Galaxy Formation and Evolution: What to Expect from Hierarchical Clustering Models

Abstract

In hierarchical clustering theories of galaxy formation, galaxies form by gas cooling and condensing into dark matter halos which, in turn, form by a hierarchy of mergers (White & Rees 1978). The context in which this process takes place is specified by a cosmological model that determines the spectrum of primordial density fluctuations and the rate at which they grow by gravitational instability. The best known example of such a model is the cold dark matter (CDM) model (see Frenk 1991 for a review), but a number of alternatives (mostly variants of CDM), have recently become popular in response to new data on large-scale structure and the COBE detection of anisotropies in the microwave background radiation. Regardless of the specific cosmological model that one wishes to consider, there are at least six distinct physical processes that need to be included in any theory of galaxy formation:

• - 1. The growth of dark matter halos by accretion and mergers.

• - 2. The dynamics of cooling gas.

• - 3. Star formation.

• - 4. Energy feedback into prestellar gas from the products of stellar evolution.

• - 5. Evolution of the stellar populations that form.

• - 6. Galaxy mergers.