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.

中文翻译：

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[α/Fe] -[Fe/H] 分布的双峰性是银河系型星系耗散坍缩和圆盘生长的自然结果

我们提出了一组 MW 型星系形成的自洽化学动力学模拟，以研究恒星族群中 $\alpha$ 元素双峰性的起源。我们探索双峰性如何与几何和运动学定义的星盘、气体吸积和径向迁移相关。我们发现这两个 $\alpha$-序列是在完全不同的物理环境中形成的。高$\alpha$序列是由形成厚盘的湍流致密气态盘中的恒星形成（SF）爆发早期形成的。低 $\alpha$ 星族是静止 SF 的结果，由富集气体缓慢吸积到径向延伸的薄盘上支持。厚盘形成过程中恒星反馈驱动的外流是导致周围气晕富集的原因，随后在更长的时间范围内为光盘提供能量。在薄盘阶段，化学演化达到金属丰度和丰度的平衡，恒星在此堆积。这种平衡金属丰度朝着外盘减小，产生形成低$\alpha$序列的脊线。我们确定了能够在我们的一个模拟中创建低 $\alpha$ 序列的第二种机制。由厚盘阶段结束时的反馈引起的 SF 快速关闭，抑制了晕气体的化学富集，一旦吸积到恒星形成盘上，就会在薄盘形成开始时稀释 ISM . 两种机制都可以在星系中运行，但当 SF 效率不再受厚盘形成的支配时，预计前者会发生，而后者可以发生在内部区域。由于存在低和高气体密度环境，双峰性与任何特定的合并历史无关，这表明它可能比所声称的更广泛。