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 \sigma _ { v,los } \simeq 160 { km\ > s ^ { -1 } } , at 30 - 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 \sim 100 - 200 { km\ > s ^ { -1 } } , 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 .