Statistical analysis of bound companions in the Coma cluster

TL;DR

This study investigates the substructure of the Coma galaxy cluster by identifying potential bound companions around giant galaxies, revealing a low frequency of such companions consistent with the cluster's regular structure.

Contribution

It introduces a novel application of over-density and nearest neighbor analyses to identify bound companions in the Coma cluster, extending previous methods used in Virgo.

Findings

Evidence of excess companions around luminous late-type giants.

Most satellites are faint dwarfs near high-luminosity primaries.

Estimated bound companions constitute 2-4% of the cluster, lower than Virgo.

Abstract

The rich and nearby Coma cluster of galaxies is known to have substructure. We aim to create a more detailed picture of this substructure by searching directly for bound companions around individual giant members. We have used two catalogs of Coma galaxies, one covering the cluster core for a detailed morphological analysis, another covering the outskirts. The separation limit between possible companions (secondaries) and giants (primaries) is chosen as M_B = -19 and M_R = -20, respectively for the two catalogs. We have created pseudo-clusters by shuffling positions or velocities of the primaries and search for significant over-densities of possible companions around giants by comparison with the data. This method was developed and applied first to the Virgo cluster by Ferguson (1992). In a second approach we introduced a modified nearest neighbor analysis using several interaction parameters for all galaxies. We find evidence for some excesses due to possible companions for both catalogs. Satellites are typically found among the faintest dwarfs (M_B < -16) around high-luminosity primaries. The most significant excesses are found around very luminous late-type giants (spirals) in the outskirts, which is expected in an infall scenario of cluster evolution. A rough estimate for an upper limit of bound galaxies within Coma is 2 - 4 percent, to be compared with ca. 7 percent for Virgo. The results agree well with the expected low frequency of bound companions in a regular cluster such as Coma.