银河系质量的多途径测定

Approaches for Measurements of the Mass of the Milky Way Galaxy

对测定银河系质量的一些主要方法及若干重要相关问题做了简要介绍,包括运动学方法(含自转曲线法)、伴天体的潮汐半径、逃逸速度法和计时推理法等。给出近40年来较有代表性的银河系质量的实测结果,并加以比较和讨论。迄今为止银河系质量的测定值仍有相当大的不确定性,未来的空间观测资料有望对此做出明显的改进。

As one of the important parameters of the Milky Way Galaxy, the Galactic mass has been determined or estimated in many different ways since 1920s, including kinematics of tracers, tidal radius of both globular clusters and dwarf satellite galaxies of the Galaxy, local escape velocity, timing argument theory, etc. Different kinds of tracers can be used for estimating the Galactic mass, including globular clusters, satellite galaxies and some other bright halo population objects, such as field horizontal branch stars, RR Lyraes variables, red giant stars, etc. In order to measure the whole dynamical mass of the galaxy, tracers with galactocentric distances as far as possible should be used. In order to determine the Galactic mass based on kinematics of tracers, it is necessary to gain space velocities of the tracers, both line-of-sight velocities and proper motions （transverse velocities）. However, in earlier time it was very dimcult and even impossible to measure proper motions of tracers because they are far away from us. In this case, only observational data which can be used for mass determination are line-of-sight velocities of tracers, and some theoretical assumptions for tracers＇ orbits are necessary to be introduced. As VLBI and space techniques become applicable to astrometry, proper motion data have been available for some satellite galaxies to estimate the Galactic mass. Besides the above method, some other approaches can also be used for estimation of the Galactic mass. （1） There is a relation among the tidal （limiting） radius of a satellite, its mass and Galactocentric distance and the Galactic mass, which can be used to estimate the Galactic mass. （2） In the solar neighborhood, there is a simple relation between the Galactic mass and the local escape velocity, which has been used for determination of the Galactic mass. （3） If a pair of galaxies, such as the Galaxy and a dwarf satellite, is well represented by point masses and isolated, their total mass may be estimated by measuring their separation, relative velocity, and time since the expansion of the universe began. The total mass is nearly equal to the Galactic mass, because the mass of the satellite is much less than the Galactic one. This is so-called timing argument. The major results for the Galactic mass obtained from different approaches since 1970s are collected and briefly discussed. It is shown from these results that many of values of the Galactic mass are with significant statistical uncertainties and in the quite wide range of （2 - 30） × 10^11M. So far as the Galactic mass is concerned, one expects that the future will belong to joint analyse of data sets of both the line-of-sight and proper motions of the satellites together with large samples of distant BHB stars. On the other side, the Gaia mission was successfully launched in December of 2013, and the mission is expected to measure the proper motions of the dwarf spherodial satellites of the Galaxy to an accuracy of a few to tens of km·s^-1, which will significantly reduce the uncertainties of values of the Galactic mass.