# Estimating the dark matter velocity anisotropy to the cluster edge

## TL;DR

This paper estimates the dark matter velocity anisotropy in the galaxy cluster Perseus using a novel method, extending measurements to near five times the previous radii, supporting the collisionless dark matter hypothesis.

## Contribution

It applies a new estimation method to measure dark matter velocity anisotropy at larger radii in a galaxy cluster, extending previous observational limits.

## Key findings

- Dark matter velocity anisotropy is consistent with simulations.
- Anisotropy is non-zero at half the virial radius at 1.7 sigma.
- Results support the collisionless nature of dark matter.

## Abstract

Dark matter dominates the properties of large cosmological structures such as galaxy clusters, and the mass profiles of the dark matter have been measured for these equilibrated structures for years using X-rays, lensing or galaxy velocities. A new method has been proposed, which should allow us to estimate a dynamical property of the dark matter, namely the velocity anisotropy. For the gas a similar velocity anisotropy is zero due to frequent collisions, however, the collisionless nature of dark matter allows it to be non-trivial. Numerical simulations have for years found non-zero and radially varying dark matter velocity anisotropies. Here we employ the method proposed by Hansen and Pifaretti (2007), and developed by Host et al. (2009) to estimate the dark matter velocity anisotropy in the bright galaxy cluster Perseus, to near 5 times the radii previously obtained. We find the dark matter velocity anisotropy to be consistent with the results of numerical simulations, however, still with large error-bars. At half the virial radius we find the velocity anisotropy to be non-zero at 1.7 standard deviations, lending support to the collisionless nature of dark matter.