The Formation of Fossil Galaxy Groups
by Elenda D'Onghia
A fossil group is defined as a group where a giant elliptical embedded in a X-ray halo with a luminosity 10-60% of the Virgo X-ray halo dominates the galaxy luminosity function, the next brightest group member being at least 2 magnitudes fainter. These systems were discovered by thanks to the X-ray satellite ROSAT. The archetype is RX J1340.6+4018, at z = 0.171, with an isolated, bright (MR=-22.7) elliptical galaxy surrounded by a halo of hot gas and dark matter. Fossil groups host nearly all field galaxies brighter than MR=-22.5, have mass-to-light ratios comparable to Virgo (M/LR ~ 300) With a number density ~ 2.4 ·10 -7 Mpc -3, they are not rare: they corresponds to ~ 20% of all clusters and groups with an X-ray luminosity larger than 2.5 ·1042 h50- 2 erg s-1. We use a set of twelve high-resolution N-body/hydrodynamical simulations in the LCDM cosmology to investigate the origin and formation rate of fossil groups (FGs), which are X-ray bright galaxy groups dominated by a large elliptical galaxy, with the second brightest galaxy being at least two magnitudes fainter. The simulations invoke star formation, chemical evolution with non-instantaneous recycling, metal dependent radiative cooling, strong star burst driven galactic super winds, effects of a meta-galactic UV field and full stellar population synthesis. All the selected groups have similar mass but random merging history. We find an interesting correlation between the magnitude gap between the first and second brightest galaxy and the formation time of the group. It is found that FGs have assembled half of their final dark matter mass already at z>1, and subsequently typically grow by minor merging only, wheras non-FGs on average form later.