The zCOSMOS survey: the role of the environment in the evolution of the luminosity function of different galaxy types

Abstract

Aims. An unbiased and detailed characterization of the galaxy luminosity function (LF) is a basic requirement in many astrophysical issues: it is of particular interest in assessing the role of the environment in the evolution of the LF of different galaxy types. Methods. We studied the evolution in the B band LF to redshift z ~ 1 in the zCOSMOS 10k sample, for which both accurate galaxy classifications (spectrophotometric and morphological) and a detailed description of the local density field are available. Results. The global B band LF exhibits a brightening of ~0.7 mag in M^* from z ~ 0.2 to z ~ 0.9. At low redshifts (z<0.35), spectrophotometric late types dominate at faint magnitudes (M_B_(AB) > -20), while the bright end is populated mainly by spectrophotometric early types. At higher redshift, spectrophotometric late-type galaxies evolve significantly and, at redshift z ~ 1, the contribution from the various types to the bright end of the LF is comparable. The evolution for spectrophotometric early-type galaxies is in both luminosity and normalization: M* brightens by ~0.6 mag but φ^∗ decreases by a factor ~1.7 between the first and the last redshift bin. A similar behaviour is exhibited by spectrophotometric late-type galaxies, but with an opposite trend for the normalization: a brightening of ~0.5 mag is present in M^*, while φ^∗ increases by a factor ~1.8. Studying the role of the environment, we find that the global LF of galaxies in overdense regions has always a brighter M^* and a flatter slope. In low density environments, the main contribution to the LF is from blue galaxies, while for high density environments there is an important contribution from red galaxies to the bright end. The differences between the global LF in the two environments are not due to only a difference in the relative numbers of red and blue galaxies, but also to their relative luminosity distributions: the value of M^* for both types in underdense regions is always fainter than in overdense environments. These results indicate that galaxies of the same type in different environments have different properties. We also detect a differential evolution in blue galaxies in different environments: the evolution in their LF is similar in underdense and overdense regions between z ~ 0.25 and z ~ 0.55, and is mainly in luminosity. In contrast, between z ~ 0.55 and z ~ 0.85 there is little luminosity evolution but there is significant evolution in φ^∗, that is, however, different between the two environments: in overdense regions φ^∗ increases by a factor ~1.6, while in underdense regions this increase reaches a factor ~2.8. Analyzing the blue galaxy population in more detail, we find that this evolution is driven mainly by the bluest types. Conclusions. The "specular" evolution of late- and early-type galaxies is consistent with a scenario where a part of blue galaxies is transformed in red galaxies with increasing cosmic time, without significant changes in the fraction of intermediate-type galaxies. The bulk of this tranformation in overdense regions probably happened before z ~ 1, while it is still ongoing at lower redshifts in underdense environments.

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