The Formation of the Hubble Sequence of Disk Galaxies: The Effects of Early Viscous Evolution

Abstract

We investigate a model of disk galaxies whereby viscous evolution of the gaseous disk drives material inwards to form a proto-bulge. We start from the standard picture of disk formation through the settling of gas into a dark halo potential well, with the disk initially coming into centrifugal equilibrium with detailed conservation of angular momentum. We derive generic analytic solutions for the disk-halo system after adiabatic compression of the dark halo, with free choice of the input virialized dark halo density profile and of the specific angular momentum distribution. We derive limits on the final density profile of the halo in the central regions. Subsequent viscous evolution of the disk is modelled by a variation of the specific angular momentum distribution of the disk, providing analytic solutions to the final disk structure. The assumption that the viscous evolution timescale and star formation timescale are similar leads to predictions of the properties of the stellar components. Focusing on small `exponential' bulges, ones that may be formed through a disk instability, we investigate the relationship between the assumed initial conditions, such as halo `formation', or assembly, redshift $z_f$, spin parameter $λ$, baryonic fraction $F$, and final disk properties such as global star formation timescale, gas fraction, and bulge-to-disk ratio. We find that the present properties of disks, such as the scale length, are compatible with a higher initial formation redshift if the re-distribution by viscous evolution is included than if it is ignored. We also quantify the dependence of final disk properties on the ratio $F/λ$, thus including the possibility that the baryonic fraction varies from galaxy to galaxy, as perhaps may be inferred from the observations.