使用LAMOST大样本巨星研究银河系的质量分布

The study of the mass distribution of the Milky Way has never been beyond the solar neighborhood, except the central region of the Galaxy and the stellar halo. Particularly for the Galactic disk, our knowledge about the mass profile and its dynamical features are very limited. Therefore, we propose to research how the mass distributes in the disk far beyond the solar neighborhood. This requests a large data set covering a broad region of the Galactic disk. The LAMOST survey plans to observe around 700 thousands of disk giant stars (including red clump and K giant stars) in its 5-year schedule. These targets are approximately statistically unbiased and ready for the study of the disk dynamics. Their kinematic information extracted from the low-resolution spectra will be used for measuring the Galactic rotation curve in the range of 5-20 kpc in Galactocentric radius. As the most accurate measurement has ever been made, the spatial resolution will be better than 100 pc and the velocity uncertainty will be lower than 5 km/s. It is sufficient for not only decomposing the disk and halo components, but also a quantitative detection of the potential massive substructure in the outer disk. For the first time, we will measure the lopsidedness, which is critical in understanding the interaction between the disk and the dark matter halo in secular evolution, of the Galactic stellar disk from the variation of the rotation curve in different directions.

银河系物质分布的研究当前仅限于太阳邻域等少数特殊区域，对于天空中最为显著的银盘的物质分布和动力学特性所知甚少。研究银盘的动力学特性，探测银河系物质分布，进而追问银盘和暗物质晕之间的关联、动力学演化历史等基本问题需要超出太阳邻域，在广阔的银盘上观测大量恒星位置和运动信息。LAMOST巡天将在5年内提供约70万统计无偏的银盘红团簇和K型巨星光谱，正适合此类研究的要求。通过它们的运动特征，我们将在半径5-20 kpc内绘制迄今最为详细和精确的银河系自转曲线，距离精度好于100 pc，速度精度达到5 km/s。如此精确估计的自转曲线将可以准确分解出银盘和暗物质晕这两个最主要的质量成分，并将定量研究外盘中可能存在的高密度子结构。本项目首次提出在自转曲线的空间变化中探测银河系恒星盘的不对称性，这对于理解暗物质晕在银盘动力学演化中所起的作用是至关重要的。

本课题通过使用LAMOST巡天观测的红巨星研究银河系的质量分布。经过四年研究，团队在银河系结构方面取得了突破性进展。首次展示了旋转曲线在外盘呈现的S星震荡，目前倾向于认为这个震荡可能是由于银盘的非平衡结构产生的，同质量分布关系不大。通过恒星计数研究发现银盘比原来认为的要大四分之一，这必将影响银河系的质量分布特征。同时我们还首次证实了银河系的恒星晕具有复杂的形态，其内晕呈现扁球形，外晕则逐渐过渡成球形。这样的复杂形态预示其可能有非常独特的质量分布、演化历史。对银盘外盘的非对称运动开展了系统研究，揭示了外盘恒星具有显著的非对称运动。这些新的观测发现对于进一步了解银河系的结构和演化历史具有重要的科学意义。这些成果同时也是近年来LAMOST巡天项目所产生的重要科学成果之一。