Nearby Small Groups of Galaxies

Abstract

To date, our view of the universe has largely been two-dimensional. Velocity data, the basis for a look in the third dimension, have been too incomplete and uneven in quality to provide a clear picture. Nonetheless, the pioneering work by de Vaucouleurs (1975) has given us a rough idea of what the universe is like locally. At least a good fraction of galaxies are improbably close to their nearest neighbours compared with expectations based on statistical fluctuations of a random distribution. Our vocabulary to describe these associations includes the words: binary, group, cloud, cluster and supercluster. Does the real universe indeed have characteristic scales that make these terms meaningful? Or, as Peebles and his co-workers (Davis, Groth & Peebles 1977, and reference therein) would have us believe, is there structure on all scales, at least up to about 15 Mpc? And associated galaxies aside, are there galaxies truly randomly distributed: are there field galaxies? Looking two-dimensionally, it has been possible to arrive at remarkably different conclusions. Turner & Gott (1975) concluded that roughly 40% of all galaxies are randomly distributed while Soneira & Peebles (1977) set an upper limit of 18%. It was roughly this latter figure that de Vaucouleurs (1975) derived with his early look into the third dimension.