Properties of Intra-group Stars and Galaxies in Galaxy Groups: "Normal" versus "Fossil" Groups

Abstract

Cosmological LCDM simulations of 12 M_vir~10^14 Msun galaxy groups have been performed, invoking star formation, chemical evolution with non-instantaneous recycling, metallicity dependent radiative cooling, strong star-burst driven galactic super-winds and effects of a meta-galactic UV field. At z=0, intra-group light (IGL) fractions are found to be 12-45%. Low values refer to groups with only a small difference between the R-band magnitudes of the first and second ranked group galaxy, large are typical of "fossil" groups (FGs). The IG stars in the 4 FGs are 0.3-0.5 Gyr older than in the 8 nonFGs. For the IGL, B-R=~1.4, in good agreement with observations. For FGs/nonFGs the iron abundance of the IG stars is slightly sub-solar in the central parts (r~100 kpc) decreasing to about 40% solar at about 0.5 r_vir The IG stars are alpha-element enhanced with [O/Fe] increasing with r, and an overall [O/Fe]~0.45, indicating predominant SNII enrichment. The velocity distributions of the IG stars and group galaxies are, at r>~30 kpc, significantly more radially anisotropic for FGs than for nonFGs. So a characteristic of FG formation, apart from formation time (D'Onghia et al.), may be the "initial" velocity distribution of the group galaxies. For FGs one can dynamically infer the (dark matter dominated) mass distribution of the groups all the way to r_vir, from the kinematics of the IG stars or group galaxies. For the nonFGs this method overestimates the group mass at r>~200 kpc, by up to a factor of two at r_vir. This is interpreted as FGs being, in general, more relaxed than nonFGs. Finally, FGs of the above M_vir should host ~500 planetary nebulae at projected distances between 100 and 1000 kpc from the first ranked galaxy. All results appear consistent with the FG formation scenario of D'Onghia et al.