On the Formation and Evolution of Disk Galaxies: Cosmological Initial Conditions and the Gravitational Collapse

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Abstract
We use a semianalytical approach and the standard σ8 = 1 cold dark matter (SCDM) cosmological model to study the gravitational collapse and virialization, the structure, and the global and statistical properties of isolated dark matter galactic halos that emerge from primordial Gaussian fluctuations. First, from the statistical properties of the primordial density fluctuation field, the possible mass aggregation histories (MAHs) are generated. Second, these histories are used as the initial conditions of the gravitational collapse. To calculate the structure of the virialized systems, we have generalized the secondary infall model to allow arbitrary MAHs and internal thermal motions. The average halo density profiles we obtained agree with the profile derived as a fitting formula to results of N-body cosmological simulations by Navarro, Frenk, & White. The comparison of the density profiles with the observational data is discussed, and some possible solutions to the disagreement found in the inner regions are proposed. The results of our approach, after considering the gravitational dragging of the baryon matter that forms a central disk in centrifugal equilibrium, show that the empirical Tully-Fisher (TF) relation and its scatter can be explained through the initial cosmological conditions, at least for the isolated systems. The σ8 = 1 SCDM model produces galaxies with high velocities when compared with observations, but when the SCDM power spectrum is normalized to σ8 = 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 relation is practically invariant with respect to the spin parameter λ.