On the Cluster Physics of Sunyaev-Zel'dovich and X-ray Surveys IV: Characterizing Density and Pressure Clumping due to Infalling Substructures

TL;DR

This study uses cosmological hydrodynamical simulations to analyze the density and pressure clumping in galaxy cluster outskirts, revealing how infalling substructures influence measurements crucial for cosmology.

Contribution

It provides new insights into the radial, mass, and redshift dependence of gas clumping and introduces the concept of super-clumping driven by large substructures.

Findings

Clumping bias remains below 20% within R200.

Clumping increases steeply beyond R200 due to infalling structures.

Large substructures dominate the clumping signal, indicating super-clumping.

Abstract

Understanding the outskirts of galaxy clusters at the virial radius (R200) and beyond is critical for an accurate determination of cluster masses and to ensure unbiased cosmological parameter estimates from cluster surveys. This problem has drawn renewed interest due to recent determinations of gas mass fractions beyond R200, which appear to be considerably larger than the cosmic mean, and because the clusters' total Sunyaev-Zel'dovich flux receives a significant contribution from these regions. Here, we use a large suite of cosmological hydrodynamical simulations to study the clumpiness of density and pressure and employ different variants of simulated physics, including radiative gas physics and thermal feedback by active galactic nuclei. We find that density and pressure clumping closely trace each other as a function of radius, but the bias on density remains on average < 20% within the virial radius R200. At larger radius, clumping increases steeply due to the continuous infall of coherent structures that have not yet passed the accretion shock. Density and pressure clumping increase with cluster mass and redshift, which probes on average dynamically younger objects that are still in the process of assembling. The angular power spectra of gas density and pressure show that the clumping signal is dominated by comparably large substructures with scales >R200/5, signaling the presence of gravitationally-driven "super-clumping". In contrast, the angular power spectrum of the dark matter (DM) shows an almost uniform size distribution due to unimpeded subhalos. The quadrupolar anisotropy dominates the signal and correlates well across different radii as a result of the prolateness of the DM potential. We provide a synopsis of the radial dependence of the clusters' non-equilibrium measures (kinetic pressure support, ellipticity, and clumping) that all increase sharply beyond R200.