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Published March 2011 | Published
Journal Article Open

A survey of HC_3N in extragalactic sources: Is HC_3N a tracer of activity in ULIRGs?

Abstract

Context. HC_3N 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 HC_3N, when combining earlier data from the literature with six new single-dish detections, and to compare HC_3N 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 HC_3N rotational lines (mainly the J = 10–9 transition) and of the J = 1–0 transitions of HCN and HNC. Our combined HC_3N 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 "HC_3N-luminous galaxy" is made based upon the HC_3N/HCN ratio. Most (~80%) HC_3N-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 HC_3N and OH megamasers need warm dust for their excitation. Alternatively, the dust that excites the OH megamaser offers protection against UV destruction of HC_3N. A high silicate absorption strength is also found in several of the HC_3N-luminous objects, which may help the HC3N to survive. Finally, we find that a high HC_3N/HCN ratio is related to a high dust temperature and a low C_(II) flux.

Additional Information

© 2011 ESO. Received 11 August 2010, Accepted 16 December 2010, Published online 15 February 2011. Appendix is only available in electronic form at http://www.aanda.org. Many thanks to the IRAM, OSO, and SEST staff for their help during the observations. We are grateful to H. W. W. Spoon for sharing his PAH and silicate data with us. We would also like to thank the anonymous referee for several useful suggestions which improved the manuscript. J.L. wishes to thank Instrument center for Danish Astrophysics (IDA) for grant support. S.A. wishes to thank the Swedish Research Council for grant support. F.C. wishes to thank the EU ESTRELA programme for support.

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August 19, 2023
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