Non Thermal Support for the Outer Intracluster Medium

TL;DR

This paper models how turbulence driven by gas inflows provides non-thermal support in the outer regions of galaxy clusters, affecting observable signals and mass estimates, especially at redshifts below 0.5.

Contribution

It offers an analytical model predicting turbulence development in cluster outskirts and its impact on X-ray and microwave observations, a novel approach to understanding cluster support.

Findings

Turbulence increases in cluster outskirts at z<0.5 due to weaker shocks.

Non-thermal support causes a negative bias in X-ray mass estimates.

Model predicts observable effects on X-ray and microwave signals.

Abstract

We submit that non thermalized support for the outer intracluster medium in relaxed galaxy clusters is provided by turbulence, driven by inflows of intergalactic gas across the virial accretion shocks. We expect this component to increase briskly during the cluster development for z<1/2, due to three factors. First, the accretion rates of gas and dark matter subside, when they feed on the outer wings of the initial perturbations in the accelerating Universe. Second, the infall speeds decrease across the progressively shallower gravitational potential at the shock position. Third, the shocks eventually weaken, and leave less thermal energy to feed the intracluster entropy, but relatively more bulk energy to drive turbulence into the outskirts. The overall outcome from these factors is physically modeled and analytically computed; thus we ascertain how these concur in setting the equilibrium of the outer intracluster medium, and predict how the observables in X rays and microwaves are affected, so as to probe the development of outer turbulence over wide cluster samples. By the same token, we quantify the resulting negative bias to be expected in the total mass evaluated from X-ray measurements.