What Do the Hitomi Observations Tell Us About the Turbulent Velocities in the Perseus Cluster? Probing the Velocity Field with Mock Observations

TL;DR

This study uses synthetic Hitomi observations of a simulated galaxy cluster to analyze gas motions, revealing that sloshing can produce observed velocities and emphasizing the importance of modeling observational biases.

Contribution

The paper demonstrates that sloshing motions can replicate Hitomi's velocity measurements and highlights the impact of observational biases and viewing angles on velocity inferences.

Findings

Sloshing motions can produce line shifts similar to Hitomi measurements.

PSF effects can bias velocity measurements by up to 50 km/s.

Total kinetic energy is less than 10% of thermal energy, consistent with observations.

Abstract

Hitomi made the first direct measurements of galaxy cluster gas motions in the Perseus cluster, which implied that its core is fairly "quiescent", with velocities less than $\sim$200 km s$^{-1}$, despite the presence of an active galactic nucleus and sloshing cold fronts. Building on previous work, we use synthetic Hitomi/SXS observations of the hot plasma of a simulated cluster with sloshing gas motions and varying viscosity to analyze its velocity structure in a similar fashion. We find that sloshing motions can produce line shifts and widths similar to those measured by Hitomi. We find these measurements are unaffected by the value of the gas viscosity, since its effects are only manifested clearly on angular scales smaller than the SXS $\sim$1' PSF. The PSF biases the line shift of regions near the core as much as $\sim 40-50$ km s$^{-1}$, so it is crucial to model this effect carefully. We also infer that if sloshing motions dominate the observed velocity gradient, Perseus must be observed from a line of sight which is somewhat inclined from the plane of these motions, but one that still allows the spiral pattern to be visible. Finally, we find that assuming isotropy of motions can underestimate the total velocity and kinetic energy of the core in our simulation by as much as $\sim$60%. However, the total kinetic energy in our simulated cluster core is still less than 10% of the thermal energy in the core, in agreement with the Hitomi observations.