The ESO Nearby Abell Cluster Survey

Abstract

We study the orbits of the various types of galaxies observed in the ESO Nearby Abell Cluster Survey. We combine the observed kinematics and projected distributions of galaxies of various types with an estimate of the mass density profile of the ensemble cluster to derive velocity-anisotropy profiles. Galaxies within and outside substructures are considered separately. Among the galaxies outside substructures we distinguish four classes, on the basis of their projected phase-space distributions. These classes are: the brightest ellipticals (with ), the other ellipticals together with the S0's, the early-type spirals (Sa–Sb), and the late-type spirals and irregulars (Sbc-Irr) together with the emission-line galaxies (except those of early morphology). The mass profile was determined from the distribution and kinematics of the early-type (i.e. elliptical and S0) galaxies 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. We apply the method described by Binney & Mamon as implemented by Solanes & Salvador-Solé to derive, to our knowledge for the first time, the velocity anisotropy profiles of all galaxy classes individually (except, of course, the early-type class). We check the validity of the solutions for , where and are the mean squared components of the radial and tangential velocity, respectively, by comparing the observed and predicted velocity-dispersion profiles. 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. For both the early and the late spirals, as well as for the galaxies in substructures, the data allow equilibrium solutions. The data for the early spirals are consistent with isotropic orbits (), although there is an apparent radial anisotropy at . For the late spirals an equilibrium solution with isotropic orbits is rejected by the data at the >99% confidence level. While within , increases linearly with radius to a value 1.8 at . Taken at face value, the data for the galaxies in substructures indicate that isotropic solutions are not acceptable, and tangential orbits are indicated. Even though the details of the tangential anisotropy remain to be determined, the general conclusion appears robust. We briefly discuss the possible implications of these velocity-anisotropy profiles for current ideas of the evolution and transformation of galaxies in clusters.