本星系群动力学质量之估测

Estimates of the Dynamical Mass of the Local Group of Galaxies

本星系群的动力学质量M_(LG)可以通过多种途径来进行估测,如计时质量法、零速度面法、位力质量法等,对之做了简要的说明,并且介绍了河外星系自行测定在相关研究中的重要性及其现状。对现有测定值进行了分析和讨论,结果表明,目前所知本星系群之动力学质量的合理估值可取为M_(LG)=(2.73±0.44)×10^(12)M_⊙。

The Milky Way（MW） is a member of a small group of galaxies called as the Local Group（LG）. The LG is dominated by its two largest members, the MW and the Andromeda galaxy（M31）. The other members can be separated into one of two categories： dwarf satellites accompanying the MW or M31, and the dozen or so independent galaxies sprinkled in space up to ~1.5 Mpc away. This group presents a unique example where sufficient data are available to derive its mass and dynamics, which has been the topic of many previous studies.Since 1959 the dynamical mass of the LG has been estimated in different ways, among which three independent methods have been most frequently used： the timing argument,the zero velocity surface and the virial theorem. The timing argument is to use the relative motion of MW and M31 to estimate the total mass of the LG. The methods of zero velocity surface and virial theorem use kinematic data of all satellite galaxies to estimate the LG mass.Furthermore, in the zero velocity surface method the mass of the LG is determined from the external Hubble velocity field surrounding the group, and the virial mass is derived from the internal motions of member galaxies of the LG.In order to estimate the dynamical mass of the LG by using the timing argument, one must know the three dimensional kinematical data of M31 with respect to the MW. While the light-of-sight velocity of M31 is well known from Doppler measurements, determining its proper motions has been elusive for some half century or longer, because M31 is very faraway from us. Recently, the first proper motion measurements were obtained with the HST equipments and have been used to derive the mass of the LG.The major results for the mass of the LG, MLG, obtained from different approaches since 1959, are collected in Table 1 and briefly discussed. It can be seen from the table that the range of MLG is 1.2〈 MLG〈 5.6, discrepancies being quite big. However, the range of M LG published in the recent years（2009—2014） has been obviously narrowed to1.88〈 MLG〈 4.27, from which an optimum estimator MLG=（2.73 ± 0.44） × 10^12M⊙can be found.