Constraining hydrostatic mass bias of galaxy clusters with
  high-resolution X-ray spectroscopy
  Gas motions in galaxy clusters play important roles in determining the
properties of the intracluster medium (ICM) and in the constraint of
cosmological parameters via X-ray and Sunyaev-Zel'dovich effect observations of
galaxy clusters. The Hitomi measurements of gas motions in the core of the
Perseus Cluster have provided insights into the physics in galaxy clusters. The
XARM mission, equipped with the Resolve X-ray micro-calorimeter, will continue
Hitomi's legacy by measuring ICM motions through Doppler shifting and
broadening of emission lines in a larger number of galaxy clusters, and at
larger radii. In this work, we investigate how well we can measure bulk and
turbulent gas motions in the ICM with XARM, by analyzing mock XARM simulations
of galaxy clusters extracted from cosmological hydrodynamic simulations. We
assess how photon counts, spectral fitting methods, multiphase ICM structure,
deprojections, and region selection affect the measurements of gas motions. We
first show that XARM is capable of recovering the underlying spherically
averaged turbulent and bulk velocity profiles for dynamically relaxed clusters
to within $\sim 50\%$ with a reasonable amount of photon counts in the X-ray
emission lines. We also find that there are considerable azimuthal variations
in the ICM velocities, where the velocities measured in a single azimuthal
direction can significantly deviate from the true value even in dynamically
relaxed systems. Such variation must be taken into account when interpreting
data and developing observing strategies. We will discuss the prospect of using
the upcoming XARM mission to measure non-thermal pressure and to correct for
the hydrostatic mass bias of galaxy clusters. Our results are broadly
applicable for future X-ray missions, such as Athena and Lynx.
