Volume 570, October 2014
|Number of page(s)||20|
|Published online||09 October 2014|
CO in Hickson compact group galaxies with enhanced warm H2 emission: Evidence for galaxy evolution?⋆,⋆⋆
1 Departamento de Física Teórica y del Cosmos, Universidad de Granada, Spain
2 Instituto Universitario Carlos I de Física Teórica y Computacional, Facultad de Ciencias, 18071 Granada, Spain
3 NASA Herschel Science Center, California Institute of Technology, Pasadena, CA 91125, USA
4 Department of Astronomy, University of Cape Town, Private Bag X3, 7701 Rondebosch, South Africa
5 ARC Super Science Fellow, Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia
6 Institut d’Astrophysique Spatiale, CNRS, UMR 8617, Université Paris-Sud, Bât. 121, 91405 Orsay Cedex, France
7 Spitzer Science Center, IPAC, California Institute of Technology, Pasadena, CA 91125, USA
8 NASA Extragalactic Database, IPAC, California Institute of Technology, Pasadena, CA 91125, USA
Received: 13 February 2014
Accepted: 9 July 2014
Context. Galaxies in Hickson Compact Groups (HCGs) are believed to experience morphological transformations from blue, star-forming galaxies to red, early-type galaxies. Galaxies with a high ratio between the luminosities of the warm H2 to the 7.7 μm PAH emission (so-called Molecular Hydrogen Emission Galaxies, MOHEGs) are predominantly in an intermediate phase, the green valley. Their enhanced H2 emission suggests that the molecular gas is affected in the transition.
Aims. We study the properties of the molecular gas traced by CO in galaxies in HCGs with measured warm H2 emission in order to look for evidence of the perturbations affecting the warm H2 in the kinematics, morphology and mass of the molecular gas.
Methods. We observed the CO(1–0) emission of 20 galaxies in HCGs and complemented our sample with 11 CO(1–0) spectra from the literature. Most of the galaxies have measured warm H2 emission, and 14 of them are classified as MOHEGs. We mapped some of these galaxies in order to search for extra-galactic CO emission. We analyzed the molecular gas mass derived from CO(1–0), MH2, and its kinematics, and then compared it to the mass of the warm molecular gas, the stellar mass and star formation rate (SFR).
Results. Our results are the following. (i) The mass ratio between the CO-derived and the warm H2 molecular gas is in the same range as found for field galaxies. (ii) Some of the galaxies, mostly MOHEGs, have very broad CO linewidths of up to 1000 km s-1 in the central pointing. The line shapes are irregular and show various components. (iii) In the mapped objects we found asymmetric distributions of the cold molecular gas. (iv) The star formation efficiency (=SFR/MH2) of galaxies in HCGs is very similar to isolated galaxies. No significant difference between MOHEGs and non-MOHEGs or between early-type and spiral galaxies has been found. In a few objects the SFE is significantly lower, indicating the presence of molecular gas that is not actively forming stars. (v) The molecular gas masses, MH2, and ratios MH2/LK are lower in MOHEGs (predominantly early-types) than in non-MOHEGs (predominantly spirals). This trend remains when comparing MOHEGs and non-MOHEGs of the same morphological type.
Conclusions. We found differences in the molecular gas properties of MOHEGs that support the view that they have suffered (or are presently suffering) perturbations of the molecular gas, as well as a decrease in the molecular gas content and associated SFR. Higher resolution observations of the molecular gas are needed to shed light on the nature of these perturbations and their cause.
Key words: galaxies: evolution / galaxies: groups: general / galaxies: ISM / galaxies: interactions / infrared: galaxies / intergalactic medium
Table 5 and Appendix A are only available in electronic form at http://www.aanda.org
Spectra from Figs. A.1 and A.2 and the FITS files of the reduced spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/570/A24
© ESO, 2014
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