Conclusions

We have started a project to study the properties of the neutral hydrogen for a new sample of compact groups of galaxies. Parkes observations of a first sub-sample confirm earlier observations that a significant fraction of compact groups are poor in HI, suggesting that at least a third of the groups that appear compact on the sky, are in fact true compact systems. ATCA observations of some of these groups suggest that a low HI content of a compact group does indeed indicate that this group is a compact system, possibly in an advanced stage of evolution. However, the ATCA observations also show that HI-rich groups exist that are likely to be true compact systems. We conclude that a significant fraction of the groups that are selected as compact groups are genuine compact systems. This high number of true compact groups provides an important constraint on the models for the formation of structure in the universe.

© Copyright Astronomical Society of Australia 1997