The environmental dependence of the red galaxy sequence

The dependence of the sequence of red galaxies (RS) with the environment is investigated using field, group, and cluster galaxies drawn from the SDSS. Our work focuses in studying the mean colour ($μ_R$) and the scatter ($σ_R$) of the RS as a function absolute magnitude in different environments characterised either by the mass of the system in which the galaxies are located or by the distance to the system's centre. The same analysis is carried out using red early type galaxies. For a given luminosity, $μ_R$ of field galaxies is bluer and $σ_R$ 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 $μ_R$ and the smallest $σ_R$. These differences almost disappear when red early type galaxies alone are considered. Galaxies in the core and in the outskirts of groups have similar $μ_R$, whereas galaxies in clusters show a strong dependence on cluster centric distance. Red early type galaxies in the outskirts of clusters have $σ_R$ values as large as field galaxies', while galaxies in the inner regions of clusters have lower values. We find that bright red early type galaxies have reached nearly the same evolutionary stage in all environments. Our results suggest that 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.

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