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.
Highlights
IntroductionThe term ‘fossil group’ (FG) was first introduced by Ponman et al (1994) for an apparently isolated elliptical galaxy surrounded by an X-ray halo, with an X-ray luminosity typical of a group of galaxies. Ponman et al (1994) made the hypothesis that FGs could be the fossil relics of old groups of galaxies, in which the L∗ galaxies (where L∗ is the characteristic magnitude of the cluster luminosity function) have had enough time to merge with the central one (BCG)
Ponman et al (1994) made the hypothesis that FGs could be the fossil relics of old groups of galaxies, in which the L∗ galaxies have had enough time to merge with the central one (BCG)
A different orbital distribution of galaxies in fossil and non-fossil systems could naturally explain the increased growth of the central galaxy in FGs at the expense of disrupted satellites approaching on radial orbits
Summary
The term ‘fossil group’ (FG) was first introduced by Ponman et al (1994) for an apparently isolated elliptical galaxy surrounded by an X-ray halo, with an X-ray luminosity typical of a group of galaxies. Ponman et al (1994) made the hypothesis that FGs could be the fossil relics of old groups of galaxies, in which the L∗ galaxies (where L∗ is the characteristic magnitude of the cluster luminosity function) have had enough time to merge with the central one (BCG). For a galaxy system to be classified as an FG, it must have an X-ray luminosity LX ≥ 1042 h−502 erg s−1 and a magnitude gap ∆m12 ≥ 2 in the r-band, between the BCG and the second brightest group member within 0.5 r200 from the BCG With this definition, even some clusters can enter the FG class (Cypriano et al 2006; Zarattini et al 2014). D’Onghia et al (2005) claimed that the infall of L∗ galaxies along filaments with small impact parameters is required to explain the existence of FGs in numerical simulations Testing this scenario requires determining the orbits of FG galaxies
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