On the Self-Similar Appearance of Galaxy Clusters in X-rays

TL;DR

This paper reviews and refines the scaling laws of galaxy clusters in X-ray observations, incorporating hydrodynamic effects and redshift evolution to improve the accuracy of cosmological measurements.

Contribution

It provides a comprehensive review of cluster scaling laws, tests formation approximations with simulations, and proposes a modified scaling scheme including hydrodynamics and redshift effects.

Findings

Fiducial radius based on fixed overdensity is more precise than redshift-dependent overdensity.

Modified scaling laws with a mass-dependent gas fraction better match observations.

Current data are consistent with the proposed scaling modifications.

Abstract

The largest uncertainty for cosmological studies using clusters of galaxies is introduced by our limited knowledge of the statistics of galaxy cluster structure, and of the scaling relations between observables and cluster mass. A large effort is therefore undertaken to compile global galaxy cluster properties in particular obtained through X-ray observations and to study their scaling relations. However, the scaling schemes used in the literature differ. The present paper aims to clarify this situation by providing a thorough review of the scaling laws within the standard model of large-scale structure growth and to discus various steps of practical approximations. We derive the scaling laws for X-ray observables and cluster mass within the pure gravitational structure growth scenario. Using N-body simulations we test the recent formation approximation used in earlier analytic approaches which involves a redshift dependent overdensity parameter.We find this approximation less precise than the use of a fiducial radius based on a fixed overdensity with respect to critical density. Inspired by the comparison of the predicted scaling relations with observations we propose a first order modification of the scaling scheme to include the observed effects of hydrodynamics in structure formation. This modification involves a cluster mass dependent gas mass fraction. We also discuss the observational results of the reshift evolution of the most important scaling relations and find that also a redshift dependence of the gas mass to total mass relation has to be invoked within our modification scheme. We find that the current observational data are within their uncertainties consistent with the proposed modified scaling laws.