CLASH-VLT velocity anisotropy profiles in a stack of massive galaxy clusters

TL;DR

This study measures the velocity anisotropy profiles of galaxy cluster members using advanced techniques, revealing how orbital anisotropy varies with radius and galaxy population, with implications for cluster dynamics and cosmology.

Contribution

It provides the first detailed comparison of anisotropy profiles across multiple galaxy populations using both parametric and non-parametric methods, with unprecedented data quality.

Findings

Anisotropy increases from the center to the virial radius.

Different galaxy populations show similar anisotropy profiles.

A notable drop in anisotropy occurs at about 250 kpc from the center.

Abstract

We measure the velocity anisotropy profile $\beta(r)$ of different galaxy cluster member populations by analysing the stacked projected phase space of nine massive ($M_\mathrm{200c}>7\times10^{14}$ M$_\odot$) galaxy clusters at intermediate redshifts ($0.18 < z < 0.45$). We select our sample of galaxy clusters by choosing the most round and virialised objects among the targets of the CLASH-VLT spectroscopic program, which offers a large spectral database. Complementary MUSE observations on most of these clusters allowed us to identify an unprecedented number of cluster members, strongly enhancing the precision of our measurement with respect to previous studies. Our sample of cluster members is divided in four classes: red and blue by colour, and high- and low-mass by stellar mass. We employ two parallel techniques, namely the MAMPOSSt method (parametric in $\beta(r)$) and the Jeans equation inversion (non parametric in $\beta(r)$). The results from both techniques are found in agreement for any given cluster member population, and suggest that the orbital anisotropy in galaxy clusters grows from the centre (where $\beta\approx 0.2-0.4$) to the virial radius ($\beta\gtrsim 0.8$), and it is similar for the different cluster member populations. We also find an interesting dynamical feature in the Jeans inversion results, that is a drop in $\beta(r)$ at a distance of $\sim 250$ kpc from the cluster centre. We provide robust anisotropy estimates by exploring a highly significant number of model combinations: 72 with MAMPOSSt (varying the mass, surface number density, $\beta(r)$ model, and galaxy population) and 18 (varying total mass model and galaxy population) in the Jeans inversion. Such an extensive investigation of the $\beta(r)$ profile in galaxy clusters is a wide basis for future studies on cluster dynamical masses and cluster cosmology in the era of large spectroscopic surveys