Dwarf galaxies coherently orbit Andromeda

The international PANDAS (Pan-Andromeda Archaeological Survey) collaboration has been intensively examining the enormous spherical halo of dark matter and satellite dwarf galaxies that envelops the Andromeda galaxy. Shown in the photo, Andromeda is the Milky Way’s nearest full-grown neighbor. The team’s principal instrument is a wide-field camera on the Canada-France-Hawaii Telescope atop Hawaii’s Mauna Kea. Having completed a census of dwarf satellites within the halo, PANDAS reports a surprising result: Fifteen of Andromeda’s 27 known dwarf satellites lie in a vast, astonishingly thin plane through the galaxy’s center, and they appear to orbit in concert. Though the plane of satellites is at least a million light-years in diameter, its thickness is only 5 ×10⁴ light-years. The result presents a challenge to standard scenarios of large-galaxy formation, in which dark nonbaryonic matter plays the dominant role in the stages that precede the accumulation of enough hydrogen to ignite star formation. Andromeda’s starry disk does of course define a select plane within the surrounding spherical halo. But dark matter, interacting only gravitationally, is impervious to the dissipative forces that flatten spinning concentrations of gas. So one would expect the small dark-matter halos (the progenitors of dwarf galaxies), captured one at a time by their mighty neighbor, to be trapped in independent orbits that prefer no particular plane. Prior, less definitive evidence of planar formation among dwarf galaxies in the Milky Way’s halo had already instigated discussion of modified or alternative scenarios that might explain coplanarity of satellite-galaxy orbits. (R. A. Ibata et al., Nature 493, 62, 2013 .)—Bertram Schwarzschild