Deep spectroscopy in nearby galaxy clusters - III. Orbital structure of galaxies in Abell 85

Galaxies in clusters are strongly affected by their environment. They evolve according to several physical mechanisms that are active in clusters. Their efficiency can strongly depend on the orbital configuration of the galaxies. Our aim is to analyse the orbits of the galaxies in the cluster Abell 85, based on the study of the galaxy velocity anisotropy parameter. We have solved the Jeans equation under the assumption that the galaxies in A 85 are collisionless objects, within the spherically symmetric gravitational potential of the virialized cluster. The mass of the cluster was estimated with X-ray and caustic analyses. We find that the anisotropy profile of the full galaxy population in A 85 is an increasing monotonic function of the distance from the cluster centre: on average, galaxies in the central region (r/r(200) < 0.3) are on isotropic orbits, while galaxies in the outer regions are on radial orbits. We also find that the orbital properties of the galaxies strongly depend on their stellar colour. In particular, blue galaxies are on less radial orbits than red galaxies. The different families of cluster galaxies considered here have the pseudo phase-space density profiles Q(r) and Q(r)(r) consistent with the profiles expected in virialized dark matter haloes in N-body simulations. This result suggests that the galaxies in A 85 have reached dynamical equilibrium within the cluster potential. Our results indicate that the origin of the blue and red colours of the different galaxy populations is the different orbital shape rather than the accretion time.