© CAMBRIDGE UNIVERSITY PRESS 1999

1.4.6 Cluster Morphology and Evolution

Cluster Morphology. Richstone, Loeb, and Turner (1992) showed that clusters are expected to be evolved - i.e. rather spherical and featureless - in low- cosmologies, in which structures form at relatively high redshift, and that clusters should be more irregular in = 1 cosmologies, where they have formed relatively recently and are still undergoing significant merger activity. There are few known clusters that seem to be highly evolved and relaxed, and many that are irregular - some of which are obviously undergoing mergers now or have recently done so (see e.g. Burns et al. 1994). This disfavors low- models, but it remains to be seen just how low. Recent papers have addressed this. In one (Mohr et al. 1995) a total of 24 CDM simulations with = 1 or 0.2, the latter with = 0 or 0.8, were compared with data on a sample of 57 clusters. The conclusion was that clusters with the observed range of X-ray morphologies are very unlikely in the low- cosmologies. However, these simulations have been criticized because the ₀ = 0.2 ones included rather a large amount of ordinary matter: _b = 0.1. (This is unrealistic both because h 0.8 provides the best fit for ₀ = 0.2 CDM, but then the standard BBN upper limit is _b < 0.02h² = 0.03; and also because observed clusters have a gas fraction of ~ 0.15(h / 0.5)^-3/2.) Another study (Jing et al. 1995) using dissipationless simulations and not comparing directly to observational data found that CDM with ₀ = 0.3 and h = 0.75 produced clusters with some substructure, perhaps enough to be observationally acceptable (cf. Buote & Xu 1997). Clearly, this important issue deserves study with higher resolution hydrodynamic simulations, with a range of assumed _b, and possibly including at least some of the additional physics associated with the galaxies which must produce the metallicity observed in clusters, and perhaps some of the heat as well. Better statistics for comparing simulations to data may also be useful (Buote & Tsai 1996).

Cluster Evolution. There is evidence on the evolution of clusters at relatively low redshift, both in their X-ray properties (Henry et al. 1992, Castander et al. 1995, Ebeling et al. 1995) and in the properties of their galaxies. In particular, there is a strong increase in the fraction of blue galaxies with increasing redshift (the ``Butcher-Oemler effect''), which may be difficult to explain in a low-density universe (Kauffmann 1995). Field galaxies do not appear to show such strong evolution; indeed, a recent study concludes that over the redshift range 0.2 z 1.0 there is no significant evolution in the number density of ``normal'' galaxies (Steidel, Dickinson, & Persson 1994). This is compatible with the predictions of various models, including CHDM with two neutrinos sharing a total mass of about 5 eV (see below), but the dependence of the number of clusters on redshift can be a useful constraint on theories (Jing & Fang 1994, Bryan et al. 1994, Walter & Klypin 1996, Eke, Cole & Frenk 1996).