4.3. Drivers of the disk Hubble sequence

The main properties, correlations and evolution features of disk galaxies formed within CDM halos, under the assumption mentioned at the beginning of this section, arise from the combination of three cosmological factors and their statistical distributions: the virial mass, the MAH, and the spin parameter . The former determines the scale. The latter ones determine the SF history and the intensive properties, and are at the basis of the disk HS correlations, suggesting a biparametrical nature for this sequence (Avila-Reese & Firmani 2000). Observations tend to confirm this biparametrical nature (e.g, de Blok & McGaugh 1997). The MAH drives the gas infall rate, which determines mainly the galaxy color indexes. The parameter determines mainly the disk SB, and strongly influences the rotation curve shape and the bulge-to-disk ratio (secular bulge). The mass galaxy fraction, f_gal, which influences some disk properties, is an astrophysical parameter determined mainly by the halo gas dissipation and feedback efficiencies; it may vary from galaxy to galaxy, but its value on average is around 0.03 (see Section 3). The shapes of the rotation curves and the SB distribution are better reproduced jointly just for this value (Avila-Reese et al. 2002).

An important ingredient, which begins now to be included in the CDM models of disk evolution, is the dynamics related to bars, spiral arms and minor mergers (e.g., see the contributions by Valenzuela, Athanassoula, Curir, Bertschik and others in this volume). This ingredient, highly connected to the main disk properties and processes described above, is at the basis of the "showy" disk morphological signatures, which characterize the tuning fork of the disk HS, and may play some role in the morphological transformation due to secular processes.