Hydrostatic and Caustic Mass Profiles of Galaxy Clusters

TL;DR

This study compares X-ray and caustic mass profiles of 16 galaxy clusters, finding good agreement and suggesting minimal hydrostatic bias, which supports the reliability of these methods in mass estimation.

Contribution

It provides the first direct comparison of hydrostatic and caustic mass profiles across a sample of clusters without assuming a common functional form.

Findings

Hydrostatic and caustic masses agree within 20% across the radial range.

The mass ratio at R₅₀₀ is approximately 1.20, indicating small bias.

No dependence of mass ratio on cluster X-ray morphology.

Abstract

We compare X-ray and caustic mass profiles for a sample of 16 massive galaxy clusters. We assume hydrostatic equilibrium in interpreting the X-ray data, and use large samples of cluster members with redshifts as a basis for applying the caustic technique. The hydrostatic and caustic masses agree to better than $\approx20\%$ on average across the radial range covered by both techniques $(\sim[0.2-1.25]R_{500})$. The mass profiles were measured independently and do not assume a common functional form. Previous studies suggest that, at $R_{500}$, the hydrostatic and caustic masses are biased low and high respectively. We find that the ratio of hydrostatic to caustic mass at $R_{500}$ is $1.20^{+0.13}_{-0.11}$; thus it is larger than 0.9 at $\approx3\sigma$ and the combination of under- and over-estimation of the mass by these two techniques is $\approx10\%$ at most. There is no indication of any dependence of the mass ratio on the X-ray morphology of the clusters, indicating that the hydrostatic masses are not strongly systematically affected by the dynamical state of the clusters. Overall, our results favour a small value of the so-called hydrostatic bias due to non-thermal pressure sources.