the dark matter halo structure of ``fossil groups candidates

It is generally considered that compact galaxy group (e.g. Hickson Compact Groups) members merge to form a single giant elliptical galaxy within a few billion years expected from the numerical simulations. Such a giant elliptical galaxy is called the "fossil" group, which consist of a large elliptical galaxy embedded at center of an extended X-ray emitted gas halo of galaxy group size. We investigated X-ray gas properties and the structure of dark matter halos of seven fossil groups candidates with ASCA or Chandra, and confirmed extension of hot gas are 200-400 h-1 kpc, temperature and metal abundance are 1-2 keV, 0.2-0.5 solar, 75 respectively. These value resemble those of compact groups. Our fossil group samples show higher mass-to-light ratio (M200 /LB 200-900) in comparison with compact groups and clusters of galaxies (M200 /LB 100-400). One possible explanation of this trend is that these fossil groups has the low star formation rate. These results suggest that the fossil groups do not always evolve from compact groups.