The formation of fossil galaxy groups in the hierarchical universe

We use a set of 12 high-resolution N-body/hydrodynamical simulations in the Lambda CDM 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 2 mag fainter. The simulations invoke star formation, chemical evolution with noninstantaneous recycling, metal-dependent radiative cooling, strong starburst-driven galactic superwinds, effects of a metagalactic UV field, and full stellar population synthesis. We find an interesting correlation between the magnitude gap between the brightest and second-brightest galaxy and the formation time of the group. It is found that FGs have already assembled half of their final dark matter mass at, and subsequently they typically grow by minor merging only, whereas non-FGs on average form later. z greater than or similar to 1 The early assembly of FGs leaves sufficient time for galaxies of to merge into the central one by dynamical L similar to L-* friction, resulting in the large magnitude gap at z = 0. About 33% +/- 16% of the groups simulated are found to be FGs, whereas the observational estimate is similar to 10%-20%. The FGs are found to be overluminous in the X-ray relative to non-FGs of the same optical luminosity, in qualitative agreement with observations. Finally, from a dynamical friction analysis, we find that FGs exist at all only because infall of galaxies happens along L similar to L-* filaments with small impact parameters.