Inside-Out Bulge Formation and the Origin of the Hubble Sequence

Abstract

A simple, inside-out formation scenario for bulges is presented. Stability arguments are used to suggest that bulges form out of the low-angular momentum baryons that cool inside a virialized dark halo: a self-regulating mechanism transforms part of the baryonic matter into a bulge component, such that the remainder of the baryons can settle in a stable disk component. We couple this scenario to the Fall & Efstathiou theory of disk formation and search for the parameters and physical processes that determine the disk-to-bulge ratio, and therefore explain to a large extent the origin of the Hubble sequence. This simple galaxy formation scenario can account for both spirals and S0s, but fails to incorporate more bulge dominated systems.

4 pages, 1 figure (LaTeX, cupconf.sty). To appear in the proceedings of the STScI workshop "When and How do Bulges Form and Evolve", eds. C. M. Carollo, H. C. Ferguson, and R. F. G. Wyse, Cambridge: CUP