# Physical Origins of Gas Motions in Galaxy Cluster Cores: Interpreting Hitomi Observations of the Perseus Cluster

## TL;DR

This study uses simulations to interpret Hitomi's measurements of gas motions in the Perseus galaxy cluster core, revealing that gentle AGN feedback and cosmic accretion explain the observed quiescent velocities.

## Contribution

It demonstrates that realistic simulations can reproduce Hitomi's observations, linking gas motions to AGN feedback mode and cosmic accretion processes.

## Key findings

- Gas motions are consistent with gentle AGN feedback and no recent major mergers.
- Large-scale shear is mainly caused by cosmic accretion and mergers.
- Simulations match observed velocity dispersions of 100-200 km/s.

## Abstract

The Hitomi X-ray satellite has provided the first direct measurements of the plasma velocity dispersion in a galaxy cluster. It finds a relatively "quiescent" gas with a line-of-sight velocity dispersion ~ 160 km/s, at 30 kpc to 60 kpc from the cluster center. This is surprising given the presence of jets and X-ray cavities that indicates on-going activity and feedback from the active galactic nucleus (AGN) at the cluster center. Using a set of mock Hitomi observations generated from a suite of state-of-the-art cosmological cluster simulations, and an isolated but higher resolution simulation of gas physics in the cluster core, including the effects of cooling and AGN feedback, we examine the likelihood of Hitomi detecting a cluster with the observed velocities. As long as the Perseus has not experienced a major merger in the last few gigayears, and AGN feedback is operating in a "gentle" mode, we reproduce the level of gas motions observed by Hitomi. The frequent mechanical AGN feedback generates net line-of-sight velocity dispersions ~100-200 km/s, bracketing the values measured in the Perseus core. The large-scale velocity shear observed across the core, on the other hand, is generated mainly by cosmic accretion such as mergers. We discuss the implications of these results for AGN feedback physics and cluster cosmology and progress that needs to be made in both simulations and observations, including a Hitomi re-flight and calorimeter-based instruments with higher spatial resolution.