Star Formation in Cosmological Simulations of Spiral Galaxies

Observational constraints on spiral galaxies were used to choose dark haloes from a dissipationless N-body simulation of a standard CDM model. High resolution initial conditions were produced by applying a nested hierarchy algorithm to individual dark haloes which have circular speeds comparable to those observed and which are isolated from other large, condensing perturbations. We show that the angular momentum catastrophe can be avoided if feedback processes prevent gas from collapsing until late epochs when haloes are relatively smooth and evolve slowly. Further results are presented for a cosmological, gas-dynamical simulation of the formation of an individual galaxy which led to a distribution of matter similar to observed sprial galaxies. For this model, in which gas collapsed at late times, producing an angular momentum comparable to real spirals, the resolution is increased by a factor of two. The spatial and time dependences of the formation of stars and depletion of gas are analyzed. The distributions of stellar ages show that older, redder stellar bulge/halo/disk components form by z=0.8, whereas a younger, bluer component of the disk and bulge appears as gas in the disk cools and continues to form stars. Both the face-on and edge-on evolution of the stellar and gas disks are presented. These results account for the observed ages of older stars in the bulge, halo, and disk and for the younger stars in spiral disks.