Fossil groups in the Millennium simulation. From the brightest to the
  faintest galaxies during the past 8 Gyr
  We investigate the evolution of bright and faint galaxies in fossil and
non-fossil groups. We used mock galaxies constructed based on the Millennium
run simulation II. We identified fossil groups at redshift zero according to
two different selection criteria, and then built reliable control samples of
non-fossil groups that reproduce the fossil virial mass and assembly time
distributions. The faint galaxies were defined as having r-band absolute
magnitudes in the range [-16,-11]. We analysed the properties of the bright and
faint galaxies in fossil and non-fossil groups during the past 8 Gyr. We
observed that the brightest galaxy in fossil groups is typically brighter and
more massive than their counterparts in control groups. Fossil groups developed
their large magnitude gap between the brightest galaxies around 3.5 Gyr ago.
The brightest galaxy stellar masses of all groups show a notorious increment at
that time. By analysing the behaviour of the magnitude gap between the first
and the second, third, and fourth ranked galaxies, we found that at earlier
times, fossil groups comprised two large brightest galaxies with similar
magnitudes surrounded by much fainter galaxies, while in control groups these
magnitude gaps were never as large as in fossils. At early times, fossil groups
in the faint population were denser than non-fossil groups, then this trend
reversed, and finally they became similar at the present day. The mean number
of faint galaxies in non-fossil systems increases in an almost constant rate
towards later times, while this number in fossil groups reaches a plateau at
$z\sim0.6$ that lasts $\sim 2$ Gyr, and then starts growing again more rapidly.
The formation of fossil groups is defined at the very beginning of the groups
according to their galaxy luminosity sampling, which could be determined by
their merging rate at early times.
