Galaxy clustering and the dark-matter problem

Abstract

Recent observational and theoretical results on galaxy clustering are reviewed. A major difficulty in relating observations to theory is that the former refer to luminous material whereas the latter is most directly concerned with the gravitationally dominant but invisible dark matter. The simple assumption that the distribution of galaxies generally follows that of the mass appears to conflict with evidence suggesting that galaxies of different kinds are clustered in different ways. If galaxies are indeed biased tracers of the mass, then dynamical estimates of the mean cosmic density, which give Ω « 0.2 may underestimate the global value of Ω. There are now several specific models for the behaviour of density fluctuations from very early times to the present epoch. The late phases of this evolution need to be followed by N -body techniques; simulations of scale-free universes and of universes dominated by various types of elementary particles are discussed. In the former case, the models evolve in a self-similar way; the resulting correlations have a steeper slope than that oberved for the galaxy distribution unless the primordial power spectral index n « 2. Universes dominated by light neutrinos acquire a large coherence length at early times. As a result, an early filamentary phase develops into a present day distribution that is more strongly clustered than observed galaxies and is dominated by a few clumps with masses larger than those of any known object. If the dark matter consists of ‘cold’ particles such as photinos or axions, then structure builds up from subgalactic scales in a roughly hierarchical way. The observed pattern of galaxy clustering can be reproduced if either Ω « 0.2 and the galaxies are distributed as the mass, or if Ω — 1, H 0 = 50 km s -1 Mpc -1 and the galaxies form only at high peaks of the smoothed linear density field. The open model, however, is marginally ruled out by the observed small-scale isotropy of the microwave background, whereas the flat one is consistent with such observations. With no further free parameters a flat cold dark-matter universe produces the correct abundance of rich galaxy clusters and of galactic halos; the latter have flat rotation curves with amplitudes spanning the observed range. Preliminary calculations indicate that the properties of voids may be consistent with the data, but the correlations of rich clusters appear to be somewhat weaker than those reported for Abell clusters.