Ultimate age-dating method for galaxy groups; clues from the Millennium
  Simulations
  There have been a number of studies dedicated to identification of fossil
galaxy groups, arguably groups with a relatively old formation epoch. Most of
such studies identify fossil groups, primarily based on a large luminosity gap,
which is the magnitude gap between the two most luminous galaxies in the group.
Studies of these types of groups in the millennium cosmological simulations
show that, although they have accumulated a significant fraction of their mass,
relatively earlier than groups with a small luminosity gap, this parameter
alone is not highly efficient in fully discriminating between the "old" and
"young" galaxy groups, a label assigned based on halo mass accumulation
history.
  We study galaxies drawn from the semi-analytic models of Guo et al. (2011),
based on the Millennium Simulation. We establish a set of four observationally
measurable parameters which can be used in combination, to identify a subset of
galaxy groups which are old, with a very high probability. We thus argue that a
sample of fossil groups selected based on luminosity gap will result in a
contaminated sample of old galaxy groups. By adding constraints on the
luminosity of the brightest galaxy, and its offset from the group luminosity
centroid, we can considerably improve the age-dating.
