The Quiescent Intracluster Medium in the Core of the Perseus Cluster

TL;DR

Hitomi X-ray observations of the Perseus cluster core reveal a remarkably quiescent hot gas atmosphere with minimal turbulence, supporting the accuracy of hydrostatic mass estimates and providing insights into intracluster medium dynamics.

Contribution

This study provides the first direct measurement of the velocity dispersion and turbulence in the intracluster medium of the Perseus cluster core using Hitomi X-ray data.

Findings

Gas velocity dispersion of 164 km/s in the cluster core

Turbulent pressure support is less than 4% of total pressure

Minimal correction needed for hydrostatic mass estimates

Abstract

Clusters of galaxies are the most massive gravitationally-bound objects in the Universe and are still forming. They are thus important probes of cosmological parameters and a host of astrophysical processes. Knowledge of the dynamics of the pervasive hot gas, which dominates in mass over stars in a cluster, is a crucial missing ingredient. It can enable new insights into mechanical energy injection by the central supermassive black hole and the use of hydrostatic equilibrium for the determination of cluster masses. X-rays from the core of the Perseus cluster are emitted by the 50 million K diffuse hot plasma filling its gravitational potential well. The Active Galactic Nucleus of the central galaxy NGC1275 is pumping jetted energy into the surrounding intracluster medium, creating buoyant bubbles filled with relativistic plasma. These likely induce motions in the intracluster medium and heat the inner gas preventing runaway radiative cooling; a process known as Active Galactic Nucleus Feedback. Here we report on Hitomi X-ray observations of the Perseus cluster core, which reveal a remarkably quiescent atmosphere where the gas has a line-of-sight velocity dispersion of 164+/-10 km/s in a region 30-60 kpc from the central nucleus. A gradient in the line-of-sight velocity of 150+/-70 km/s is found across the 60 kpc image of the cluster core. Turbulent pressure support in the gas is 4% or less of the thermodynamic pressure, with large scale shear at most doubling that estimate. We infer that total cluster masses determined from hydrostatic equilibrium in the central regions need little correction for turbulent pressure.