Spatial Anisotropy of Galaxy Kinematics in Sloan Digital Sky Survey Galaxy Clusters

TL;DR

This paper reports the first observational detection of anisotropic galaxy kinematics within clusters, revealing that galaxy velocity dispersion varies with azimuthal angle and depends on the cluster's orientation, aligning with simulation predictions.

Contribution

It introduces the first measurement of azimuthal dependence of galaxy kinematics in clusters, highlighting the impact of cluster orientation on observed anisotropy.

Findings

Velocity dispersion is higher along the major axis.

Anisotropy depends strongly on line-of-sight orientation.

Results agree with numerical simulation predictions.

Abstract

Measurements of galaxy cluster kinematics are important in understanding the dynamical state and evolution of clusters of galaxies, as well as constraining cosmological models. While it is well established that clusters exhibit non-spherical geometries, evident in the distribution of galaxies on the sky, azimuthal variations of galaxy kinematics within clusters have yet to be observed. Here we measure the azimuthal dependence of the line-of-sight velocity dispersion profile in a stacked sample of 1743 galaxy clusters from the Sloan Digital Sky Survey (SDSS). The clusters are drawn from the SDSS DR8 redMaPPer catalog. We find that the line-of-sight velocity dispersion of galaxies lying along the major axis of the central galaxy is larger than those that lie along the minor axis. This is the first observational detection of anisotropic kinematics of galaxies in clusters. We show that the result is consistent with predictions from numerical simulations. Furthermore we find that the degree of projected anisotropy is strongly dependent on the line-of-sight orientation of the galaxy cluster, opening new possibilities for assessing systematics in optical cluster finding.