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