The XMM Cluster Survey: The Stellar Mass Assembly of Fossil Galaxies

TL;DR

This study identifies fossil galaxy systems in the XMM Cluster Survey, analyzing their stellar populations and mass assembly, revealing that fossil galaxies are more massive and dominate their groups, indicating early formation and merger history.

Contribution

It provides the first comprehensive catalog of fossil systems from X-ray and optical data and compares their stellar properties to normal brightest cluster galaxies.

Findings

Fossil galaxies are more massive than typical BCGs at fixed group/cluster mass.

Fossil systems contain up to 85% of the group's optical luminosity within 0.5R200.

Fossil systems likely formed early in high-density regions of the universe.

Abstract

This paper presents both the result of a search for fossil systems (FSs) within the XMM Cluster Survey and the Sloan Digital Sky Survey and the results of a study of the stellar mass assembly and stellar populations of their fossil galaxies. In total, 17 groups and clusters are identified at z < 0.25 with large magnitude gaps between the first and fourth brightest galaxies. All the information necessary to classify these systems as fossils is provided. For both groups and clusters, the total and fractional luminosity of the brightest galaxy is positively correlated with the magnitude gap. The brightest galaxies in FSs (called fossil galaxies) have stellar populations and star formation histories which are similar to normal brightest cluster galaxies (BCGs). However, at fixed group/cluster mass, the stellar masses of the fossil galaxies are larger compared to normal BCGs, a fact that holds true over a wide range of group/cluster masses. Moreover, the fossil galaxies are found to contain a significant fraction of the total optical luminosity of the group/cluster within 0.5R200, as much as 85%, compared to the non-fossils, which can have as little as 10%. Our results suggest that FSs formed early and in the highest density regions of the universe and that fossil galaxies represent the end products of galaxy mergers in groups and clusters. The online FS catalog can be found at http://www.astro.ljmu.ac.uk/~xcs/Harrison2012/XCSFSCat.html.