Fossil group origins

Abstract

Context. Fossil systems are galaxy associations dominated by a relatively isolated, bright elliptical galaxy, surrounded by a group of smaller galaxies lacking L * objects. Because of this extreme environment, fossil groups (FGs) are ideal laboratories to study the mass assembly of brightest group galaxies (BGGs). Aims. We analyzed the near-infrared photometric and structural properties of a sample of 20 BGGs present in FGs in order to better understand their formation mechanisms. This represent the largest sample studied to date. Methods. K s -band deep images were used to study the structural properties of our sample galaxies. Their surface-brightness distribution was fitted to a Sérsic profile using the GASP2D algorithm. Then, the standard scaling relations were derived for the first time for these galaxies and compared with those of normal ellipticals and brightest cluster galaxies in non-fossil systems.Results. The BGGs presented in this study represent a subset of the most massive galaxies in the Universe. We found that their ellipticity profiles are continuously increasing with the galactocentric radius. Our fossil BCGs follow closely the fundamental plane described by normal ellipticals. However, they depart from both the log σ 0 vs. log L Ks and log r e vs. log L Ks relations described by intermediate mass ellipticals. This occurs in the sense that our BGGs have larger effective radii and smaller velocity dispersions than those predicted by these relations. We also found that more elliptical galaxies systematically deviate from the previous relations while more rounder object do not. No similar correlation was found with the Sérsic index. Conclusions. The derived scaling relations can be interpreted in terms of the formation scenario of the BGGs. Because our BGGs follow the fundamental plane tilt but they have larger effective radii than expected for intermediate mass ellipticals, we suggest that they only went through dissipational mergers in a early stage of their evolution and then assembled the bulk of their mass through subsequent dry mergers, contrary to previous claims that BGGs in FGs were formed mainly by the merging of gas-rich galaxies.

A normal abundance of faint satellites in the fossil group NGC 6482

“…Such a red color was also found for NGC 6482 by Alamo-Martínez et al (2012) (g−z = 1.85 at r 50 ). Another finding of our study is the relatively low Sérsic index of n ∼ 2.82 for NGC 6482, in agreement with the studies of the FOGO group, who find Sérsic indices around n ∼ 3 for FGs (Aguerri et al 2011;Méndez-Abreu et al 2012). They argue based on Hopkins et al (2009) and Kormendy et al (2009) that low Sérsic indices for giant ellipticals (n ∼ 2.5) are tracers for dissipational mergers.…”

“…In particular it is smaller than reported by Alamo-Martínez et al (2012) who find values of ∼3.9 in gand z-band. However, smaller values of n for fossil group central galaxies were already reported by the FOGO collaboration (Aguerri et al 2011;Méndez-Abreu et al 2012), who obtain a mean Sérsic index of n ∼ 3 for a sample of 21 FGs.…”

Fossil group origins

“…In particular, for the BGGs of our 34 FG candidates, a two-phase merger scenario was proposed to explain their scaling relations. Indeed, Méndez-Abreu et al (2012) claim that these objects went through dissipational mergers in an early stage of their evolution and assembled the bulk of their mass through subsequent dry mergers. This process seems similar to the one proposed by Díaz-Giménez et al (2008), in which the BGGs in fossil systems have undergone their last major merger later than in non-fossil systems.…”

“…Some observations (Khosroshahi et al 2006) show that these objects are different from both isolated elliptical galaxies and central galaxies in non-fossil clusters in the sense that they have disky isophotes in the center and their luminosity correlates with velocity dispersion, while other authors (La Barbera et al 2009;Méndez-Abreu et al 2012) found no differences in isophotal shapes between fossil and non-fossil central galaxies. In Méndez-Abreu et al (2012) we analyzed deep K-band images of 20 BGGs in fossil and non-fossil systems and showed that these galaxies follow the tilted fundamental plane of normal ellipticals (see Bernardi et al 2011). This suggests that BGGs are dynamically relaxed systems that suffer dissipational mergers during their formation.…”

“…The details of the project are summarized in the first paper of the series ). The structural properties of the BGGs in fossil and non-fossil systems were shown in the second paper (Méndez-Abreu et al 2012). The L X −L opt relations of fossil and normal systems are analyzed in Girardi et al (2014), the third paper.…”