Are compact groups of galaxies special?

¹ 17, rue des plantes, 75014 Paris, France
² Institut d’Astrophysique de Paris (UMR 7095: CNRS & Sorbonne Université), 98 bis boulevard Arago, 75014 Paris, France
³ CONICET. Instituto de Astronomía Teórica y Experimental (IATE), Laprida 854, X5000BGR Córdoba, Argentina
⁴ Universidad Nacional de Córdoba (UNC). Observatorio Astronómico de Córdoba (OAC), Laprida 854, X5000BGR Córdoba, Argentina

^⋆ Corresponding author: matthieu.tricottet@gmail.com

Received: 29 July 2024
Accepted: 23 April 2025

Abstract

It is often believed that isolated compact groups (CGs) of galaxies are special systems, but only a few studies have compared CGs to regular groups. We study the global properties and internal correlations of a volume- and luminosity-complete subsample of 78 groups of four members (CG₄s) within the HMCG Hickson-like sample of compact groups. We compared these CGs to those of a similarly built subsample (including the three-magnitude range of CG₄s) of the Lim regular groups. The latter were split into three control samples: one with the four brightest members (Control_4Bs), one with the four closest members to the brightest group galaxy (BGG; Control_4Cs), and one with exactly four members (RG₄s). The vast majority of the CG₄s are located within regular groups, and a large preponderance of the BGGs of these CG₄s are the same as those of their host groups. The CG₄s are smaller than the groups of all other samples and more luminous than RG₄s. Both results are a consequence of their selection as high surface brightness systems. However, the CG₄s (especially those split among several regular groups) have luminosities similar to Control_4Cs. The CG₄s also have higher velocity dispersions, probably because of a too-permissive redshift accordance criterion. The BGGs of the CG₄s are not any more dominant in luminosity than those of RG₄s, but they are significantly more offset relative to the group size because the Lim groups are built around their BGGs. In summary, compact groups have similar properties to regular groups of four galaxies and to the cores of regular groups once the selection criteria of CGs are considered. A large fraction of the CGs are the cores of regular groups, which are isolated on the sky by construction but rarely isolated in real space (from simulations), indicating that they are often plagued by chance alignments of host group galaxies along the line of sight.