The environmental dependence of the red galaxy sequence

Abstract

The dependence of the sequence of red galaxies (RS) with the environment is investigated using volume-limited samples of field, group and cluster galaxies drawn from the Sloan Digital Sky Survey. Our work focuses on studying the mean colour (μR) and the scatter (σR) of the RS as a function of 0.1r-band absolute magnitude in different environments characterized either by the mass of the system in which the galaxies are located or by the distance to the system's centre. These results are compared with the RS of field galaxies. The same analysis is carried out using subsamples of red galaxies classified as early types according to their concentration parameter. For a given luminosity, μR of field galaxies is bluer and σR is larger than their group and cluster counterparts irrespective of mass and position within the systems. Among systems of galaxies, high-mass groups and clusters have the reddest μR and the smallest σR. These differences almost disappear when red early-type galaxies alone are considered. Galaxies in the core and in the outskirts of groups have similar μR, whereas galaxies in clusters show a strong dependence on cluster-centric distance, the inner galaxies being the reddest objects. Red early-type galaxies in the outskirts of clusters have σR values as large as those of field galaxies, while galaxies in the inner regions of clusters have lower values, comparable to those of group galaxies. We find that bright red early-type galaxies have reached nearly the same evolutionary stage in all environments. Our results suggest that, although effective in drifting galaxies of intermediate luminosities towards redder colours, the cluster environment is not necessary to populate the RS. We propose a scenario in which the RS in massive systems is populated by two different star formation history galaxies: red early-type galaxies that formed the bulk of their stars during the early stages of massive halo assembly and red galaxies that passed most of their lives inhabiting poor groups or the field and fell into massive systems at lower redshifts.