Halo Radius (Splashback Radius) of Groups and Clusters of Galaxies on small scales

TL;DR

This study measures the splashback radius of galaxy groups and clusters, revealing it as a new boundary that correlates with the virial radius, providing insights into the structure of dark matter halos.

Contribution

Introduces a new observed boundary, the splashback radius, for galaxy clusters and groups, and quantifies its relation to the virial radius using observational data.

Findings

The splashback radius is proportional to the virial radius.

Mean splashback radius for groups is 1.14 Mpc; for clusters, 2.00 Mpc.

The ratio of splashback to virial radius is approximately 1.4.

Abstract

We report the results of a study of the distribution of galaxies in the projection along the radius ($R \leq 3R_{200c}$) for 157~groups and clusters of galaxies in the local Universe (0.01 < $z$ < 0.10) with line-of-sight velocity dispersions~200~km~s$^{-1}$ < $ \sigma$ < 1100~km~s$^{-1}$. We introduce a new observed boundary for the halos of clusters of galaxies, which we identify with the splashback radius $R_{\rm{sp}}$. We also identified the core of groups/clusters of galaxies with the radius$R_c$. These radii are determined by the observed integrated distribution of the number of galaxies as a function of squared angular radius from the center of the group/cluster, which (usually) coincides with the brightest galaxy. We found for the entire sample that the boundary of dark matter $R_{\rm{sp}}$ for groups/clusters of galaxies is proportional to the radius $R_{\rm{200c}}$ of the virialized region. We measured the mean radius $\langle R_{\rm{sp}} \rangle = 1.14\pm0.02$~Mpc for groups of galaxies ($\sigma \leq 400$~km~s$^{-1}$) and $\langle R_{\rm{sp}} \rangle = 2.00\pm0.07$~Mpc for clusters of galaxies ($\sigma > 400$~km~s$^{-1}$). The mean ratio of radii is $\langle R_{\rm{sp}}/R_{\rm{200c}} \rangle = 1.40\pm0.02$}, or $\langle R_{\rm{sp}}/R_{\rm{200m}} \rangle = 0.88\pm0.02$.