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

We analyze the chemical properties of a set of solar vicinity stars, and show that the small dispersion in abundances of α-elements at all ages provides evidence that the star formation history (SFH) has been uniform throughout the thick disk. In the context of long-timescale 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 SFH due to other processes governing the interstellar medium in the early disk, suggesting that infall cannot be a determining parameter of the chemical evolution at these epochs. We argue 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 either 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. The progressive, continuous process usually invoked may not have actually existed in the Milky Way