银河系卫星星系研究进展

Progress on the study of the Milky Way satellite galaxies

对银河系内卫星星系进行全面的"人口普查"具有重要的意义。目前已经发现了二十几个卫星星系,其光度范围分布很广,最暗的矮星系比球状星体还暗。叙述了卫星星系的光度分布、空间分布和动力学性质。总结了观测和理论研究进展,并讨论了星流和伽玛射线在研究银河系结构和暗物质性质方面的贡献。表明了卫星星系的统计分布能用来很好地限制冷暗物质理论和星系形成的相关物理过程,同时指出当前研究的局限性和可能的发展方向。

In recent years, great progress has been achieved on studies of the satellite galaxies in the Milky Way （MW）. With the help of the Sloan Digital Sky Survey （SDSS）, more than dozen faint satellites have been discovered, doubling the known satellites to more than twenty. The luminosity of these satellites ranges from 10^3L⊙ to 10^10L⊙, and the faintest dwarf has stellar mass close to the globular cluster, but with much more extend spatial distribution. Using the enriched data of stellar spectra, more accurate information about the satellite galaxies （luminosity, chemical abundance, kinematics） can be obtained and it allows for comprehensive comparison between model predictions and observations. In this paper, we review those studies which can put strong constraints on the CDM scenario and galaxy formation physics. The notorious puzzle about the MW is the＂Missing Satellite Problem＂ explicitly shown by Moore et al. （1999） that the number of substructures in N-body simulation surpasses the observed satellites by a factor of ten. This problem has triggered modifications of the CDM model, either by suppressing the perturbation amplitude on small scales or using the warm dark matter instead. However, most work tries to rescue the standard CDM scheme by invoking astrophysics processes to solve this discrepancy. In this aspect, the luminosity function of satellite is especially useful as it allows for a direct comparison between model predictions and the data. By quantifying the detectability and completeness of the SDSS, the luminosity function of MW satellites can be well described by a power law down to magnitude My = -2. It is found that the MW should contains more than hundreds of faint dwarf satellites. Combining the formation history of the MW from high-resolution simulation with those astrophysics process, such as gas cooling, star formation , feedback and cosmic re-ionization, it is shown that galaxy formation model can reproduce the luminosity function of satellites. The UV background has the effect to suppress gas cooling in small haloes after re-ionization. Thus only a biased population of substructures in N-body simulation can form stars. The distribution of satellites around the Milky Way is also weird that most of them are distributed in a flattened plane, which is almost perpendicular to the MW stellar disk. Such a spatial configuration is though to challenge the CDM theory as the distribution of substructures is isotropic. Using simulation with star formation included, it is found that most satellites reside in dark matter haloes which are massive before accreted by host halo, and they are predominately accreted along the major axes of the host haloes. Thus the satellites tend to distribute in a flattened disk. It is also suggested that flattened structure are formed because most satellites are in a group before accretion. However, It is still puzzle why the flattened disk is perpendicular to the MW disk. It is found that most satellites are dark matter dominated inside the luminous radii. Surprisingly, it is found that the total mass inside 300 pc is almost a constant of 10^7M⊙ independent of luminosity. Theoretical work finds that as there is a low mass limit below which gas cooling is very inefficient, and the halo of satellites spans a narrow range of mass before accretion. As the satellites are aecreted into the MW at different redshift, the effect of cosmic re-ionization differs significantly, thus producing a wide range of luminosity. Some argue that this flat relation is only a selection effect from surface brightness limit of the SDSS. We also briefly review other related issues including stellar stream from satellites, and gamma ray from dark matter annihilation. More and more stellar streams are discovered by the SDSS, and using the kinematics and chemical information of stars in the streams, the progenitors of these stream can be identified, and most are related to known dwarf satellites and globular clusters. Satellite galaxies are the best place for gamma ray from dark matter annihilation due to their proximity and lower contamination from point sources. We finally note that current survey of satellites still suffers detection limit （surface brightness）. More faint satellites are expected to be found by the next generation of facilities, such as LSST, GAIA and TMT. For theoretical modeling of the MW, we need to be aware of the cosmic variance as we have observed only one galaxy. High-resolution simulation with star formation is needed to simultaneously model the formation of the MW itself and its satellites. It is definitely a challenge to our understanding of astrophysics and computational power.