[论文解读] Birth of the Galactic Disk Revealed by the H3 Survey

Charlie Conroy, David H. Weinberg+13 more|arXiv (Cornell University)|Apr 6, 2022

Stellar, planetary, and galactic studies被引用 36

一句话总结

论文使用 H3 调查光谱学和 Gaia 数据来识别原位与被俘获的恒星，揭示了非单调的 [alpha/Fe] 对 [Fe/H] 轨迹，并推断在银河系圆盘诞生的约 z~4 时刻出现了星形成效率的显著上升。

ABSTRACT

We use chemistry ([alpha/Fe] and [Fe/H]), main sequence turnoff ages, and kinematics determined from H3 Survey spectroscopy and Gaia astrometry to identify the birth of the Galactic disk. We separate in-situ and accreted stars on the basis of angular momenta and eccentricities. The sequence of high-alpha in-situ stars persists down to at least [Fe/H]=-2.5 and shows unexpected non-monotonic behavior: with increasing metallicity the population first declines in [alpha/Fe], then increases over the range -1.3<[Fe/H]<-0.7, and then declines again at higher metallicities. The number of stars in the in-situ population rapidly increases above [Fe/H]=-1. The average kinematics of these stars are hot and independent of metallicity at [Fe/H]<-1 and then become increasingly cold and disk-like at higher metallicities. The ages of the in-situ, high-alpha stars are uniformly very old (13 Gyr) at [Fe/H]<-1.3, and span a wider range (8-12 Gyr) at higher metallicities. Interpreting the chemistry with a simple chemical evolution model suggests that the non-monotonic behavior is due to a significant increase in star formation efficiency, which began 13 Gyr ago. These results support a picture in which the first 1 Gyr of the Galaxy was characterized by a "simmering phase" in which the star formation efficiency was low and the kinematics had substantial disorder with some net rotation. The disk then underwent a dramatic transformation to a "boiling phase", in which the star formation efficiency increased substantially, the kinematics became disk-like, and the number of stars formed increased tenfold. We interpret this transformation as the birth of the Galactic disk at z~4. The physical origin of this transformation is unclear and does not seem to be reproduced in current galaxy formation models.

研究动机与目标

• 使用化学丰度、动力学和年龄来识别银河系中的原位与被俘获恒星群体。
• 在广泛金属丰度范围内刻画高α 原位群体，以理解圆盘形成历史。
• 用化学演化建模解释观测到的丰度模式，以推断星形成历史和圆盘诞生时代。

提出的方法

• 使用 H3 光谱学和 Gaia 天体测定来推导 [Fe/H]、[alpha/Fe]、距离和动力学。
• 通过轨道参数（偏心率 e 和角动量 LZ）定义原位与被俘获样本。
• 使用 MINESweeper 流水线在平坦年龄先验 4–14 Gyr 的条件下为主序翻转和次生星进行年龄推断。
• 计算原位高α 序列的运动学趋势（V_phi、sigma_Vz）和金属丰度分布。
• 应用一个简单的化学演化模型（VICE），设定固定的物质输入/输出参数，以将 [alpha/Fe]-[Fe/H] 轨迹解释为星形成效率 (SFE) 的体现。
• 探讨模型参数的变化以检验以 SFE 驱动的非单调轨迹的鲁棒性。

实验结果

研究问题

• RQ1 银河系中原位与被俘获恒星的化学、动力学和年龄特征是什么？
• RQ2 高α 原位群体是否在金属丰度上展现非单调的化学演化，这对圆盘形成意味着什么？
• RQ3 是否能用随星形成效率变化的化学演化模型再现观测的 [alpha/Fe] 对 [Fe/H] 轨迹，并指示银河圆盘形成的时间点？
• RQ4 可以推断出标志银河圆盘诞生的时代和物理转变是什么？

主要发现

• 高α 原位序列延伸至 [Fe/H] ~ -2.5，呈现非单调的 [alpha/Fe] 演化：下降至 [Fe/H] ~ -1.3，随后在 [Fe/H] ~ -0.7 处上升，在更高金属丰度处再次下降。
• 在 [Fe/H] < -1.3 时，原位高α 群体以极高年龄为主（平均约 13 Gyr），在更高丰度下逐渐变年轻（8–12 Gyr）。
• 约在 13 Gyr 之前星形成效率的迅速提升解释了非单调轨迹，暗示从低活动阶段向高活跃阶段的转变，导致盘状动力学并带来星形成的十倍提升。
• 这一转变被解读为银河圆盘在 z ~ 4 的诞生，目前的模型尚未完全再现这一起源。