Properties of intra‐group stars and galaxies in galaxy groups: ‘normal’ versus ‘fossil’ groups

Cosmological [cold dark matter (ΛCDM)] TreeSPH simulations of the formation and evolution of 12 galaxy groups of virial mass ∼10 14  M ⊙ have been performed. The simulations invoke 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 ultraviolet (UV) field. The intra‐group (IG) stars are found to contribute 12–45 per cent of the total group B ‐band luminosity at z = 0 . The lowest fractions are found for groups with only a small difference between the R ‐band magnitudes of the first and second ranked group galaxy (Δ m 12, R ≲ 0.5) , the larger fractions are typical of ‘fossil’ groups (FGs, Δ m 12, R ≥ 2 ). A similar conclusion is obtained from BVRIJK surface brightness profiles of the IG star populations. The IG stars in the four FGs are found to be older than the ones in the eight ‘normal’ groups (non‐FGs), on average by about 0.3–0.5 Gyr. The typical colour of the IG stellar population is B − R = 1.4–1.5 , for both types of systems in good agreement with observations. The mean iron abundance of the IG stars is slightly sub‐solar in the central part of the groups ( r ∼ 100 kpc) decreasing to about 40 per cent solar at about half the virial radius. The IG stars are α‐element enhanced with a trend of [O/Fe] increasing with r and an overall [O/Fe]∼ 0.45 dex, indicative of dominant enrichment from Type II supernovae. The abundance properties are similar for both types of systems. The velocity distributions of the IG stars are, at r ≳ 30 kpc , significantly more radially anisotropic for FGs than for the non‐FGs; this also holds for the velocity distributions of the group galaxies. This indicates that an important characteristic determining whether a group becomes fossil or not, apart from its formation time, as discussed by D'Onghia et al., is 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 the virial radius, from the kinematics of the IG stars or group galaxies. For the non‐FGs, this method overestimates the group mass at r ≳200 kpc , by up to a factor of 2 at the virial radius. This is interpreted as FGs being, in general, more relaxed than non‐FGs. Finally, FGs of the above virial mass should host ∼500 planetary nebulae at projected distances between 100 and 1000 kpc from the first ranked galaxy. All results obtained appear consistent with the tidal stripping and merging scenario for the formation of FGs, put forward by D'Onghia et al.