The Cold Dark Matter (CDM) cosmological model, inspired by the inflationary theory, has triggered the development of a provocative deductive scenario of galaxy formation and evolution. According to this scenario, galaxies form within non-dissipative dark matter halos, which assemble hierarchically from primordial density fluctuations (White & Rees 1978). The deductive approach offers a valuable theoretical frame for interpreting the galaxy phenomena and motivates new confrontations between theory and observations. In parallel, from the observed properties of Milky Way (MW) and nearby galaxies, chemical and spectrophotometric evolutionary models allowed us to infer, by an inductive approach, fundamental aspects of galaxy evolution. More recently, the observation of galaxies at different redshifts is making possible to trace directly the luminous, structural and dynamical evolution of whole populations of galaxies.
The question of how the Hubble morphological sequence originated and evolved in its complex relation with the environment is one of the most exciting challenges of extragalactic astronomy. The Hubble sequence (HS) of galaxies is based on a morphological classification, but several physical properties also change along this sequence (Roberts & Haynes 1994), a relevant morphological indicator being the bulge-to-disk ratio (Hubble 1936; Simien & de Vaucouleurs 1986). Therefore, it is natural to concentrate our attention on the physical nature of disks and spheroids, and on how galaxy structures do emerge in the context of a unified cosmological scenario.
While the evolution of disks appears gradual and extended in time, spheroids (elliptical galaxies and bulges) seem to concentrate their star formation activity upon a violent, early phase of their evolution. According to hierarchical galaxy formation schemes, most of the stars originated in disks, while subsequent processes of dynamical nature transformed some of these disks into spheroids. Nowadays, the increasing information of the high-redshift universe is opening new perspectives for the understanding of the formation of spheroids, its connection with ultra luminous infrared galaxies (ULIGs), quasars (QSOs), active galactic nuclei (AGNs), as well as for the interplay of galaxy formation with the intergalactic medium.
In this review our attention will be focused on the formation and evolution of disks and spheroids in the hierarchical CDM scenario. Some recent reviews on observational and evolution aspects of the HS can be found in Roberts & Haynes (1994), Ellis (1999, 2001), Freeman (1999), van den Bergh (2002). Previous reviews on the physical origin of the HS can be found in Silk & Wyse (1993) and Pfenniger (1996).
The plan of this review is first to discuss the origin and properties of the galactic dark matter halos, as well as the gas infall and galaxy formation processes within them (Sections 2 and 3). Then, the status of models of disk and spheroid formation in the hierarchical scenario and their comparison with observations will be presented in Sections 4 and 5, respectively. In Section 6 a sketch of the unified phenomenon of galaxy formation in the cosmological context is presented, and in Section 7 the concluding remarks as well as some perspectives for the future are enumerated.