基于LAMOST巡天的银河系结构演化研究新进展

Progress on the structure and evolution of the Milky Way based on LAMOST survey

银河系至少是目前唯一能够获得单个恒星三维空间和运动信息的星系,这使得它成为我们得以详细研究的最重要的星系.更好地了解银河系的组成及其演化在星系天文学中起着至关重要的作用.自Hipparcos时代以来,人们开始以数以万计的恒星为样本研究银河系,这使我们能够在太阳附近发现许多新现象.SDSS和2MASS等巡天将我们的视野扩展到银河系的更大体积,在恒星晕中发现了丰富的子结构.LAMOST巡天提供了1000多万个恒星光谱,极大地推动了我们对银河系的了解.本文综述了近年来基于LAMOST巡天观测数据开展的银河系研究新进展,特别是在银河系恒星晕的结构与演化、银河系外盘的形态与非稳态运动,以及银河系动力学质量和本地暗物质密度等方面进行了重点介绍.

The Milky Way is,at least for now,the only galaxy that we can obtain the full 3 dimensional spatial and kinematical information of individual stars.This makes it the most important galaxy sample that we can study in details.In principle,better understanding on how the Milky Way ensembles and how it evolves plays a vital role in Galactic astronomy.Since Hipparcos ages,people started to study the Galaxy with tens of thousands of stars as samples.The observations allow us to discover a lot of new phenomena in the solar neighborhood.SDSS and 2 MASS surveys extend our vision to a larger volume of the Galaxy,in which rich substructures in the stellar halo have been unveiled.LAMOST survey provides 10 more million stellar spectra,substantially pushes our knowledge of the Milky Way forward.In this article,we briefly review the substantial progress on several fields,including the structure and evolution of the stellar halo,the structure and non-equilibrium kinematics of the Galactic outer disk,and the dynamical mass and local dark matter density of the galaxy,based on the LAMOSTsurvey data.We know that the stellar halo has a complicated shape with oblate stellar density profile in the inner part and spherical profile in the outer part.We find that the inner stellar halo rotates with a velocity of around20 km/s,which is very likely associated with the secular evolution between the Galactic disk and the halo.New tidal debris originated from merged dwarf galaxies has been found in the phase space rather than in the physical space.This implies that the stellar halo may be dominated by the tidal debris and favor a pure merged origin of the stellar halo.Thanks to the deep survey of LAMOST in the Galactic anti-center direction,we are able to detect the edge of the disk.We find that the Galactic stellar disk does not show end or truncation at least at the galactocentric distance of 20 kpc.This means that the Milky Way must have a quite large disk.By carefully measuring the radial stellar density profile of the outer Galactic disk,we found that the radial profile shows two breaks.This may be evident that the Milky Way disk is affected by the merging event some years ago.Tons of evidences have been revealed from stellar density and kinematics to support that the stellar disk is far from dynamical equilibrium.In fact,it may be severely perturbed by a passing-by dwarf satellite a few hundred million years ago.This event injected energy and angular momenta to the orbits of many stars.As a consequence,these stars show significant asymmetric motion in all three dimensions.Moreover,the mean velocity of the disk stars shows spiral patterns in Z-VZphase plane.This spiral pattern and complicated asymmetric motions in other projected phase plane,may be all the results of the perturbation induced by the passing-by dwarf satellite,although the identification of these dwarf galaxies is still unclear.Finally,measuring the dark matter mass of the Milky Way is a non-trivial task.We tried to lock down the local dark matter density using the vertical motion and vertical distribution of nearby stars.After a few tries,we reduced the uncertainty of the local dark matter density from around 50%to about 20%and obtained that the dark matter density at solar vicinity is 0.0133-0.0022+0.0024 M⊙pc-3.