Compact groups from semi-analytical models of galaxy formation – I. A comparative study of frequency and nature

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 σ 8 leads to fewer dense groups, while the higher Ω 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.

Compact Groups of Galaxies in Sloan Digital Sky Survey and LAMOST Spectral Survey. I. The Catalogs

“…As a result, the physical nature and the boundness of CGs have been debated for several decades. Both theoretical models (e.g., Mamon 1986;Tovmassian et al 2006) and simulation studies (e.g., Hernquist et al 1995;Díaz-Giménez & Mamon 2010;Hartsuiker & Ploeckinger 2020;Díaz-Giménez et al 2020) have suggested that CGs are mixture of virialized groups, chance alignments in filaments, and collapsing groups as bound substructures within clusters. An alternative scenario has also been proposed that the large groups are the birth-place of the embedded CGs (Diaferio et al 1994;Ramella et al 1994;Andernach & Coziol 2005).…”

A Fast Radio Burst in a Compact Galaxy Group at z ∼ 1

“…galaxies comprise 1% of all galaxies out to at least z ≈ 2 (Wiens et al 2019;Díaz-Giménez et al 2020), making this a fairly rare and noteworthy host environment. The large majority of observational CG studies are at low redshifts of z  0.2.…”