Photochemical evolution of elliptical galaxies - I. The high-redshift formation scenario

In this paper we compute new multizone photochemical evolution models for elliptical galaxies, taking into account detailed nucleosynthetic yields, feedback from supernovae and an initial infall episode. By comparing model predictions with observations, we derive a picture of galaxy formation in which the higher the mass of the galaxy, the shorter the infall and the star formation time-scales. Therefore, in this scenario, the most massive objects are older than the less massive ones, in the sense that larger galaxies stop forming stars at earlier times. Each galaxy is created outside-in, i.e. the outermost regions accrete gas, form stars and develop a galactic wind very quickly, compared with the central core in which the star formation can last up to similar to1.3 Gyr. In particular, we suggest that both the duration of the star formation and the infall time-scale decrease with galactic radius. In order to convert theoretical predictions into line-strength indices, different calibrations are adopted and discussed, focusing in particular on their dependence on the alpha enhancement. By means of our model, we are able to match the observed mass-metallicity and colour-magnitude relations for the centre of the galaxies as well as to reproduce the overabundance of Mg relative to Fe, observed in the nuclei of bright ellipticals, and its increase with galactic mass. Furthermore, we find that the observed Ca underabundance relative to Mg can be real, owing to the non-negligible contribution of type Ia SNe to the production of this element. We predict metallicity and colour gradients inside the galaxies that are in good agreement with the mean value of the observed ones. Finally, we conclude that models with a Salpeter initial mass function (IMF) are the best ones in reproducing the majority of the properties of ellipticals, although a slightly flatter IMF seems to be required in order to explain the colours of the most massive galaxies.