The inside-out formation of the Galactic bulge

Abstract

We developed a chemical evolution model that incorporates the mass distribution in the bulge and disc to determine the radial dependence of the time-scale in the Galaxy. The model was used to test a scenario in which the bulge forms inside-out. The model successfully reproduces several key features of the data: the spread in the [ α /Fe] versus [Fe/H] relation, the shape of the metallicity distribution function across different regions of the bulge, the [Fe/H] radial gradient within the bulge, the age-metallicity relationship and the evolution of [ α /Fe] with age. The findings suggest a scenario in which most of the bulge stars formed before the development of the boxy/peanut X-shaped bar structure. Consequently, the classical origin of the Galactic bulge cannot be excluded, and this scenario provides a plausible explanation for the observed chemical properties of the bulge.