The nature of the nearest compact group of galaxies from precise distance measurements

Context: Compact groups (CGs) of galaxies, similar to those catalogued by Hickson, appear to be the densest galaxy structures in the Universe. Redshift information is insufficient to determine whether a CG is roughly as dense in three dimensions as it appears in projection, or whether it is caused by a chance alignment along the line of sight within a larger galaxy system.
Aims: Recent precise distance measurements help probe the nature of the nearest CG, situated in the Virgo cluster, whose dominant member is M 60.
Methods: The isolated status of the CG is reassessed with recent photometry and a statistical analysis is performed on the surface brightness fluctuation (SBF) distances measured by Mei et al. in Virgo, for 4 of the 5 CG members.
Results: The neighboring galaxy NGC 4606 appears (with 80-90% confidence) to be too faint to affect the isolated status of the CG. Taken at face value, the SBF distances suggest that M 59 and NGC 4660 lie roughly 2 Mpc to the foreground of M 60, NGC 4638, and the bulk of the Virgo cluster. The statistical analysis confirms that the CG is, indeed, the result of a chance alignment of its galaxies, with NGC 4638 lying at least 800 kpc further away (with 99% confidence) than either M 59 or NGC 4660. The first two galaxy distances are consistent with M 59 and NGC 4660 constituting a tight pair. The dominant galaxy, M 60, is at least 440 kpc more distant (95% confidence) than the M 59+NGC 4660 pair, and over 1 Mpc (99% confidence) more distant if one uses the broken linear calibration of the SBF distances.
Conclusions: This work constitutes the first direct analysis of the nature of a compact group of galaxies. Chance alignments of galaxies represent a realistic alternative to truly dense groups to explain the nature of CGs. With current SBF distance accuracies, one could determine the nature of HCG 68 in the same way.