Properties of fossil groups in cosmological simulations and galaxy formation models

we investigate fossil groups identified in the hydrodynamical simulations of the GIMIC project, which consists of resimulations of five regions in the Millennium Simulation (MS) that are characterized by different large-scale densities, ranging from a deep void to a proto-cluster region. For comparison, we also consider semi-analytic models built on top of the MS, as well as a conditional luminosity function approach. We focus on comparing the properties of fossil groups in the theoretical models and observational results, highlighting the differences between them, and trying to identify possible dependencies on environment for which our approach is particularly well set-up. We find that the optical fossil fraction shows no clear environmental dependence in either of the semi-analytic models, but there is evidence for a weak effect in the GIMIC hydrodynamic simulations. Interestingly, the X-ray luminosity distribution of groups in GIMIC also shows an environmental effect, making X-ray bright groups disproportionally rare in low-density regions. Combining the optical and X-ray selection criteria for fossil groups, we find that the fossil fraction exhibits a peak as a function of halo mass, at a mass scale of around $10^{13} -10^{14} \hMsun$. Over the GIMIC halo mass range we resolve best, $9.0 \times 10^{12} \sim 4.0 \times 10^{13} h^{-1}\Msun$, the central galaxies in the fossil groups show similar properties as those in ordinary groups, in terms of age, metallicity, color, concentration, and mass-to-light ratio. These results support an interpretation of fossil groups as transient phases in the evolution of ordinary galaxy groups rather than forming a physically distinct class of objects. [abridged]