Compact groups from the Millennium Simulations: I. Nature and properties
Eugenia Díaz-Giménez, Cinthia Ragone-Figueroa, H. Muriel, G. A. Mamon · arXiv (Cornell University) · 2008
There is much debate on whether compact groups of galaxies (CGs) are mostly physically dense systems of 4 or more luminous galaxies or instead caused by chance alignments of galaxies along the line of sight (CAs) within looser groups or larger structures. For a more statistical assessment of their reality, we identify CGs within mock galaxy catalogs constructed on top of the Millennium Simulation at z = 0 with three different semi-analytic models of galaxy formation by Croton et al. (2006, C06), Bower et al. (2006, B06), and De Lucia & Blaizot (2007, DLB). CGs are identified using the same two dimensional criteria as those visually applied by Hickson (1982) to his CGs (HCGs). We identify several hundred of CGs in projected space, among which roughly half (55%, 56 % and 29 % for C06, B06 and DLB, respectively) constitute our mock velocity sample (mvCGs), where each CG contains at least 4 galaxies with velocities within 1000 kms −1 from the group median velocity. The space density of mvCGs is 50 times the observed one, indicating that the HCG sample completeness is only 2%, as Hickson appeared to have missed CGs that were either faint, low surface