Constraining Gas Motions in the Intra-Cluster Medium
  The detailed velocity structure of the diffuse X-ray emitting intra-cluster
medium (ICM) remains one of the last missing key ingredients in understanding
the microphysical properties of these hot baryons and constraining our models
of the growth and evolution of structure on the largest scales in the Universe.
Direct measurements of the gas velocities from the widths and shifts of X-ray
emission lines were recently provided for the central region of the Perseus
Cluster of galaxies by $Hitomi$, and upcoming high-resolution X-ray
microcalorimeters onboard $XRISM$ and $Athena$ are expected to extend these
studies to many more systems. In the mean time, several other direct and
indirect methods have been proposed for estimating the velocity structure in
the ICM, ranging from resonant scattering to X-ray surface brightness
fluctuation analysis, the kinematic Sunyaev-Zeldovich effect, or using optical
line emitting nebulae in the brightest cluster galaxies as tracers of the
motions of the ambient plasma. Here, we review and compare the existing
estimates of the velocities of the hot baryons, as well as the various
overlapping physical processes that drive motions in the ICM, and discuss the
implications of these measurements for constraining the viscosity and
identifying the source of turbulence in clusters of galaxies.
