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.

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