星系形成理论预测较低质量的星系之间会发生合并。但尽管小星系向银河系这样的大星系上的吸积已被间接观测到，但尚未有低质量星系 (总质量小于10亿颗太阳类恒星质量的星系) 之间的合并被发现。现在Nicola Amorisco等人报告了在 “仙女座”的卫星星系之一的矮球形星系 “仙女座-II” 中对一个恒星流所做的运动检测 (kinematic detection) 。他们的结论是，他们观测到了两个矮星系之间一次合并的残留物。这反映了星系形成的无标度 (scale-free) 特点，它们可以小至最低的星系质量规模。
Driven by gravity, massive structures like galaxies and clusters of galaxies are believed to grow continuously through hierarchical merging and accretion of smaller systems. Observational evidence of accretion events is provided by the coherent stellar streams crossing the outer haloes of massive galaxies, such as the Milky Way or Andromeda. At similar mass scales, around 1011 solar masses in stars, further evidence of merging activity is also ample. Mergers of lower-mass galaxies are expected within the hierarchical process of galaxy formation6, but have hitherto not been seen for galaxies with less than about 109 solar masses in stars. Here we report the kinematic detection of a stellar stream in one of the satellite galaxies of Andromeda, the dwarf spheroidal Andromeda II, which has a mass of only 107 solar masses in stars9. The properties of the stream show that we are observing the remnant of a merger between two dwarf galaxies. This had a drastic influence on the dynamics of the remnant, which is now rotating around its projected major axis10. The stellar stream in Andromeda II illustrates the scale-free character of the formation of galaxies, down to the lowest galactic mass scales.