Finding Fossil Groups: Optical Identification and X-ray Confirmation
  We report the discovery of 12 new fossil groups of galaxies, systems
dominated by a single giant elliptical galaxy and cluster-scale gravitational
potential, but lacking the population of bright galaxies typically seen in
galaxy clusters. These fossil groups (FGs), selected from the maxBCG optical
cluster catalog, were detected in snapshot observations with the Chandra X-ray
Observatory. We detail the highly successful selection method, with an 80%
success rate in identifying 12 FGs from our target sample of 15 candidates. For
11 of the systems, we determine the X-ray luminosity, temperature, and
hydrostatic mass, which do not deviate significantly from expectations for
normal systems, spanning a range typical of rich groups and poor clusters of
galaxies. A small number of detected FGs are morphologically irregular,
possibly due to past mergers, interaction of the intra-group medium (IGM) with
a central AGN, or superposition of multiple massive halos. Two-thirds of the
X-ray-detected FGs exhibit X-ray emission associated with the central BCG,
although we are unable to distinguish between AGN and extended thermal galaxy
emission using the current data. This sample, a large increase in the number of
known FGs, will be invaluable for future planned observations to determine FG
temperature, gas density, metal abundance, and mass distributions, and to
compare to normal (non-fossil) systems. Finally, the presence of a population
of galaxy-poor systems may bias mass function determinations that measure
richness from galaxy counts. When used to constrain power spectrum
normalization and {\Omega}_m, these biased mass functions may in turn bias
these results.
