[论文解读] Bimodality of [\alpha/Fe]-[Fe/H] distributions in Milky Way-type galaxies with thick and thin discs

Sergey Khoperskov, M. Haywood+6 more|arXiv (Cornell University)|Jun 17, 2020

Galaxies: Formation, Evolution, Phenomena参考文献 8被引用 8

一句话总结

本研究利用自洽的化学-动力学模拟表明，银河系类星系中[$\alpha$/Fe]-[Fe/H]分布的双峰性源于两个不同的形成阶段：早期、湍流、紧凑的盘形成高[$\alpha$/Fe]序列，通过星暴形成；后期、平静、扩展的盘通过缓慢的气体吸积形成低[$\alpha$/Fe]序列。双峰性由不同的气体密度环境驱动，与并合历史无关，表明其在旋涡星系中具有普遍性。

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.

研究动机与目标

• 理解银河系类星系中观测到的双峰[$\alpha$/Fe]-[Fe/H]分布的起源。
• 研究几何和运动学盘结构、气体吸积以及径向迁移如何影响高[$\alpha$/Fe]和低[$\alpha$/Fe]星族的形成。
• 确定双峰性是否源于并合历史，或源于盘形成中的内在物理过程。
• 识别导致厚盘与薄盘星族之间化学分离的物理机制。

提出的方法

• 使用自洽的银河系类星系形成化学-动力学模拟，以模拟恒星形成、气体动力学和化学演化。
• 模拟追踪气体、恒星和金属度的演化，包括恒星反馈和外流对晕区的金属增益。
• 模型包含径向迁移和可变的恒星形成效率，以评估其对化学丰度模式的影响。
• 测试了低[$\alpha$/Fe]序列形成的两种不同机制：(1) 来自晕区的富集气体缓慢吸积，(2) 厚盘反馈停止后由未富集气体稀释。
• 模拟区分了高[$\alpha$/Fe]（早期、紧凑、湍流盘）和低[$\alpha$/Fe]（后期、扩展、平静盘）的形成环境。
• 在空间和运动学分箱中分析化学丰度模式，以识别低[$\alpha$/Fe]序列的脊线及双峰结构。

实验结果

研究问题

• RQ1 何种物理条件导致银河系类星系中高[$\alpha$/Fe]星族的形成？
• RQ2 低[$\alpha$/Fe]星族如何形成，其化学平衡与径向金属度梯度由何种过程主导？
• RQ3 双峰[$\alpha$/Fe]-[Fe/H]分布在多大程度上依赖于并合历史，而非盘形成中的内在物理过程？
• RQ4 是否存在多种机制可生成低[$\alpha$/Fe]星族？这些机制在何种条件下起作用？

主要发现

• 高[$\alpha$/Fe]星族在早期由湍流、紧凑的气体盘通过星暴形成，导致α元素快速富集。
• 低[$\alpha$/Fe]星族源于由晕区缓慢吸积富集气体供给的径向扩展薄盘中的平静恒星形成。
• 厚盘形成期间的恒星反馈使气体晕区富集，该富集气体随后供给薄盘，维持其化学平衡。
• 低[$\alpha$/Fe]星族因外盘气体供给的径向梯度导致平衡金属度降低，从而形成脊线。
• 存在第二种低[$\alpha$/Fe]星族形成机制：恒星形成迅速停止会抑制晕区富集，导致星际介质稀释，从而降低α元素增强程度。
• 双峰性与并合历史无关，而是由气体密度环境的差异引起，表明其是旋涡星系中的普遍特征。