Title: Early Co-formation of the Milky Way's Thin and Thick Disks at Redshift z > 2 Show Chinese title

Title: 红移 z > 2 时银河系薄盘和厚盘的同时形成

Abstract: The Milky Way serves as a template for understanding the formation and evolution of late-type massive disk galaxies since we can obtain detailed chemical and kinematic information for large samples of individual stars. However, the early formation of the disk and the dichotomy between the chemical thick and thin disks remain under intense debate. Some mechanisms have been proposed to explain the formation of this dichotomy, such as the injection of metal-poor gas by a gas-rich merger such as Gaia-Sausage Enceladus (GSE), or by cosmic gas filaments, radial migration, and the presence of star-forming clumps at high redshift ($z > 2$). In this work, we combine astrometric data from the Gaia mission, chemical abundances from APOGEE and LAMOST spectroscopic surveys, and StarHorse ages to map the evolution of our Galaxy. The Bayesian isochrone-fitting code StarHorse can estimate ages for thousands of stars in the solar neighborhood, being most reliable for main sequence turnoff and sub-giants, computing distances and extinction simultaneously. From these samples, we show that (i) there is an old thin disk population ($>11$ Gyr) that indicates a period of co-formation between the thick and thin disks of the Milky Way before the GSE merger, i.e. the Galaxy itself could initiate the formation of a low-alpha disk without the need for a gas-rich merger, and (ii) this merger would have been important to stop the formation of stars in the thick disk. Show Chinese abstract

Abstract: 银河系由于可以为大量单个恒星获得详细的化学和动力学信息，因此成为理解晚期型大质量盘星系形成与演化的模板。然而，盘早期的形成以及化学厚盘和薄盘之间的二分性仍然存在激烈争论。 一些机制已经被提出用来解释这种二分性的形成，例如富含气体的合并（如盖亚香肠结构恩克拉多斯，GSE）注入贫金属气体会导致这种二分性，或者由宇宙气体细丝、径向迁移以及高红移时恒星形成团块的存在引起（$z > 2$）。 在这项工作中，我们结合了盖亚任务的天文学数据、APOGEE 和 LAMOST 光谱巡天的化学丰度以及 StarHorse 年龄来绘制银河系的演化图。贝叶斯等年龄拟合代码 StarHorse 可以估计太阳邻域数千颗恒星的年龄，对于主序拐点和次巨星最为可靠，并同时计算距离和消光。从这些样本中，我们展示了（i）存在一个古老的薄盘群体（$>11$Gyr），这表明在 GSE 合并之前，银河系厚盘和薄盘之间有一个共同形成的时期，即银河系本身可能在没有富含气体合并的情况下启动了低 α 盘的形成，以及（ii）这个合并可能对停止厚盘中的恒星形成起到了重要作用。