Disc galaxy evolution models in a hierarchical formation scenario: structure and dynamics

We predict the internal structure and dynamics of present-day disc galaxies using galaxy evolution models within a hierarchical formation scenario. Th e halo mass aggregation histories, for a flat cold dark matter model with c osmological constant, were generated and used to calculate the virializatio n of dark matter haloes. A diversity of halo density profiles were obtained , the most typical one being close to that suggested by Navarro, Frenk & Wh ite. We modelled the way in which discs in centrifugal equilibrium are buil t within the evolving dark haloes, using gas accretion rates proportional t o the halo mass aggregation rates, and assuming detailed angular momentum c onservation. We calculated the gravitational interactions between halo and disc - including the adiabatic contraction of the halo due to disc formatio n - and the hydrodynamics, star formation and evolution of the galaxy discs . We find that the slope and zero-point of the Tully-Fisher (TF) relation i n the infrared bands may be explained as a direct consequence of the cosmol ogical initial conditions. This relation is almost independent of the assum ed disc mass fraction, when the disc component in the rotation curve decomp osition is non-negligible. Thus, the power spectrum of fluctuations can be normalized at galaxy scales through the TF relation independently of the di sc mass fraction assumed. The rms scatter of the model TF relation originat es mainly from the scatter in the dark halo structure and, to a minor exten sion, from the dispersion of the primordial spin parameter lambda. The scat ter obtained from our models does not disagree with the observational estim ates. Our models allow us to understand why the residuals of the TF relatio n do not correlate significantly with disc size or surface brightness. We c an also explain why low and high surface brightness galaxies have the same TF relation; the key point is the dependence of the star formation efficien cy on the disc surface density. The correlations between gas fraction and s urface brightness, and between scalelength and V-max obtained with our mode ls agree with those observed. Discs formed within the growing haloes, where lambda is assumed to be time independent, have nearly exponential surface density distributions. The shape of the rotation curves changes with disc s urface brightness and is nearly flat for most cases. The rotation curve dec ompositions show a dominance of dark matter down to very small radii, in co nflict with some observational inferences. The introduction of shallow core s in the dark halo attenuates this difficulty and produces haloes with slig htly smaller rotation velocities. Other features of our galaxy models are n ot strongly influenced by the shallow core.