Compact groups from semi-analytical models of galaxy formation -- I: a comparative study of frequency and nature

TL;DR

This study compares semi-analytical galaxy formation models to understand how cosmological parameters and model recipes influence the frequency and physical nature of compact galaxy groups, revealing significant dependencies and sensitivities.

Contribution

It provides a systematic comparison of multiple SAMs and cosmologies, highlighting how these factors affect the predicted properties of compact galaxy groups.

Findings

CG frequency decreases with lower $\sigma_8$ and higher $\Omega_{ m m}$

Fraction of physically dense CGs varies with cosmology and resolution

SAM recipe differences impact CG properties, especially orphan galaxy treatment

Abstract

Compact groups (CGs) of galaxies are defined as isolated and dense galaxy systems that appear to be a unique site of multiple galaxy interactions. Semi-analytical models of galaxy formation (SAMs) are a prime tool to understand CGs. We investigate how the frequency and the three-dimensional nature of CGs depends on the SAM and its underlying cosmological parameters. Extracting 9 lightcones of galaxies from 5 different SAMs and selecting CGs as in observed samples, we find that the frequency and nature of CGs depends strongly on the cosmological parameters. Moving from the WMAP1 to the WMAP7 and Planck cosmologies (increasing density of the Universe and decreasing normalisation of the power spectrum), the space density of CGs is decreased by a factor 2.5, while the fraction of CGs that are physically dense falls from 50 to 35 percent. The lower $\sigma_8$ leads to fewer dense groups, while the higher $\Omega_{\rm m}$ causes more chance alignments. However, with increased mass and spatial resolution, the fraction of CGs that are physically dense is pushed back up to 50 percent. The intrinsic differences in the SAM recipes also lead to differences in the frequency and nature of CGs, particularly those related to how SAMs treat orphan galaxies. We find no dependence of CG properties on the flux limit of the mock catalogues nor on the waveband in which galaxies are selected. One should thus be cautious when interpreting a particular SAM for the frequency and nature of CGs.