Galaxy formation theory predicts mergers between low-mass galaxies. Whereas the accretion of small systems onto large ones such as the Milky Way has been observed indirectly, no mergers between small galaxies - which have masses of less than a billion Sun-like stars - have been identified. Now Nicola Amorisco et al. report the kinematic detection of a stellar stream in one of the satellite galaxies of Andromeda, the dwarf spheroidal Andromeda Ⅱ. They conclude that they are observing the remnant of a merger between two dwarf galaxies. This illustrates the scale-free character of galaxy formation, down to the lowest galactic mass scales.
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