Gas thermodynamics meets galaxy kinematics: Joint mass measurements for eROSITA galaxy clusters

The mass of galaxy clusters is a critical quantity for probing cluster cosmology and testing theories of gravity, but its measurement could be biased, given that assumptions are inevitable in order to make use of any approach. In this paper, we employ and compare two mass proxies for galaxy clusters: thermodynamics of the intracluster medium and kinematics of member galaxies. We selected 22 galaxy clusters from the cluster catalog in the first SRG/eROSITA All-Sky Survey (eRASS1) that have sufficient optical and near-infrared observations. We generated multiband images in the energy range of (0.3, 7) keV for each cluster, and derived their temperature profiles, gas mass profiles, and hydrostatic mass profiles using a parametric approach that does not assume dark matter halo models. With spectroscopically confirmed member galaxies collected from multiple surveys, we numerically solved the spherical Jeans equation for their dynamical mass profiles. Our results quantify the correlation between dynamical mass and the line-of-sight velocity dispersion, log M_dyn = (1.296 ± 0.001)log(σ_los²r_proj/G)‑(3.87 ± 0.23), with a root mean square (rms) scatter of 0.14 dex. We find that the two mass proxies lead to roughly the same total mass, with no observed systematic bias. As such, the σ₈ tension is not specific to hydrostatic mass or weak lensing shears, but also appears with galaxy kinematics. Interestingly, the hydrostatic-to-dynamical mass ratios decrease slightly toward large radii, which could possibly be evidence for accreting galaxies in the outskirts. We also compared our hydrostatic masses with the latest weak lensing masses inferred with scaling relations. The comparison shows that the weak lensing mass is significantly higher than our hydrostatic mass by ∼110%. This might explain the significantly larger value of σ₈ from the latest measurement using eRASS1 clusters than almost all previous estimates in the literature. Finally, we tested the radial acceleration relation established in disk galaxies. We confirm the missing baryon problem in the inner region of galaxy clusters using three independent mass proxies for the first time. As ongoing and planned surveys are providing deeper X-ray observations and more galaxy spectra for cluster members, we expect to extend the study to cluster outskirts in the near future.