Mass-Velocity Dispersion Relation in HIFLUGCS Galaxy Clusters

TL;DR

This study reveals a tight empirical relation between baryonic mass and velocity dispersion in galaxy clusters, showing similarities to galaxy dynamics but with notable differences in acceleration scales and baryon fraction discrepancies.

Contribution

It presents the first detailed measurement of the mass-velocity dispersion relation in galaxy clusters, highlighting a universal relation with low scatter and exposing baryon fraction discrepancies.

Findings

Discovery of a tight MVDR relation with low intrinsic scatter.

Residuals uncorrelated with other cluster properties.

Baryon fraction deficit increases in lower-mass clusters.

Abstract

We investigate the mass-velocity dispersion relation (MVDR) in 29 galaxy clusters in the HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS). We measure the spatially resolved, line-of-sight velocity dispersion profiles of these clusters, which we find to be mostly flat at large radii, reminiscent of the rotation curves of galaxies. We discover a tight empirical relation between the baryonic mass $M_\mathrm{bar}$ and the flat velocity dispersion $\sigma$ of the member galaxies, i.e. MVDR, $\log(M_\mathrm{bar}/M_\odot)=4.1^{+0.4}_{-0.4}\,\log(\sigma/\mathrm{km}\,\mathrm{s}^{-1})+1.6^{+1.0}_{-1.3}$, with the lognormal intrinsic scatter of $12^{+3}_{-3}\%$. The residuals of the MVDR are uncorrelated with other cluster properties like temperature, cluster radius, baryonic mass surface density, and redshift. These characteristics are reminiscent of the MVDR for individual galaxies, albeit about ten times larger characteristic acceleration scale. The cluster baryon fraction falls short of the cosmic value, exposing a problem: the discrepancy increases systematically for clusters of lower mass and lower baryonic acceleration.