Measuring galaxy cluster mass profiles into the low acceleration regime with galaxy kinematics

TL;DR

This study uses galaxy kinematics to measure mass profiles of galaxy clusters in low acceleration regimes, comparing dynamical and X-ray masses, revealing deviations from galaxy acceleration relations and implications for cosmology.

Contribution

Introduces a model-independent method to derive galaxy cluster mass profiles and anisotropy profiles, addressing the mass-velocity anisotropy degeneracy with virial shape parameters.

Findings

Mass profiles consistent with X-ray data for relaxed clusters

Unrelaxed clusters show 50% higher dynamical mass than X-ray estimates

Clusters deviate from the galaxy radial acceleration relation

Abstract

We probe the dynamical mass profiles of 10 galaxy clusters from the HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS) using galaxy kinematics. We numerically solve the spherical Jeans equation, and parameterize the dynamical mass profile and the galaxy velocity anisotropy profile using two general functions to ensure that our results are not biased towards any specific model. The mass-velocity anisotropy degeneracy is ameliorated by using two "virial shape parameters" that depend on the fourth moment of velocity distribution. The resulting velocity anisotropy estimates consistently show a nearly isotropic distribution in the inner regions, with an increasing radial anisotropy towards large radii. We compare our derived dynamical masses with those calculated from X-ray gas data assuming hydrostatic equilibrium, finding that massive and rich relaxed clusters generally present consistent mass measurements, while unrelaxed or low-richness clusters have systematically larger total mass than hydrostatic mass by an average of 50\%. This might help alleviate current tensions in the measurement of $\sigma_8$, but it also leads to cluster baryon fractions below the cosmic value. Finally, our approach probes accelerations as low as $10^{-11}$ m s$^{-2}$, comparable to the outskirts of individual late-type galaxies. We confirm that galaxy clusters deviate from the radial acceleration relation defined by galaxies.