Fossil Groups in the course of Galaxy Evolution

Abstract

Fossil groups are X-ray bright galaxy groups characterized by a central elliptical galaxy that dominates the total light of the group in the optical. We present here a photometric analysis of the nearest fossil group NGC6482 down to $M_R\gsim-10.5$ mag — to our knowledge the deepest fossil group study yet, which can probe its faint satellite system in a meaningful way. We find signatures that the brightest group galaxy must have undergone a gas-rich merger in the past, favoring the cannibalism scenario for NGC6482, i.e., the brightest galaxies in the group center have merged to form the dominant central elliptical.We find the faint-end slope of the luminosity function to be within the range of values typically found in ordinary cluster environments. We thus conclude that the NGC6482 fossil group shows photometric properties consistent with those of regular galaxy clusters and groups, including a normal abundance of faint satellites. We additionally investigate fossil groups in a state-of-the-art semi-analytical model. From a sample of 59 fossil groups with masses comparable to NGC6482 we find that their properties are similar to non-fossil systems. Both reside in similarly dense environments and have similar number density distributions of dwarf galaxies. We do not find a missing satellite problem in the semi-analytical model. The faint-end slopes of the luminosity functions cover a range that is covered by observations. In particular, the faint-end slope of NGC6482 is in good agreement with the slopes determined in the 59 fossil groups of the model. Specifically, we confirm the picture of a transient fossil phase as both fossil and non-fossil systems spent similar time periods in the fossil phase. Therefore, this suggests that fossil and non-fossil groups are representations of the same evolutionary track of ordinary galaxy groups, supporting the cannibalism scenario. From the perspective of galaxy evolution there is no difference between fossil and non-fossil systems.