Galaxy formation by gravitational collapse

Galaxies may form by the collapse of density perturbations in a universe dominated by dark matter. In this scenario, the growth of density and velocity distributions around collapsed mass peaks is investigated, taking into account the random velocities imparted by substructure. Structures resembling galactic halos arise naturally from density perturbations in the cold dark matter scenario. When the halo structures are adiabatically compressed by the dissipating baryonic component, the resulting rotation curves are flat, over the full range of amplitudes studied here. In all cases the baryons are dynamically important in the centers of the structures, but in no case do they dominate. A peak of amplitude somewhat less than three sigma on the scale 10 to the ninth solar masses produces a structure with a rotation velocity comparable to that of the Galaxy; it is argued that peaks of about this amplitude and smaller should account for most of the mass in bound structures.