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

TL;DR

This paper compares chemical evolution models with new stellar abundance data to understand the formation history of the Galactic bulge, concluding it formed rapidly with a flat initial mass function and specific nucleosynthesis characteristics.

Contribution

It introduces new predictions for Ba, Cr, and Ti abundances and constrains the bulge's star formation history and initial mass function using the latest observational data.

Findings

Bulge formed rapidly within the first 0.5 Gyr.

A flat IMF better fits the bulge data than the solar vicinity IMF.

The model reproduces the decrease of [O/Mg] at high metallicity.

Abstract

The chemical abundances measured in stars of the Galactic bulge offer an unique opportunity to test galaxy formation models as well as impose strong constraints on the history of star formation and stellar nucleosynthesis. 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 on previous paper (O, Mg, Si, S and Ca) but some other predictions are new (Ba, Cr and Ti). We compute several chemical evolution models by adopting different initial mass functions for the Galactic bulge and then compare 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. We find that in order to reproduce at best the metallicity distribution function one should assume a flat IMF for the bulge not steeper than the Salpeter one. The initial mass function derived for the solar vicinity provides instead a very poor fit to the data. The [el/Fe] vs. [Fe/H] relations in the bulge are well reproduced 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. Our results strongly suggest that the new data, and in particular the MDF of the bulge, confirm what concluded before and in particular 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 so flat as previously thought. Finally, our model can also reproduce the decrease of the [O/Mg] ratio for [Mg/H] > 0 in the bulge, which is confirmed by the new data and interpreted as due to mass loss in massive stars.