Cold accretion flows and chemical bimodality of the Milky Way galaxy

Abundance of chemical elements in the stars provides important clues regarding galaxy formation. The most powerful diagnostics is the relative abundance of {\alpha}-elements (O, Mg, Si, S, Ca, and Ti) with respect to iron (Fe), [{\alpha}/Fe], each of which is produced by different kinds of supernovae. The existence of two distinct groups of stars in the solar neighbourhood, one with high [{\alpha}/Fe] and another with low [{\alpha}/Fe], suggests that the stars in the solar vicinity have two different origins. However, the specific mechanism of the realization of this bimodality is unknown. Here, we show that the cold flow hypothesis recently proposed for the accretion process of primordial gas onto forming galaxies predicts two episodes of star formation separated by a hiatus 6-7 Gyr ago and naturally explains the observed chemical bimodality. We found that the first phase of star formation that forms high [{\alpha}/Fe] stars is caused by the 'genuine' cold flow, in which unheated primordial gas accretes to the galactic disk in a freefall fashion. The second episode of star formation that forms low [{\alpha}/Fe] stars is sustained by much slower gas accretion as the once-heated gas gradually cools by radiation. The cold flow hypothesis can also explain the large-scale variation in the abundance pattern observed in the Milky Way galaxy in terms of the spatial variation of gas accretion history.