The dawn is quiet here: Rise in [ $\alpha$ /Fe] is a signature of massive gas accretion that fueled the proto-Milky Way

Abstract The proto-Milky Way epoch forms the earliest stars in our galaxy and sets the initial conditions for the subsequent disk formation. Recent observations showed that the [ $\alpha$ /Fe] ratio among in situ metal-poor stars declined between [Fe/H] $=-3$ and $-1.3$ until it reached the lowest value ( $\sim$ 0.25) and rose to the traditional value associated with the high-[ $\alpha$ /Fe] thick disk ( $\sim$ 0.3) at [Fe/H] $\approx$ -1.0. It was suggested that the rise in [ $\alpha$ /Fe] could be caused by an increase in the star formation efficiency (SFE), known as the ‘simmering’ phase scenario. However, gas inflow also plays a vital role in shaping the star formation history and chemical evolution of galaxies, especially during the earliest epoch of the universe. We investigate this unexpected [ $\alpha$ /Fe]-rise with an experiment involving a galactic chemical evolution model. Our model has five free parameters: the mass of the initial reservoir of the cold interstellar medium (ISM) at birth, the frequency of Type Ia supernovae (SNe Ia), the cooling timescale of the warm ISM, the SFE, and the inflow rate of fresh gas. The last two free parameters were allowed to change after [ $\alpha$ /Fe] reached its lowest value, dividing the proto-Galaxy epoch into two phases. The models that reproduced the observed [Fe/H]-[ $\alpha$ /Fe]-track provided estimates for these fundamental parameters of the proto-Milky Way. We find that the rise in [ $\alpha$ /Fe] could also be caused by a large inflow of high-[ $\alpha$ /Fe] gas and conclude that the [ $\alpha$ /Fe]-rise could be a signature of the gas accretion that fuelled the formation of the Milky Way disk.