Annu. Rev. Astron. Astrophys. 1993. 31: 689-716 Copyright © 1993 by Annual Reviews. All rights reserved

3.5. Low Redshift Phenomena

The accumulating evidence from galaxy counts (cf Guhathakurta et al 1990, and references therein) is fascinating, but at this point too difficult to interpret (with, for example, the blue and infra-red N(M) relations significantly discordant) to shed any light on the structure formation issue. As redshifts are measured for a larger fraction of the faint sample, this may provide an important discriminant among theoretical models. The clearly apparent, relatively rapid evolution seen in the blue counts appears to be occurring at redshifts z 0.5. It may indicate changing rates of merging, an alteration in the IGM which promotes galaxy formation at this epoch (Babul & Rees 1992), or a myriad of other possibilities.

The merging phenomenon itself is an important testing ground for CDM models. In an = 1 universe, merging is continuous, since all objects are bound to all other objects. It was noted in the early N-body work that, to a first approximation, the simulations looked self-similar; aside from questions of scale, the universe at one epoch looks very much as it does at a later one with smaller objects steadily collecting into larger ones and with concomitant growth in the sizes of voids.

What is the observational evidence on this issue? While, unfortunately, the subject has been mired in polemics, certain facts are clear. There is direct local evidence for mergers among bright galaxies, as the early work of the Toomres (1972) and many recent studies (cf Schweizer 1987) have indicated. Both relatively isolated galaxies and cluster galaxies (Hausman & Ostriker 1978) show signs of cannibalism. But is this a phenomenon which, at low redshifts, affects a small fraction of galaxies in a major way and a large fraction in a minor way? Or is it of major significance for most galaxies in the redshift interval 0 < z < 0.5? Events at higher redshift are less relevant. On the one hand, observational traces of such events are obscured by time; on the other hand, all theories envision galaxies originating by an accumulation of subunits.

Two recent studies have come to somewhat different conclusions on this matter. Richstone et al (1992), examining the substructure seen in clusters of galaxies, argued for a large and a good deal of recent merging, while Toth & Ostriker (1992), looking at the thinness and coldness of normal spiral discs, argued for little recent merging on average. The latter study set a limit of < 4% in the last 5 billion years, which is apparently inconsistent with the standard CDM models.

This is a critical area for further research, as it directly tests for in dense lumps, so both observational and theoretical studies on a careful quantitative level would be well rewarded.