Tracing the non-thermal pressure and hydrostatic bias in galaxy clusters

TL;DR

This paper models non-thermal pressure in galaxy clusters, relates it to hydrostatic bias, and uses observational data to estimate the bias distribution, finding most clusters have low bias levels insufficient to resolve cosmological discrepancies.

Contribution

It introduces a model linking non-thermal pressure support to hydrostatic bias and applies it to X-ray and SZ data to estimate bias distributions in galaxy clusters.

Findings

50% of clusters have bias < 0.2

80% of clusters have bias < 0.33

Measured bias is generally below 0.2 in the sample

Abstract

We present a modelization of the non-thermal pressure, $P_{NT}$, and we apply it to the X-ray (and Sunayev-Zel'dovich) derived radial profiles of the X-COP galaxy clusters. We relate the amount of non-thermal pressure support to the hydrostatic bias, $b$, and speculate on how we can interpret this $P_{NT}$ in terms of the expected levels of turbulent velocity and magnetic fields. Current upper limits on the turbulent velocity in the intracluster plasma are used to build a distribution $\mathcal{N}(<b) - b$, from which we infer that 50 per cent of local galaxy clusters should have $b < 0.2$ ($b<0.33$ in 80 per cent of the population). The measured bias in the X-COP sample that includes relaxed massive nearby systems is 0.03 in 50% of the objects and 0.17 in 80% of them. All these values are below the amount of bias required to reconcile the observed cluster number count in the cosmological framework set from Planck.