SZ-X-ray Surface Brightness Fluctuations in the SPT-XMM clusters

TL;DR

This study analyzes surface brightness fluctuations in 60 galaxy clusters using SZ and X-ray data to infer turbulent motions, revealing subsonic turbulence in most clusters and broad Mach number distributions.

Contribution

First large sample analysis combining SZ and X-ray data to measure turbulence in galaxy clusters through surface brightness fluctuations.

Findings

Average Mach number of 0.52 indicating subsonic turbulence

Broad distribution of Mach numbers suggesting diverse dynamical states

Good agreement between pressure and density fluctuation-based turbulence estimates

Abstract

The hot plasma in galaxy clusters, the intracluster medium (ICM), is expected to be shaped by subsonic turbulent motions, which are key for heating, cooling, and transport mechanisms. The turbulent motions contribute to the non-thermal pressure which, if not accounted for, consequently imparts a hydrostatic mass bias. Accessing information about turbulent motions is thus of major astrophysical and cosmological interest. Characteristics of turbulent motions can be indirectly accessed through surface brightness fluctuations. This study expands on our pilot investigations of surface brightness fluctuations in the SZ and X-ray by examining, for the first time, a large sample of 60 clusters using \textit{both} SPT-SZ and XMM-Newton data and span the redshift range $0.2 < z < 1.5$, thus constraining the respective pressure and density fluctuations within 0.6~$R_{500}$. We deem density fluctuations to be of sufficient quality for 32 clusters, finding mild correlations between the peak of the amplitude spectra of density fluctuations and various dynamical parameters. We infer turbulent velocities from density fluctuations with an average Mach number $\mathcal{M}_{\text{3D}} = 0.52 \pm 0.14$, in agreement with numerical simulations. For clusters with inferred turbulent Mach numbers from both pressure, $\mathcal{M}_{\text{P}}$ and density fluctuations, $\mathcal{M}_{\rho}$, we find broad agreement between $\mathcal{M}_{\text{P}}$ and $\mathcal{M}_{\rho}$. Our results suggest either a bimodal or skewed unimodal Mach number distribution, with the majority of clusters being turbulence-dominated (subsonic) while the remainder are shock-dominated (supersonic).