Clustering of Galaxies in a Hierarchical Universe: II. Evolution to High Redshift

Abstract

In hierarchical cosmologies the evolution of galaxy clustering depends both on cosmological quantities such as Omega and Lambda, which determine how dark matter halos form and evolve, and on the physical processes - cooling, star formation and feedback - which drive the formation of galaxies within these merging halos. In this paper, we combine dissipationless cosmological N-body simulations and semi-analytic models of galaxy formation in order to study how these two aspects interact. We focus on the differences in clustering predicted for galaxies of differing luminosity, colour, morphology and star formation rate and on what these differences can teach us about the galaxy formation process. We show that a "dip" in the amplitude of galaxy correlations between z=0 and z=1 can be an important diagnostic. Such a dip occurs in low-density CDM models because structure forms early and dark matter halos of 10**12 solar masses, containing galaxies with luminosities around L*, are unbiased tracers of the dark matter over this redshift range; their clustering amplitude then evolves similarly to that of the dark matter. At higher redshifts bright galaxies become strongly biased and the clustering amplitude increases again. In high density models, structure forms late and bias evolves more rapidly. As a result, the clustering amplitude of L* galaxies remains constant from z=0 to 1. The strength of these effects is sensitive to sample selection. The dip is weaker for galaxies with lower star formation rates, redder colours, higher luminosities and earlier morphological types. We explain why this is the case and how it is related to the variation with redshift of the abundance and and environment of the observed galaxies. Studies of clustering evolution as a function of galaxy properties should place strong constraints on models.