Clues to the formation of the Milky Way's thick disk

TL;DR

This study investigates the chemical properties of stars in the Milky Way's thick disk, suggesting a uniform star formation history and a pre-assembled gas disk, challenging inside-out formation models.

Contribution

It provides evidence that the thick disk's formation was likely a rapid, pre-assembled process with minimal inside-out growth, contrasting with traditional models.

Findings

Small dispersion in alpha-element abundances across ages.

Lack of radial metallicity gradient in the thick disk.

Evidence for a pre-assembled, closed-box like early disk.

Abstract

We analyse the chemical properties of a set of solar vicinity stars, and show that the small dispersion in abundances of \alpha-elements at all ages provides evidence that the SFH has been uniform throughout the thick disk. In the context of long time scale infall models, we suggest that this result points either to a limited dependence of the gas accretion on the Galactic radius in the inner disk (R<10 kpc), or to a decoupling of the accretion history and star formation history due to other processes governing the ISM in the early disk, suggesting that infall cannot be a determining parameter of the chemical evolution at these epochs. We argue however that these results and other recent observational constraints -- namely the lack of radial metallicity gradient and the non-evolving scale length of the thick disk -- are better explained if the early disk is viewed as a pre-assembled gaseous system, with most of the gas settled before significant star formation took place -- formally the equivalent of a closed-box model. In any case, these results point to a weak, or non-existent inside-out formation history in the thick disk, or in the first 3-5 Gyr of the formation of the Galaxy. We argue however that the growing importance of an external disk whose chemical properties are distinct from those of the inner disk would give the impression of an inside-out growth process when seen through snapshots at different epochs. However, the progressive, continuous process usually invoked may not have actually existed in the Milky Way.