Fossil group origins

Abstract

Aims. We aim to study how the orbits of galaxies in clusters depend on the prominence of the corresponding central galaxies. Methods. We divided our data set of ∼100 clusters and groups into four samples based on their magnitude gap between the two brightest members, Δm12. We then stacked all the systems in each sample in order to create four stacked clusters and derive the mass and velocity anisotropy profiles for the four groups of clusters using the MAMPOSSt procedure. Once the mass profile is known, we also obtain the (non-parametric) velocity anisotropy profile via the inversion of the Jeans equation. Results. In systems with the largest Δm12, galaxy orbits are generally radial, except near the centre, where orbits are isotropic (or tangential when also the central galaxies are considered in the analysis). In the other three samples with smaller Δm12, galaxy orbits are isotropic or only mildly radial. Conclusions. Our study supports the results of numerical simulations that identify radial orbits of galaxies as the cause of an increasing Δm12 in groups.