The formation of disk - bulge - halo systems and the origin of the Hubble sequence

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. These latter systems, which consist of an elliptical spheroid with an embedded disk with a scale-length of typically a few hundred parsecs, seem to form a smooth sequence with spirals and S0s towards lower disk-to-bulge ratio. 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 subsequent layers of gas cool and form stars inside a virialized dark halo. 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 assume that the baryons that build the disk do not loose their specific angular momentum, and 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. If galaxy formation is efficient, and all available baryons are transformed into the disk-bulge system, cosmologies with Ω0 ∼< 0.3 can be excluded, since stable spiral disks would not be allowed to form. However, if we assume that the efficiency with which galaxies form depends on the formation redshift, as suggested by the small amount of scatter in the observed Tully-Fisher relation, and we assume that the probability for a certain baryon to ultimately end up in the disk or bulge is independent of its specific angular momentum, spirals are allowed to form, but only at small formation redshifts (z ∼< 1). At higher formation redshifts, stability arguments suggest the formation of systems with smaller disk-to-bulge ratios, such as S0s. Since density perturbations in clusters will generally collapse earlier than those in the field, this scenario naturally predicts a density-morphology relation, the amplitude of which depends on the baryon fraction of the Universe. Disky ellipticals are too compact to be incorporated in this scenario, and they thus do not form a continuous sequence with spirals and S0s, at least not in the sense of the galaxy formation scenario envisioned in this paper. Alternative formation scenarios for the disky ellipticals, such as gas-rich mergers or an internal mass loss origin for the embedded disks, are much more viable. Subject headings: galaxies: formation — galaxies: general — galaxies: halos — instabilities