THE FORMATION OF DISC GALAXIES IN A COSMOLOGICAL CONTEXT - STRUCTURE AND KINEMATICS

We present results concerning the internal structure and kinematics of disc galaxies formed in cosmologically motivated simulations. The calculations include dark matter, gas dynamics, radiative cooling, star formation, supernova feedback and metal enrichment. The initial model is an isolated, rigidly rotating, overdense sphere with a mass of about 8 x 10(11) M. which is perturbed by small-scale fluctuations according to a biased cold dark matter power spectrum. Converging, Jeans unstable and rapidly cooling regions are allowed to form stars. Via supernovae, metal-enriched gas is returned to the interstellar medium. From these initial conditions a galaxy forms which shows the main properties of spiral galaxies: a rotationally supported exponential disc which consists of young stars with approximately solar metallicity, a slowly rotating halo of old metal-poor stars, a bulge of old metal-rich stars and a slowly rotating extended halo of dark matter. The bulge, and the stellar and dark haloes are supported by an anisotropic velocity dispersion and have a de Vaucouleurs surface density profile. The flattening of the dark and stellar haloes is too large to be explained by rotation only. Whether the flattening of the bulge is caused by an anisotropic velocity dispersion or by its rotation cannot be answered, because of the limited numerical resolution due to gravitational softening. Considering only the young stellar component, the disc is cold (sigma = 20 km s(-1)) and thin (z < 1 kpc). The dynamical formation process ends after about 4 Gyr, when the disc reaches a quasi-stationary state. During the subsequent 8 Gyr gas is mainly transformed into stars, decreasing the gas fraction from 40 to 10 per cent. The star formation rate decreases during the quasi-stationary state from about 5 M. yr(-1) at z = 1 to less than 0.5 M. yr(-1) at the end of the simulation (z = 0).