Galaxy Formation in Hierarchical Models

Abstract

High resolution gravity plus smoothed particle hydrodynamics simulations are used to study the formation of galaxies within the context of hierarchical structure formation. The simulations have sufficient dynamic range to resolve from ten kpc scale galactic disks up to many Mpc scale filaments. Over this range of scales, we find that hierarchical structure development proceeds through a series of increasingly larger filamentary collapses. The well resolved simulated galaxies contain hundreds to thousands of particles and have varied morphologies covering the entire expected range from disks to tidally distorted objects. The epoch of galaxy formation occurs early, about redshift 2.5 for 10^12 M_sun galaxies. Hierarchical formation naturally produces correlations among the mass, age, morphology, and local density of galaxies which match the trends in the observed morphology--density relation. We also describe a method of spiral galaxy formation in which galactic disks form through the discrete accretion of gas clouds which transport most of the angular momentum to the inner regions. Such a process is characteristic of the somewhat chaotic nature of hierarchical structure formation where simple analytical ideas of spherical collapse appear incongruous.