K+a galaxies in the zCOSMOS survey: Physical properties of systems in their post-starburst phase

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