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

TL;DR

This study analyzes the X-ray gas properties of fossil systems and finds they are more similar to galaxy clusters than to ordinary galaxy groups, based on multiple gas and dark matter profile relations.

Contribution

It provides the first detailed comparison of fossil systems' gas and dark matter properties with those of galaxy clusters and groups, highlighting their similarities to clusters.

Findings

Fossil systems follow the same $M_{500}-T$ and $L_{X}-T$ relations as ordinary systems.

Gas densities within $0.1r_{200}$ are similar to galaxy clusters.

Entropy within $0.1r_{200}$ is lower than in ordinary groups but similar to clusters.

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.