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 catalogue 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 four galaxy groups were carried out with Chandra telescope with 10-20 ksec exposure time. Combining the X-ray and the optical observations we find evidence 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-ray luminosity threshold is close to the convention known 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 to be 4 to 8 × 10-6 Mpc-3, in broad agreement with the estimations from observations of X-ray selected fossils and predictions of cosmological simulations.