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Paper   IPM / Astronomy / 13764
School of Astronomy
  Title:   Optically selected fossil groups; X-ray observations and galaxy properties
1.  H.G. Khosroshahi
2.  G. Gozaliasl
3.  J. Rasmussen
4.  A. Molaeinezhad
5.  T. Ponman
6.  A.A. Dariush
7.  A.J. Sanderson
  Status:   Published
  Journal: MNRAS
  Vol.:  443
  Year:  2014
  Pages:   318-327
  Supported by:  IPM
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.

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