Galactic astroarchaeology: Reconstructing the bulge history by means of the newest data

Abstract

Context. The chemical abundances measured in stars of the Galactic bulge offer an unique opportunity to test galaxy formation models, as well as to impose strong constraints on the history of star formation and stellar nucleosynthesis. Aims. The aims of this paper are to compare abundance predictions from a detailed chemical evolution model for the bulge with the newest data. Some of the predictions have already appeared in previous papers (O, Mg, Si, S, and Ca), but some other predictions are new (Ba, Cr, and Ti). Methods. We computed several chemical evolution models by adopting different initial mass functions for the Galactic bulge and then compared the results to new data including both giants and dwarf stars in the bulge. In this way we can impose strong constraints on the star formation history of the bulge. Results. We find that to reproduce the metallicity distribution function best, one should assume a flat IMF for the bulge that is not steeper than the Salpeter one. The initial mass function derived for the solar vicinity instead provides a very poor fit to the data. The [el/Fe] vs. [Fe/H] relations in the bulge are reproduced well by a very intense star formation rate and a flat IMF, as in the case of the stellar metallicity distribution. Our model predicts that the bulge formed very quickly with the majority of stars formed inside the first 0.5 Gyr. Conclusions. Our results strongly suggest that the new data and, in particular, the MDF of the bulge confirm what have been concluded before, that the bulge formed very fast from gas shed by the halo, and that the initial mass function was flatter than in the solar vicinity and in the disk, although not as flat as previously thought. Finally, our model can also reproduce the decrease in the [O/Mg] ratio for [Mg/H] > 0 in the bulge, which is confirmed by the new data and interpreted as the result of mass loss in massive stars. © 2010 ESO.