Measuring the dark matter velocity anisotropy in galaxy clusters

TL;DR

This study measures the velocity anisotropy of dark matter in galaxy clusters using X-ray data, revealing differences from baryonic gases and supporting simulation predictions about dark matter behavior.

Contribution

It introduces a novel method to measure dark matter temperature anisotropy in galaxy clusters, providing observational evidence of non-isotropic dark matter velocities.

Findings

Dark matter temperature anisotropy beta_dm > 0 at >3 sigma confidence

Results align with cosmological N-body simulations

Dark matter equilibration differs from baryonic gases

Abstract

The Universe contains approximately 6 times more dark matter than normal baryonic matter, and a directly observed fundamental difference between dark matter and baryons would both be significant for our understanding of dark matter structures and provide us with information about the basic characteristics of the dark matter particle. We discuss one distinctive feature of dark matter structures in equilibrium, namely the property that a local dark matter temperature may depend on direction. This is in stark contrast to baryonic gases. We used X-ray observations of two nearby, relaxed galaxy clusters, under the assumptions of hydrostatic equilibrium and identical dark matter and gas temperatures in the outer cluster region, to measure this dark matter temperature anisotropy beta_dm, with non-parametric Monte Carlo methods. We find that beta_dm is greater than the value predicted for baryonic gases, beta_gas=0, at more than 3 sigma confidence. The observed value of the temperature anisotropy is in fair agreement with the results of cosmological N-body simulations and shows that the equilibration of the dark matter particles is not governed by local point-like interactions in contrast to baryonic gases.