How did dwarf galaxies form?

Dwarf spheroidal galaxies are small, are old, and contain a greater proportion of dark matter than either larger galaxies or globular clusters, which are also small and old. Only the dwarf spheroidals that surround the Milky Way and the nearby Andromeda galaxy are bright enough to study. Despite their quirks, dwarf spheroidals (such as the one shown here) are nevertheless similar enough to their galactic hosts that they could conceivably have started to form at the same time. But that formation scenario faces a challenge. About 60% of Andromeda’s dwarf spheroidals occupy a more-or-less spherical halo around their host. The rest, however, occupy a thin pancake-shaped region that encompasses Andromeda’s galactic plane (see Physics Today, March 2013, page 12). Such a confined distribution is difficult to produce if all the dwarfs began forming with the host galaxy when it first collapsed. An alternative scenario entails the on-plane...