Are compact groups of galaxies special?

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 (CG 4 s) 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 CG 4 s) of the Lim regular groups. The latter were split into three control samples: one with the four brightest members (Control 4B s), one with the four closest members to the brightest group galaxy (BGG; Control 4C s), and one with exactly four members (RG 4 s). The vast majority of the CG 4 s are located within regular groups, and a large preponderance of the BGGs of these CG 4 s are the same as those of their host groups. The CG 4 s are smaller than the groups of all other samples and more luminous than RG 4 s. Both results are a consequence of their selection as high surface brightness systems. However, the CG 4 s (especially those split among several regular groups) have luminosities similar to Control 4C s. The CG 4 s also have higher velocity dispersions, probably because of a too-permissive redshift accordance criterion. The BGGs of the CG 4 s are not any more dominant in luminosity than those of RG 4 s, 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.