Formation of galaxies in ?cold dark matter cosmologies - I. The fine structure of disc galaxies

In this work we present a detailed analysis of the global and fine structure of four middle-mass disc galaxies obtained from hydrodynamic simulations in a ?cold dark matter (?CDM) scenario. These objects have photometric disc-to-total ratios in good agreement with those observed for late-type spirals, as well as kinematic properties in agreement with the observational I-band TullyFisher relation. We identify the different dynamical components at redshift zero on the basis of both orbital parameters and the binding energy of stars in the galaxy. In this way, we recognize a slowly rotating centrally concentrated spheroid, and two disc components supported by rotation: a thin disc with stars in nearly circular orbits, and a thick disc with orbital parameters transitional between the thin disc and the spheroid. The spheroidal component is composed mainly by old, metal-poor and a-enhanced stars. The distribution of metals in this component shows, however, a clear bimodality with a low-metallicity peak, which could be related to a classical bulge formed from rapid collapse at early times, and a high-metallicity peak, which could be related to a pseudo-bulge formed from instabilities of the inner disc. The thin disc appears in our simulations as the youngest and most metal-rich component, with median stellar ages ranging from 3.8 to 6.7 Gyr. The radial distribution of ages and colours in this component is U-shaped: the new stars are forming in the inner regions, where the galaxy is bluer, and then migrate through secular processes reaching the outer parts. Finally, we also find in all simulated galaxies a thick disc containing about 16 per cent of the total stellar mass and with properties that are intermediate between those of the thin disc and the spheroid. Its low-metallicity stars are a-enhanced when compared to thin disc stars of the same metallicity. The structural parameters (e.g. the scale height) of the simulated thick discs suggest that such a component could result from the combination of different thickening mechanisms that include merger-driven processes, but also long-lived internal perturbations of the thin disc.