New Metallicity Independent Constraints of Galactic Infall From Deuterium Observations

Abstract

Local deuterium observations have shown large variations over different lines of sight. However, it has been recently proposed that such variations can be explained by strong depletion of deuterium onto dust grains. Consequently, recent Far Ultraviolet Spectroscopic Explorer (FUSE) deuterium observations represent only a lower bound on the true local deuterium abundance which has thus been estimated to be as high as ∼ 85% of the primordial D abundance, as opposed to previous estimates of ∼ 55%. Such high local deuterium abundance could be explained with Galactic infall. Within our analytical model we demonstrate that such high local D abundance in fact requires a significant infall. Our constraint comes from the FUSE deuterium observations AND Galactic gas fraction estimates, which, when used in concert, demand infall rate comparable to the star-formation rate. Moreover, our analysis also constrains the fraction of stellar mass that is returned to the ISM to a range 0.1 < R <∼ 0.4 which is just marginally consistent with modern initial mass functions. Finally, the requirement of infall is broadly consistent with hierarchical structure formation. Thus, our results offer new qualitative and quantitative ways of placing Galactic evolution in the larger cosmological context.