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 dwarfs forming after the off-plane dwarfs from material spread about the system after Andromeda merged with another galaxy. To shed light on the mystery, Michelle Collins of Yale University and her collaborators used the DEIMOS instrument at the Keck II telescope to observe two of Andromeda’s on-plane dwarf spheroidals. Having determined the dwarfs’ chemical makeup and other properties, the researchers then compared them with existing observations of 12 on-plane and 17 off-plane dwarf spheroidals in Andromeda. They found no measurable differences between the two populations, which suggests that Andromeda’s on-plane and off-plane dwarfs formed at the same time from the same material. The prevailing cosmological framework struggles to account for such a starkly bimodal distribution. (M. L. M. Collins et al., Astrophys. J. Lett. 799, 13, 2015, doi:10.1088/2041-8205/799/1/L13 .)