RX J1548.9+0851, a fossil cluster?
  Fossil galaxy groups are spatially extended X-ray sources with X-ray
luminosities above L_X,bol > 10^42 h_50^-2 ergs s^-1 and a central elliptical
galaxy dominating the optical, the second-brightest galaxy being at least 2
magnitudes 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 ~300 kpc of the system and find 31
additional redshifts from the literature, resulting in a total number of 54
spectroscopically confirmed group members within 1 Mpc. RX J1548.9+0851 is made
up of two bright ellipticals in the central region with a magnitude gap of
m_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 568 km s^-1 and a mass of ~2.5 x
10^14 M_sun for the system confirming previous studies that revealed fossils to
be massive. (abridged)
