Space Sciences, Onsala Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
⁶ California Institute of Technology, 1200 E. California Blvd., Mail Code 301-17, Pasadena, CA 91125-4700, USA

Received: 11 August 2010
Accepted: 16 December 2010

Abstract

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.