Compact groups in theory and practice - II. Comparing the observed and predicted nature of galaxies in compact groups

TL;DR

This study uses simulations to compare observed and predicted galaxy properties in compact groups, revealing that many apparent groups are contaminated by interlopers and that selection methods can improve the accuracy of identifying genuine compact group galaxies.

Contribution

It demonstrates that most observed compact groups are contaminated by interlopers and proposes selection techniques to better identify true compact group members.

Findings

Only ~30% of simulated groups are truly compact in 3D.

Genuine compact group galaxies are predominantly red, elliptical, and passive.

Selection by redshift and galaxy color reduces contamination significantly.

Abstract

We examine the properties of galaxies in compact groups identified in a mock galaxy catalogue based upon the Millennium Run simulation. The overall properties of groups identified in projection are in general agreement with the best available observational constraints. However, only ~30% of these simulated groups are found to be truly compact in 3 dimensions, suggesting that interlopers strongly affect our observed understanding of the properties of galaxies in compact groups. These simulations predict that genuine compact group galaxies are an extremely homogeneous population, confined nearly exclusively to the red sequence: they are best described as `red and dead' ellipticals. When interlopers are included, the population becomes much more heterogeneous, due to bluer, star-forming, gas-rich, late-type galaxies incorrectly identified as compact group members. These models suggest that selection of members by redshift, such that the line-of-sight velocity dispersion of the group is less than 1000 km/s, significantly reduces contamination to the 30% level. Selection of members by galaxy colour, a technique used frequently for galaxy clusters, is also predicted to dramatically reduce contamination rates for compact group studies.