Bimodality of [α/Fe]-[Fe/H] distributions is a natural outcome of dissipative collapse and disc growth in Milky Way-type galaxies

Abstract: We present a set of self-consistent chemo-dynamical simulations of MW-type galaxies formation to study the origin of the bimodality of $\alpha$-elements in stellar populations. We explore how the bimodality is related to the geometrically and kinematically defined stellar discs, gas accretion and radial migration. We find that the two $\alpha$-sequences are formed in quite different physical environments. The high-$\alpha$ sequence is formed early from a burst of star formation (SF) in a turbulent, compact gaseous disc which forms a thick disc. The low-$\alpha$ stellar populations is the result of quiescent SF supported by the slow accretion of enriched gas onto a radially extended thin disc. Stellar feedback-driven outflows during the formation of the thick disc are responsible for the enrichment of the surrounding gaseous halo, which subsequently feeds the disc on a longer time-scale. During the thin disc phase, chemical evolution reaches an equilibrium metallicity and abundance, where the stars pile-up. This equilibrium metallicity decreases towards the outer disc, generating the ridgeline that forms the low-$\alpha$ sequence. We identify a second mechanism capable of creating a low-$\alpha$ sequence in one of our simulations. Rapid shutdown of the SF, provoked by the feedback at the end of the thick disc phase, suppresses the chemical enrichment of the halo gas, which, once accreted onto the star-forming disc, dilutes the ISM at the beginning of the thin disc formation. Both mechanisms can operate in a galaxy, but the former is expected to occur when SF efficiency ceases to be dominated by the formation of the thick disc, while the latter can occur in the inner regions. Being the result of the presence of low and high gas density environments, the bimodality is independent of any particular merger history, suggesting that it could be much more widespread than has been claimed.