Galaxy subgroups in galaxy clusters

TL;DR

This study uses N-body simulations to analyze galaxy subgroups within clusters, examining their properties, influence on cluster dynamics, and the effectiveness of substructure detection methods.

Contribution

It provides new insights into the properties of galaxy subgroups, their impact on cluster measurements, and the relation to substructure detection techniques.

Findings

Large subgroups correlate with preferred directions in clusters.

Scatter in velocity dispersion measurements is strongly linked to subgroup orientation.

Clusters with large subgroups often show detectable substructure in the Dressler-Shectman test.

Abstract

Galaxies which fall into clusters as part of the same infall halo can retain correlations due to their shared origin. N-body simulations are used to study properties of such galaxy subgroups within clusters, including their richnesses and prevalence. The sizes, densities and velocity dispersions of all subgroups with >= 8 galaxies are found and compared to those of the host clusters. The largest galaxy subgroup provides a preferred direction in the cluster and is compared to other preferred directions in the cluster. Scatter in cluster mass measurements (via five observables), along ~ 96 lines of sight, is compared to the relation of the line of sight to this preferred direction: scatter in cluster velocity dispersion measurements show the strongest correlation. The Dressler-Shectman test (Dressler & Shectman 1988), is applied to these clusters, to see whether the substructure it identifies is related to these subgroups. The results for any specific line of sight seem noisy; however, clusters with large subgroups tend to have a higher fraction of lines of sight where the test detects substructure.