On the formation and evolution of disk galaxies: Cosmological initial conditions and the gravitational collapse

We use a semianalytical approach and the standard sigma(8) = 1 cold dark ma tter (SCDM) cosmological model to study the gravitational collapse and viri alization, the structure, and the global and statistical properties of isol ated dark matter galactic halos that emerge from primordial Gaussian fluctu ations. First, from the statistical properties of the primordial density fl uctuation held, the possible mass aggregation histories (MAHs) are generate d. Second, these histories are used as the initial conditions of the gravit ational collapse. To calculate the structure of the virialized systems, we have generalized the secondary infall model to allow arbitrary MAHs and int ernal thermal motions. The average halo density profiles we obtained agree with the profile derived as a fitting formula to results of N-body cosmolog ical simulations by Navarro, Frenk, & White. The comparison of the density profiles with the observational data is discussed, and some possible soluti ons to the disagreement found in the inner regions are proposed. The results of our approach, after considering the gravitational dragging o f the baryon matter that forms a central disk in centrifugal equilibrium, s how that the empirical Tully-Fisher (TF) relation and its scatter can be ex plained through the initial cosmological conditions, at least for the isola ted systems. The sigma(8) = 1 SCDM model produces galaxies with high veloci ties when compared with observations, but when the SCDM power spectrum is n ormalized to sigma(8) = 0.57, an excellent agreement with the observable TF relation is found, suggesting that this relation is the natural extension to galactic scales of the observed galaxy distribution power spectrum. The theoretical TF scatter is close to the measured one. The slope of the TF re lation is practically invariant with respect to the spin parameter lambda.