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There have been a number of studies dedicated to identification of fossilgalaxy groups, arguably groups with a relatively old formation epoch. Most ofsuch 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 simulationsshow that, although they have accumulated a significant fraction of their mass,relatively earlier than groups with a small luminosity gap, this parameteralone is not highly efficient in fully discriminating between the "old" and"young" galaxy groups, a label assigned based on halo mass accumulationhistory.  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 observationallymeasurable parameters which can be used in combination, to identify a subset ofgalaxy groups which are old, with a very high probability. We thus argue that asample of fossil groups selected based on luminosity gap will result in acontaminated sample of old galaxy groups. By adding constraints on theluminosity of the brightest galaxy, and its offset from the group luminositycentroid, we can considerably improve the age-dating.

Ultimate age-dating method for galaxy groups; clues from the Millennium Simulations