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The nature of compact groups (CGs) of galaxies, apparently so dense that thegalaxies often overlap, is still a subject of debate: Are CGs roughly as densein 3D as they appear in projection? Or are they caused by chance alignments ofgalaxies along the line-of-sight, within larger virialized groups or evenlonger filamentary structures? The nature of CGs is re-appraised using the z=0outputs of three galaxy formation models, applied to the dissipationlessMillennium Simulation. The same selection criteria are applied to mock galaxycatalogs from these models as have been applied by Hickson and co-workers inredshift space. We find 20 times as many mock CGs as the `HCGs' found byHickson within a distance corresponding to 9000 km/s. This very low (5%) HCGcompleteness is caused by Hickson missing groups that were either faint, nearthe surface brightness threshold, of small angular size, or with a dominantbrightest galaxy. We find that most velocity-filtered CGs are physically dense,regardless of the precise threshold used in 3D group size and line-of-sightelongation, and of the galaxy formation model used. This result also holds formock CGs with the same selection biases as was found for the HCGs.

The nature of compact groups of galaxies from cosmological simulations