The orbital velocity anisotropy of cluster galaxies: evolution

Context. In nearby clusters early-type galaxies follow isotropic orbits. The orbits of late-type galaxies are instead characterized by slightly radial anisotropy. Little is known about the orbits of the different populations of cluster galaxies at redshift z > 0.3. Aims. We investigate the redshift evolution of the orbits of cluster galaxies. Methods. We use two samples of galaxy clusters spanning similar (evolutionary corrected) mass ranges at different redshifts. The sample of low-redshift (z ∼ 0.0 − 0.1) clusters is extracted from the ESO Nearby Abell Cluster Survey (ENACS) catalog. The sample of high-redshift (z ∼ 0.4 − 0.8) clusters is mostly made of clusters from the ESO Distant Cluster Survey (EDisCS). For each of these samples, we solve the Jeans equation for hydrostatic equilibrium separately for two cluster galaxy populations, characterized by the presence and, respectively, absence of emission-lines in their spectra (’nELGs’ and ’ELGs’ hereafter). Using two tracers of the gravitational potential allows to partially break the mass–anisotropy degeneracy which plagues these kinds of analyses. Results. We confirm earlier results for the nearby cluster sample. The mass profile is well fitted by a Navarro, Frenk & White (NFW) profile with concentration c = 4. The mass profile of the distant cluster sample is also well fitted by a NFW profile, but with a slightly lower concentration, as predicted by cosmological simulations of cluster-sized halos. While the mass density profile becomes less concentrated with redshift, the number density profile of nELGs becomes more concentrated with redshift. In nearby clusters, the velocity anisotropy profile of nELGs is close to isotropic, while that of ELGs is increasingly radial with clustercentric radius. In distant clusters the projected phase-space distributions of both nELGs and ELGs are best-fitted by models with radial velocity anisotropy. Conclusions. No significant evolution is detected for the orbits of ELGs, while the orbits of nELGs evolve from radial to isotropic with time. We speculate that this evolution may be driven by the secular mass growth of galaxy clusters during their fast accretion phase. Cluster mass density profiles and their evolution with redshift are consistent with predictions for cluster-sized halos in Λ Cold Dark Matter cosmological simulations. The evolution of the nELG number density profile is opposite to that of the mass density profile, becoming less concentrated with time, probably a result of the transformation of ELGs into nELGs.