THEXMMCLUSTER SURVEY: THE STELLAR MASS ASSEMBLY OF FOSSIL GALAXIES

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 0.5R 200 , 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

Fossil group origins

“…Voevodkin et al (2010) suggest that results might be dubious when obtained for fossil and comparison systems treated in a nonhomogeneous way. Voevodkin et al (2010) Rather, we consider the results of Harrison et al (2012), the most recent paper on this subject and where the data were treated in a homogeneous way. We note that their X-ray luminosity estimates are based on a different data source (XMM-Newton) and methodology.…”

“…This method of density reconstruction gives as output the list of density peaks, their significance, density, and richness, as well as the relative membership. To minimize the effect of foreground/background galaxies we only worked on galaxies (hereafter likely members) having a color close to that of BCG, i.e., |(r − i) − (r − i) BCG | ≤ 0.2 (see also Harrison et al 2012), and having magnitude M r < −19, in order to sample the luminosity function down to ∼M * r + 3 mag, if possible, but not considering fainter galaxies.…”

“…Instead, Proctor et al (2011) claim atypical richnesses and optical luminosities, but this has not been found by Voevodkin et al (2010) and Harrison et al (2012). Recent studies of fossil systems have also challenged the former conclusions of an early formation of their BCGs from a gas-rich merger.…”

“…Analyzing the photometric and structural properties of BCGs in fossil systems, La Barbera et al (2009, 25 fossil systems) and Méndez-Abreu et al (2012, 20 fossil systems, hereafter Paper II) have found that they are similar to bright field ellipticals and to normal cluster BCGs, respectively. Finally, there is sparse evidence of a few fossil systems far from being dynamically relaxed (e.g., Harrison et al 2012;La Barbera et al 2012;Miller et al 2012).…”

“…For instance, on the basis of their observational results, Harrison et al (2012) suggest that fossil systems formed rather early and their galaxies represent the end products of galaxy mergers, while Proctor et al (2011) question the merging scenario, suggesting that the cannibalism of bright central galaxies is not a convincing explanation for the magnitude gap. Possible causes of the discrepancies among observational results reported in the literature might be connected with the use of very small samples, the presence of possible biases in the estimates of physical quantities, or inhomogeneities in the treatment of data of fossil and normal systems.…”