K+a galaxies in the zCOSMOS survey*
Physical properties of systems in their post-starburst phase
INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127, Bologna e-mail: email@example.com
2 Università di Bologna, Dipartimento di Astronomia, via Ranzani 1, 40127, Bologna, Italy
3 Institute of Astronomy, Swiss Federal Institute of Technology (ETH Hönggerberg), 8093, Zürich, Switzerland
4 Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue Edouard Belin, 31400 Toulouse, France
5 INAF - Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
6 INAF - IASF Milano, Milan, Italy
7 INAF - Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
8 INAF - Osservatorio Astronomico di Brera, Milan, Italy
9 Laboratoire d'Astrophysique de Marseille, Marseille, France
10 European Southern Observatory, Karl-Schwarzschild-Strasse 2, Garching, 85748, Germany
11 Dipartimento di Astronomia, Universita di Padova, Padova, Italy
12 Max-Planck-Institut für extraterrestrische Physik, 84571 Garching, Germany
13 INAF - Osservatorio Astronomico di Torino, 10025 Pino Torinese, Italy
14 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
15 LBNL & BCCP, University of California, Berkeley, CA 94720, USA
16 Centre de Physique Théorique, Marseille, France
17 Institut d'Astrophysique de Paris, UMR 7095 CNRS, Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris, France
18 Universitats-Sternwarte, Scheinerstrasse 1, 81679 Muenchen, Germany
19 Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
20 INAF - Osservatorio di Roma, Monteporzio Catone (RM), Italy
21 California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125, USA
22 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI, 96822
23 Research Center for Space and Cosmic Evolution, Ehime University, Bunkyo-cho, Matsuyama 790-8577, Japan
Accepted: 9 September 2009
Aims. The identities of the main processes triggering and quenching star-formation in galaxies remain unclear. A key stage in evolution, however, appears to be represented by post-starburst galaxies. To investigate the prevalence of these galaxies and their impact on galaxy evolution in general, we initiated a multiwavelength study of galaxies with k+a spectral features in the well-studied COSMOS field.
Methods. We examine a mass-selected sample of k+a galaxies in the COSMOS field at z = 0.48-1.2 using the spectroscopic zCOSMOS sample. To classify galaxies in their k+a phase, we use a spectroscopic criterion, based on the amplitude of the Hδ absorption line and the absence of the [OII] emission line. We develop our analysis for a well-defined sample by imposing stringent confidence levels on the spectroscopic redshifts and spectral measurements. We compare our results for two mass-selected samples of star-forming and quiescent galaxies selected using a purely spectral classification scheme from the 10 000 zCOSMOS catalogue (i.e. , based on measurements of 4000 Å break and EW[OII]).
Results. In our mass-limited sample, k+a galaxies occupy the brightest tail of the luminosity distribution. They are as massive as quiescent galaxies and populate the green valley in the colour versus luminosity (or stellar mass) distribution. A small percentage (<8%) of these galaxies have radio and/or X-ray counterparts (implying an upper limit to the SFR of ~8 yr-1). Over the entire redshift range explored, the class of post-starburst galaxies is morphologically a heterogeneous population with a similar incidence of bulge-dominated and disky galaxies. This distribution does not vary with the strength of the Hδ absorption line but instead with stellar mass in a way reminiscent of the well-known mass-morphology relation. The results about the incidence of asymmetries and the concentration of the light distribution derived from HST/ACS images imply that this galaxy population possibly represents an intermediate stage of galaxy evolution. Although k+a galaxies are also found in underdense regions, they appear to reside typically in a similarly rich environment as quiescent galaxies on a physical scale of ~2–8 Mpc, and in groups they show a morphological early-to-late type ratio similar to the quiescent galaxy class. With the current data set, we do not find evidence of statistical significant evolution in either the number/mass density of k+a galaxies at intermediate redshift with respect to the local values, or the spectral properties, although more solid results on this and other aspects will be obtained following the completion of the survey.
Conclusions. Several mechanisms related and unrelated to the environment are at work in quenching star-formation activity in galaxies on short timescales (<1 Gyr). Those galaxies, which are affected by a sudden quenching of their star-formation activity, may increase the stellar mass of the red-sequence by up to a non-negligible level of ~10%.
Key words: galaxies: evolution / galaxies: fundamental parameters / galaxies: luminosity function, mass function / cosmology: observations / galaxies: formation
© ESO, 2010