Dissertation for the degree of Doctor of Philosophy

A description and detailed tests of a numerical code for simulations of galaxy formation is presented. The dynamics is followed with fully adaptive and Lagrangian Smooth Particle Hydro-dynamics (SPH) combined with a gravitational tree method. The severe numerical problem of sub-resolution clustering is solved by merging of gas particles. The code is used for Hydrodynamical simulations of the formation of galaxies in the mass range 10 9 ? 10 13 M. The effect of an ambient UV radiation field, and the effect of metal enrichment of halo gas due to star formation and stellar evolution, are investigated. The UV radiation field is found to strongly affect galaxies with circular velocities less than 50 km/s. The effect is, however, not large enough to reconcile hierarchical clustering models with observations. I present numerical simulations of galaxy formation in a CDM scenario, including star formation , using the SPH and tree method on scales ranging from 20 Mpc down to less than 1 kpc. The gas component is treated as a two-phase medium, governed by thermal instability and supernova feedback. For a certain degree of supernova evaporation, several general propert ies of elliptical galaxies are reproduced, like shapes, r 1=4-profiles, half-light radii, slow rotation and anisotropic velocity dispersions. A Faber-Jackson-like relation is observed, being of the form M 2:6. No disk shaped objects form. The elliptical objects form through a hierarchical, star-forming " inside-out " collapse. The SFR has a rapidly raising and later exponentially decaying behaviour in individual objects. The stellar contribution to the closure density is around one percent. The co-moving average SFR shows an increasing behaviour with redshift. Low mass objects are strongly suppressed. Generally, the more massive objects are in place at z 0:5 ? 2. Although many problems and questions still remain, the overall impression is that surprisingly many properties of ellipticals are reproduced.