The ESO Nearby Abell Cluster Survey. XIII. The orbits of the different types of galaxies in rich clusters

TL;DR

This study analyzes the orbital dynamics of different galaxy types in rich clusters, revealing varied orbital anisotropies and implications for galaxy evolution within cluster environments.

Contribution

It provides a detailed comparison of galaxy orbit types in clusters, using a new mass profile and considering substructure effects, highlighting differences among galaxy types.

Findings

Early-type galaxies have isotropic orbits outside substructures.

Brightest ellipticals likely do not follow equilibrium solutions.

Late spirals exhibit increasing radial anisotropy outward.

Abstract

We study the orbits of the various types of galaxies observed in the ESO Nearby Abell Cluster Survey. Galaxies within and outside substructures are considered separately. We use the mass profile we determined from the distribution and kinematics of the early-type galaxies (i.e. ellipticals, excluding the brightest ones, and S0s) outside substructures; the latter were assumed to be on isotropic orbits, which is supported by the shape of their velocity distribution. The projected distribution and kinematics of the galaxies of other types are used to search for equilibrium solutions in the gravitational potential derived from the early-type galaxies, using the method described by Binney and Mamon as implemented by Solanes and Salvador-Sole'. For the brightest ellipticals we are not able to construct equilibrium solutions. This is most likely the result of the formation history and the special location of these galaxies at the centres of their clusters. The data for the early spirals allow equilibrium solutions and are consistent with isotropic orbits, although there is an apparent radial anisotropy at about 0.45 r200. For the late spirals an equilibrium solution with isotropic orbits is rejected by the data. The orbits are nearly isotropic within about 0.7 r200, but then become increasingly radial outwards. Finally, the data for the galaxies in substructures indicate that isotropic solutions are not acceptable, and tangential orbits are indicated. We briefly discuss the possible implications of these velocity-anisotropy profiles for current ideas of the evolution and transformation of galaxies in clusters. (Abridged)