Fossil groups in the Millennium Simulation

• Published in: Astronomy and astrophysics (Berlin) 2008-11, Vol.490 (3), p.965-973
• Main Authors: Díaz-Giménez, E., Muriel, H., Mendes de Oliveira, C.
• Format: Article
• Language: English
• Subjects: galaxies: clusters: general; galaxies: evolution; methods: N-body simulations; methods: statistical
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:200809760

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 $\times$ 10$^{13}~h^{-1}~{\cal M}_\odot$) 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 $\times$ 10$^{13}~h^{-1}~{\cal M}_\odot$. 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 ~$-23.5{-}5~\log~h$, which is typical of the observed fossil groups known in the literature; (c) first-ranked galaxies of systems with $ {\cal M} >$ 5 $\times$ 10$^{13}~h^{-1}~{\cal M}_\odot$, 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.