Compact groups from the Millennium Simulations: I. Nature and properties

EUGENIA DÍAZ-GIMENEZ, CINTHIA RAGONE-FIGUEROA, HERNAN MURIEL, GARY MAMON

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

Año: 2008

0035-8711

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 within looser groups or larger structures.  For a more statistical assessment of their reality, we identify CGs within mock galaxy catalogues constructed on top of the Millennium Simulation at $z=0$  with three different semi-analytic models (SAMs) of galaxy formation by citeauthor{Croton+06} (citeyear{Croton+06}, C06), citeauthor{Bower+06} (citeyear{Bower+06}, B06), and citeauthor{dLB07} (citeyear{dLB07}, DLB).  CGs are identified using the same two dimensional (2D) criteria as those   visually applied by cite{Hickson82}.  We identify several hundred of CGs in projected space, among which roughly half (52%, 53% and 30% for C06, B06 and DLB, respectively) constitute our redshift sample, where each CG contains at least 4 galaxies with velocities within $1000 , m km , s^{-1}$ from the group median velocity. Roughly a quarter of the CGs with 4 or more accordant-velocity members are physically dense (22%, 35% and 20% for C06, B06 and DLB, respectively).  This fraction rises to roughly 2/5 (37%, 51% and 30% for C06, B06 and DLB, respectively) for groups with mean surface magnitude  brighter than $mu_R < 24, m, mag,arcsec^{-2}$, as are 90% of the Hickson CGs (HCGs).  Between 2/5 and 2/3 of the mock CGs that have been resampled according to the completeness functions of the HCG are physically dense (58%, 65% and 42% for C06, B06 and DLB, respectively). We deduce that the number of physically dense quartets among the 100 HCGs is roughly between 22 and 35.Roughly 4/5ths of the CGs that  are produced by chance alignments of galaxies along the line of sight are from larger groups (the remaining are chance alignments extending beyond the larger group). In addition, roughly 40% of the CGs in our redshift samples have three or more physically close and possibly interacting galaxies.Among the different observational properties of CGs, the strongest correlations are 1) the anti-correlation of CG virial-theorem-mass-to-light ratio and crossing time, 2) the larger velocity dispersion for groups of higher surface brightness and distance.  Moreover, contrary to a previous report, we find no trend for the higher velocity dispersion HCGs to appear rounder in projection, and this is confirmed with our mock CGs.  However, physically dense CGs tend to have surface brightness, and lower projected radii, velocity dispersion, crossing time and mass to light ratio (and $M/L imes t_{ m cr}^{-1.3}$).