Fossil groups in the Millennium Simulation

Evolution of the brightest galaxies

¹ IATE (CONICET-UNC) & OAC (UNC), Laprida 854, Córdoba 5000, Argentina e-mail: eugeniadiazz@gmail.com; hernan@oac.uncor.edu
² IAG, USP. Rua do Matão 1226, São Paulo, Brazil e-mail: oliveira@astro.iag.usp.br

Received: 10 March 2008
Accepted: 19 August 2008

Abstract

Aims. We create a catalogue of simulated fossil groups and study their properties, in particular the merging histories of their first-ranked galaxies. We compare the simulated fossil group properties with those of both simulated non-fossil and observed fossil groups.

Methods. Using simulations and a mock galaxy catalogue, we searched for massive (>5  10) fossil groups in the Millennium Simulation Galaxy Catalogue. In addition, we attempted to identify observed fossil groups in the Sloan Digital Sky Survey Data Release 6 using identical selection criteria.

Results. Our predictions on the basis of the simulation data are: (a) fossil groups comprise about 5.5% of the total population of groups/clusters with masses larger than 5  10. This fraction is consistent with the fraction of fossil groups identified in the SDSS, after all observational biases have been taken into account; (b) about 88% of the dominant central objects in fossil groups are elliptical galaxies that have a median R-band absolute magnitude of ~, which is typical of the observed fossil groups known in the literature; (c) first-ranked galaxies of systems with 5  10, regardless of whether they are either fossil or non-fossil, are mainly formed by gas-poor mergers; (d) although fossil groups, in general, assembled most of their virial masses at higher redshifts in comparison with non-fossil groups, first-ranked galaxies in fossil groups merged later, i.e. at lower redshifts, compared with their non-fossil-group counterparts.

Conclusions. We therefore expect to observe a number of luminous galaxies in the centres of fossil groups that show signs of a recent major merger.