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We report on the X-ray and optical observations of galaxy groups selectedfrom the 2dfGRS group catalog, to explore the possibility that galaxy groupshosting a giant elliptical galaxy and a large optical luminosity gap presentbetween the two brightest group galaxies, can be associated with an extendedX-ray emission, similar to that observed in fossil galaxy groups. The X-rayobservations of 4 galaxy groups were carried out with Chandra telescope with10-20 ksec exposure time. Combining the X-ray and the optical observations wefind evidences for the presence of a diffuse extended X-ray emission beyond theoptical size of the brightest group galaxy. Taking both the X-ray and theoptical criteria, one of the groups is identified as a fossil group and one isruled out because of the contamination in the earlier optical selection. Forthe two remaining systems, the X-ay luminosity threshold is close to theconvention know for fossil groups. In all cases the X-ray luminosity is belowthe expected value from the X-ray selected fossils for a given opticalluminosity of the group. A rough estimation for the comoving number density offossil groups is obtained and found to be in broad agreement with theestimations from observations of X-ray selected fossils and predictions ofcosmological simulations.

Optically selected fossil groups; X-ray observations and galaxy properties