Structural and dynamical modeling of WINGS clusters

G A Mamon,A Biviano,B Poggianti,D Bettoni,A Cava,A Moretti

doi:10.1051/0004-6361/201935081

Abstract

The orbital shapes of galaxies of different classes are a probe of their formation and evolution. The Bayesian MAMPOSSt mass-orbit modeling algorithm is used to jointly fit the distribution of elliptical, spiral-irregular, and lenticular galaxies in projected phase space, on three pseudo-clusters (built by stacking the clusters after re-normalizing their positions and velocities) of 54 regular clusters from the Wide-field Nearby Galaxy-clusters Survey (WINGS), with at least 30 member velocities. Our pseudo-clusters (i.e., stacks) contain nearly 5000 galaxies with available velocities and morphological types. Thirty runs of MAMPOSSt with different priors are presented. The highest MAMPOSSt likelihoods are obtained for generalized Navarro-Frenk-White (NFW) models with steeper inner slope, free-index Einasto models, and double NFW models for the cluster and the brightest cluster galaxy. However, there is no strong Bayesian evidence for a steeper profile than the NFW model. The mass concentration matches the predictions from cosmological simulations. Ellipticals usually best trace the mass distribution while S0s are close. Spiral galaxies show increasingly elongated orbits at increasing radii, as do S0s on two stacks, and ellipticals on one stack. The inner orbits of all three types in the three stacks are consistent with isotropy. Spiral galaxies should transform rapidly into early-types given their much larger extent in clusters. Elongated outer orbits are expected for the spirals, a consequence of their recent radial infall into the cluster. The less elongated orbits we find for early-types could be related to the longer time spent by these galaxies in the cluster. We demonstrate that two-body relaxation is too slow to explain the inner isotropy of the early types, which suggests that inner isotropy is the consequence of violent relaxation during major cluster mergers or dynamical friction and tidal braking acting on subclusters. We propose that the inner isotropy of the short-lived spirals is a selection effect of spirals passing only once through pericenter before being transformed into early-type morphologies.

Highlights

The orbits of galaxies in galaxy clusters are a useful tool to understand the evolution of clusters
We demonstrate that two-body relaxation is too slow to explain the inner isotropy of the early types, which suggests that inner isotropy is the consequence of violent relaxation during major cluster mergers or dynamical friction and tidal braking acting on subclusters
This work represents the largest analysis of velocity anisotropy in cluster galaxies and the first to distinguish the orbits of ellipticals, spirals, and lenticulars using a Bayesian model to predict the distribution of these three morphological types in PPS

Summary

Introduction

The orbits of galaxies in galaxy clusters are a useful tool to understand the evolution of clusters. Galaxies detaching themselves from their initial Hubble expansion should enter clusters on fairly radial orbits. In the inner regions of clusters, most galaxies have arrived at early times, and the two-body relaxation time is often thought to be shorter than the age of the Universe deep inside the cluster where the crossing times are very short. The galaxies in the inner regions should have forgotten their initial trajectories and the inner population should have isotropic velocities. In the limit of very minor cluster mergers, clusters are relatively isolated systems accreting individual galaxies, whose orbits should be fairly radial on their first infall. In the opposite limit of major cluster mergers, galaxies suffer violent relaxation that isotropizes their orbits. The angular momentum of the secondary cluster will be transferred into individual galaxies, which may lead to an excess of more circular orbits

Results

Discussion

Conclusion