AGE DATING GALAXY GROUPS IN THE MILLENNIUM SIMULATION

Abstract

We study galaxies drawn from the semi-analytic models of Guo et al. (2011) based on the MillenniumSimulation. We establish a set of four observationally measurable parameters which can be used incombination to identify a subset of galaxy groups which are old, with a very high probability. We thereforeargue that a sample of fossil groups selected based on the luminosity gap will result in a contaminatedsample of old galaxy groups. By adding constraints on the luminosity of the brightest galaxy, and itso set from the group luminosity centroid, we can considerably improve the age-dating.Key words: journals: galaxies: groups : general { galaxies groups: evolution { groups: old or young{galaxies: structure1. INTRODUCTIONIn the hierarchical structure formation scenario the ageperseverance for galaxy systems is not trivial, as in thisparadigm, more massive galaxy systems such as galaxyclusters are formed through the mergers of smallergalaxy systems and are therefore generally young galaxysystems. In fact, the answer to the question of age de-termination should come from cosmological simulationswhere the evolutionary history of galaxy systems canbe studied. Cosmological dark matter simulations andthe implemented semi-analytic galaxy models o er toolsneeded to develop an insight into this subject. An ob-servational method for the identi cation of old galaxygroups was developed in a pioneering study by Ponmanet al. (1994). According to the convention introducedby Jones et al. (2003), fossil groups have a magnitudegap of at least 2 magnitudes within 0.5 virial radii andL