Properties and evolution of disk galaxies in a hierarchical formation scenario

We highlight some results from disk galaxy evolution models conceived within a cosmological context. When disk mergers and strong disk-halo feedback are omitted, several properties and correlations of disk galaxies seem to be related to initial conditions given by the CDM model. •Motivation. The inflation-inspired cold dark matter (CDM) hierarchical model has provided an invaluable theoretical framework for studies on galaxy formation and evolution. Nevertheless, several aspects of this phenomenon can not be treated as a simple (deductive) extrapolation of the hierarchical scenario, as is commonly done in some approaches. We have developed an approach where an inductive (backward) disk galaxy evolution model is combined with initial and boundary conditions calculated from the hierarchical scenario. Our main goal is to find the relevant initial factors and physical ingredients that determine the main local and global properties, correlations and evolutionary features of disk galaxies. In our approach (Avila-Reese et al. 1998; Avila-Reese & Firmani 2000a; Firmani & Avila-Reese 2000), disks form inside-out within growing CDM halos with a gentle gas accretion rate (no mergers) proportional to the hierarchical mass aggregation rate. We follow locally the overall evolution of individual disks in centrifugal equilibrium, including self-regulated star formation (SF) by feedback within the disk ISM (no disk-halo feedback) and population synthesis. Bulges are assumed to form via a secular mechanism. At the same time, as in the semianalytical approach, we are able to predict correlations and statistical properties of the disk galaxy population. • The disk Hubble sequence. We find that the main properties and correlations of disk galaxy models are mainly determined by the cosmological conditions. For a given mass, the dark halo concentration, the galaxy color index and the disk gas fraction are basically related to the mass aggregation history (MAH), while the disk surface brightness (SB), the bulge-to-disk ratio (b/d), and the shape of the rotation curve depend mainly on the spin parameter λ. The mass does not influence intensive properties. Thus, according to our models, the Hubble sequence is biparametrical, the MAH and λ being the two driving physical factors. The observational trends across the Hubble sequence are indeed reproduced: the redder and more concentrated is the disk, the smaller is the gas fraction and the larger is the b/d ratio. The disk mass fraction fd also influences the models; we constrict its value to 0.03 < fd < 0.08, otherwise the Also Osservatorio Astronomico di Brera, via E.Bianchi 46, I-23807 Merate, Italy