Colour similarity study among small galaxy groups members

ABSTRACT We applied a membership test based on the colour similarity of group members to detect the discordant galaxies in small groups (quintets) that had been determined by the Friends-of-Friends (FoF) algorithm. Our method depends on the similarity of the colour indices (u-g) and (g-r) of the group members. The chosen sample of quintets was extracted from “Flux- and volume-limited groups for SDSS galaxies” catalogue which is a spectroscopic sample of galaxies originally taken from the Sloan Digital Sky Survey – Data Release 10 (SDSS-DR10). The sample included 282 quintets with a total number of 1410 galaxies. The similarity measure used in this study is the Euclidean distance. The calculations showed that 73.4% of the group samples (207 out of 282 quintet groups) have galaxies with similar colours (u-g) and (g-r). Each of the remainder groups (75 systems) has an interloper galaxy with different colours than the other members, and hence they became quadrants. We found that group members tend to be more luminous than outliers. We conclude that using the similarities in the colour indices between group members gives better identification of group membership