The remnant of a merger between two dwarf galaxies in Andromeda II
  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 $10^{11}$ solar
masses in stars, further observational evidence of merging activity is also
ample. Mergers of lower-mass galaxies are expected within the hierarchical
process of galaxy formation, but have hitherto not been seen for galaxies with
less than about $10^9$ 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 galaxy Andromeda II, which has a mass of only $10^7$ solar
masses in stars. The properties of the stream show that we are observing the
remnant of a merger between two dwarf galaxies. This had a dramatic influence
on the dynamics of the remnant, which is now rotating around its projected
major axis. The stellar stream in Andromeda II illustrates the scale-free
character of the formation of galaxies, down to the lowest galactic mass
scales.
