2012A&A...540A.134E

Fossil galaxy groups are spatially extended X-ray sources with X-ray luminosities above L_X,bol≥10⁴²h₅₀^–2erg/s and a central elliptical galaxy dominating the optical, the second-brightest galaxy being at least 2 mag fainter in the R band. Whether these systems are a distinct class of objects resulting from exceptional formation and evolution histories is still unclear, mainly due to the small number of objects studied so far, mostly lacking spectroscopy of group members for group membership confirmation and a detailed kinematical analysis. To complement the scarce sample of spectroscopically studied fossils down to their faint galaxy populations, the fossil candidate RX J1548.9+0851 (z=0.072) is studied in this work. Our results are compared with existing data from fossils in the literature. We use ESO VLT VIMOS multi-object spectroscopy to determine redshifts of the faint galaxy population and study the luminosity-weighted dynamics and luminosity function of the system. The full-spectrum fitting package ULySS is used to determine ages and metallicities of group members. VIMOS imaging data are used to study the morphology of the central elliptical. We identify 40 group members spectroscopically within the central ∼300kpc of the system and find 31 additional redshifts from the literature, resulting in a total number of 54 spectroscopically confirmed group members within 1Mpc. RX J1548.9+0851 is made up of two bright ellipticals in the central region with a magnitude gap of Δm_1,2=1.34 in the SDSS r' band leaving the definition of RX J1548.9+0851 being a fossil to the assumption of the virial radius. We find a luminosity-weighted velocity dispersion of 568km/s and a mass of ∼2.5x10¹⁴M_☉ for the system confirming previous studies that revealed fossils to be massive. An average mass-to-light ratio of M/L∼400M_☉/L_☉ is derived from the SDSS g', r', and i' bands. The central elliptical is well-fitted by a pure deVaucouleurs r^1/4 law without a cD envelope. Symmetric shells are revealed along the major axis of the galaxy indicating a recent minor merger. RX J1548.9+0851 shows a steep, increasing luminosity function with a faint-end slope of α=-1.4±0.1. Satellite galaxies show a clear spatial segregation with respect to their stellar populations - objects with old stars are confined to an elongated, central distribution aligned with the major axis of the central elliptical. Although RX J1548.9+0851 shows similar properties compared to other fossils studied recently, it might not be a fossil at all, being dominated by 2 bright central ellipticals. Comparing RX J1548.9+0851 with scaling relations from ordinary poor groups and clusters confirm the idea that fossils might simply be normal clusters with the richness and optical luminosity of poor groups.