Optically selected fossil groups; X-ray observations and galaxy
  properties
  We report on the X-ray and optical observations of galaxy groups selected
from the 2dfGRS group catalog, to explore the possibility that galaxy groups
hosting a giant elliptical galaxy and a large optical luminosity gap present
between the two brightest group galaxies, can be associated with an extended
X-ray emission, similar to that observed in fossil galaxy groups. The X-ray
observations of 4 galaxy groups were carried out with Chandra telescope with
10-20 ksec exposure time. Combining the X-ray and the optical observations we
find evidences for the presence of a diffuse extended X-ray emission beyond the
optical size of the brightest group galaxy. Taking both the X-ray and the
optical criteria, one of the groups is identified as a fossil group and one is
ruled out because of the contamination in the earlier optical selection. For
the two remaining systems, the X-ay luminosity threshold is close to the
convention know for fossil groups. In all cases the X-ray luminosity is below
the expected value from the X-ray selected fossils for a given optical
luminosity of the group. A rough estimation for the comoving number density of
fossil groups is obtained and found to be in broad agreement with the
estimations from observations of X-ray selected fossils and predictions of
cosmological simulations.
