Chandra Studies of the X-ray Gas Properties of Fossil Systems

Abstract

We study ten galaxy groups and clusters suggested in the literature to be "fossil system (FS)" based on \chandra\ observations. According to the $M_{500}-T$ and $L_{\rm X}-T$ relations, the gas properties of FSs are not physically distinct from ordinary galaxy groups or clusters. We also first study the $f_{\rm gas,~2500}-T$ relation and find that the FS exhibits same as ordinary systems. The gas densities of FSs within $0.1r_{200}$, are $\sim 10^{-3}$ cm$^{-3}$, which is the same order as galaxy clusters. The entropies within $0.1r_{200}$ ($S_{0.1r_{200}}$) of FSs are systematically lower than those in ordinary galaxy groups which is consistent with previous report, but we find their $S_{0.1r_{200}}-T$ relation is more similar to galaxy clusters. The derived mass profiles of FSs are consistent with the Navarro, Frenk, \& White model in $(0.1-1)r_{200}$, and the relation between scale radius $r_{\rm s}$ and characteristic mass density $ta_{\rm c}$ indicates the self-similarity of dark matter halos of FSs. The range of $r_{\rm s}$ and $ta_{\rm c}$ of FSs are also close to those of galaxy clusters. Therefore, FSs share more common characteristics with galaxy clusters. The special birth place of the FS makes it as a distinct galaxy system type.