Fossil group origins. XI. The dependence of galaxy orbits on the
  magnitude gap
  We aim to study how the orbits of galaxies in clusters depend on the
prominence of the corresponding central galaxies. We divided our data set of
$\sim$ 100 clusters and groups into four samples based on their magnitude gap
between the two brightest members, $\Delta m_{12}$. 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. In systems with the largest $\Delta m_{12}$, galaxy orbits are
prevalently 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 $\Delta m_{12}$, galaxy orbits are
isotropic or only mildly radial. Our study supports the results of numerical
simulations that identify radial orbits of galaxies as the cause of an
increasing $\Delta m_{12}$ in groups.
