Rapid early gas accretion for the inner Galactic disc

Recent observations of the Milky Way and galaxies at high redshifts suggest that galaxy discs were already in place soon after the Big Bang. While the gas infall history of the Milky Way in the inner disc has long been assumed to be characterised by a short accretion time scale, this has not been directly constrained using observations. Using the unprecedented amount and quality of data of the inner regions of the Milky Way that has recently been produced by APOGEE and Gaia, we aim to derive strong constraints on the infall history of the inner (less than 6 kpc) Galaxy (with a focus on stars between 4-6 kpc, which we show is an appropriate proxy for the entire inner disc). We have implemented gas infall into a chemical evolution model of the Galaxy disc, and used a Schmidt-Kennicutt law to connect the infall to the star formation. We explore a number of models, and two different formulations of the infall law. In one formulation, the infall is non-parametric, and in the other the infall has an explicitly exponential form. We fit the model parameters to the time-[Si/Fe] distribution of solar vicinity stars, and the metallicity and [Si/Fe] distribution function of stars with a galactocentric radius between 4-6 kpc from APOGEE. Our results point to a fast early gas accretion, with an upper limit of accretion timescale of around 2 Gyr in the inner disc of the Milky Way. This suggests that at least half the baryons were in place within 2-3 Gyr of the Big Bang, and that half the stars of the inner disc formed within the first 5 Gyr, during the thick disc formation phase. This implies that the stellar mass of the inner disc is dominated by the thick disc, supporting our previous work, and that the gas accretion onto the inner disc was rapid and early.