The lowest luminosity (L < 10^5 L_sun) Milky Way satellite galaxies represent the extreme lower limit of the galaxy luminosity function. These ultra-faint dwarfs are the oldest, most dark matter-dominated, most metal-poor, and least chemically evolved stellar systems known. They therefore provide unique windows into the formation of the first galaxies and the behavior of dark matter on small scales. In this review, we summarize the discovery of ultra-faint dwarfs in the Sloan Digital Sky Survey in 2005, and the subsequent observational and theoretical progress in understanding their nature and origin. We describe their stellar kinematics, chemical abundance patterns, structural properties, stellar populations, orbits, and luminosity function, and what can be learned from each type of measurement. We conclude that: (1) in most cases, the stellar velocity dispersions of ultra-faint dwarfs are robust against systematic uncertainties such as binary stars and foreground contamination; (2) the chemical abundance patterns of stars in ultra-faint dwarfs require two sources of r-process elements, one of which can likely be attributed to neutron star mergers; (3) even under conservative assumptions, only a small fraction of ultra-faint dwarfs may have suffered significant tidal stripping of their stellar components; (4) determining the properties of the faintest dwarfs out to the virial radius of the Milky Way will require very large investments of observing time with future telescopes. Finally, we offer a look forward at the observations that will be possible with future facilities as the push toward a complete census of the Local Group dwarf galaxy population continues.

中文翻译：

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最暗的矮星系

最低光度 (L < 10^5 L_sun) 银河系卫星星系代表了星系光度函数的极端下限。这些超微弱的矮星是已知最古老、以暗物质为主、最贫金属和化学演化最少的恒星系统。因此，它们为了解第一批星系的形成和暗物质在小尺度上的行为提供了独特的窗口。在这篇综述中，我们总结了 2005 年斯隆数字巡天中发现的超微弱矮星，以及随后在理解它们的性质和起源方面的观测和理论进展。我们描述了它们的恒星运动学、化学丰度模式、结构特性、恒星种群、轨道和光度函数，以及可以从每种类型的测量中学到什么。我们得出结论：（1）在大多数情况下，超微弱矮星的恒星速度色散对双星和前景污染等系统不确定性具有很强的抵抗力；(2) 超微弱矮星中恒星的化学丰度模式需要两种 r 过程元素来源，其中之一可能归因于中子星合并；(3) 即使在保守的假设下，也只有一小部分超微弱矮星的恒星成分可能遭受了显着的潮汐剥离；(4) 确定银河系维里半径内最微弱矮星的特性将需要在未来的望远镜上投入大量的观测时间。最后，随着对本地群矮星系人口的全面普查的继续推进，我们展望了未来设施可能进行的观测。