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 ($\mu_R$) and the scatter ($\sigma_R$)
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, $\mu_R$ of field
galaxies is bluer and $\sigma_R$ 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 $\mu_R$ and
the smallest $\sigma_R$. These differences almost disappear when red early type
galaxies alone are considered. Galaxies in the core and in the outskirts of
groups have similar $\mu_R$, whereas galaxies in clusters show a strong
dependence on cluster centric distance. Red early type galaxies in the
outskirts of clusters have $\sigma_R$ 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.
