The Formation of Disk-Bulge-Halo Systems and the Origin of the Hubble Sequence

Abstract

We investigate the formation of disk-bulge-halo systems by including bulges in the Fall & Efstathiou theory of disk formation. This allows an investigation of bulge dominated disk galaxies, such as S0s and disky ellipticals. The aim of this paper is to examine whether spirals, S0s, and disky elllipticals all can be incorporated in one simple galaxy formation scenario. We investigate an inside-out formation scenario in which the inner, low angular momentum material is assumed to form the bulge. Stability arguments are used to suggest that this bulge formation is a self-regulating process in which the bulge grows until it is massive enough to allow the remaining gas to form a stable disk component. We search for the parameters and physical processes that determine the disk-to-bulge ratio, and therewith explain to a large extent the origin of the Hubble sequence. The spread in halo angular momenta coupled with a spread in the formation redshifts can explain the observed spread in disk properties and disk-to-bulge ratios from spirals to S0s. Disky ellipticals, however, are too compact to be incorporated. Upon introducing a galaxy formation efficiency that depends on redshift, as suggested by the small amount of scatter in the observed Tully-Fisher relation, this scenario naturally predicts a density-morphology relation, the amplitude of which depends on the baryon fraction of the Universe.