Compact groups in theory and practice - I. The spatial properties of compact groups

We use a mock galaxy catalogue based on the Millennium Run simulation to investigate the intrinsic spatial properties of compact groups of galaxies. We find that approximately 30 per cent of galaxy associations identified in our mock catalogue are physically dense systems of four or more galaxies with no interlopers, approximately half are close associations of two, three or four galaxies with one or more interlopers, and the remainder are not physically dense (projections of looser groups and physically unassociated galaxies). Thus, the effect of interloping galaxies is significant. However, we find that genuine compact groups are preferentially brighter and more isolated than those with interlopers; by increasing the required minimum surface brightness of a group from the canonical value of mu(e) = 26 to 22 mag arcsec(-2), we can increase the proportion of genuinely compact systems identified with no interlopers from 29 to 75 per cent. Of the genuine compact groups identified, more than half consist of a single dark matter halo with all the member galaxies deeply embedded within it. In some cases, there are other galaxies which share the same halo (typically with mass similar to 10(13) h(-1)M circle dot) but which are not identified as being members of the compact group. This implies that compact groups are associated with group environments, some or all members of which are in the compact group. For those compact groups where all galaxies are in the same halo, the three-dimensional velocity dispersion of the compact group correlates broadly with the virial velocity of the dark matter halo. However, the scale-size of the group - and hence the fraction of the halo mass which the group samples - is completely uncorrelated with the properties of the dark matter halo. This means that masses derived under the simple assumption of virial equilibrium using the observed velocity dispersions and sizes of compact groups give incorrect estimates of the true mass of the underlying dark matter.