Chemical evolution of local galaxies in a hierarchical model

We investigate the chemical properties of local galaxies within a cosmological framework in the hierarchical picture of galaxy formation. To this aim, we use a hierarchical semi-analytic model which includes the contribution from (i) low- and intermediate-mass stars, relevant producers of some important elements, such as C and N; (ii) Type Ia supernovae (SNe), for which a continuous delay-time distribution is assumed and which are important for the production of Fe and (iii) massive stars, dying as core-collapse Type II SNe and which produce the alpha elements. In this way, we can study abundances for a large set of chemical elements and in various galactic types, comparing our predictions with available observations in the Milky Way (MW), in local dwarf galaxies and in local ellipticals. For MW-like galaxies, we can successfully reproduce the major observational constraints, i.e. the [O-Fe] versus [Fe/H] relation observed in disc stars and the stellar metallicity distribution (SMD). For dwarf galaxies, the stellar metallicity versus mass relation is reproduced by assuming that a substantial fraction of the heavy elements is lost through metal-enhanced outflows and a Type Ia SN realization probability lower than the one of MW-like galaxies. The predicted abundance ratios for dwarf galaxies are comparable to observations derived locally for dwarf spheroidals. The SMDs predicted for dwarf galaxies are in agreement with the local observations. We predict a substantial presence of extremely low metallicity stars [Fe/H] < -2.5, which have been recently observed in ultrafaint dwarf galaxies. In ellipticals, the observations indicate higher [alpha/Fe] values in larger galaxies. Several previous attempts to model the [alpha/Fe] versus Sigma in ellipticals based on Lambda cold dark matter galaxy formation models predicted an anticorrelation between [alpha/Fe] versus Sigma, indicating too much extended star formation histories in high-mass galaxies. Our results computed with a standard Salpeter initial mass function (IMF) indicate a flat [alpha/Fe] versus Sigma relation. However, we suggest a possible solution to this problem and show how, by assuming a star-formation-dependent IMF with a slope x = 1.35 in systems with star formation rates < 100 M(circle dot) yr-1 and slightly flatter (i.e. with x = 1) in object with stronger star formation, the observed correlation between [alpha/Fe] and Sigma can be accounted for on a large velocity dispersion range. Fundamental roles are played also by interaction-triggered starbursts and active galactic nuclei feedback. Finally, a star-formation-dependent IMF seems necessary also to reproduce the stellar metallicity-Sigma relation observed in local early-type galaxies.