The giant spiral galaxy Andromeda and the slightly smaller Milky Way are king and queen of our place in space, a gathering of some 75 galaxies called the Local Group. Andromeda and the Milky Way each rule an empire of dozens of lesser galaxies that orbit them the way moons do a giant planet. Recently astronomers have discovered many more of these galactic runts. In that time, a surprising disparity has emerged. Andromeda is only 30 percent more luminous than our galaxy and so should boast a retinue only slightly grander than our own, yet most of Andromeda's newfound satellites are much brighter than ours. Clearly, we are missing something in our own backyard.
“There should be more bright satellites around the Milky Way,” says Basilio Yniguez, an astronomer at the University of California, Irvine. His team's computer simulations of how galaxies form suggest the Milky Way should host eight to 20 additional bright satellites that are as yet undiscovered.
“Bright” is defined relative to the faintest galaxies now known: Yniguez does not think the unseen satellites rival our galaxy's most flamboyant companions, the Large and Small Magellanic Clouds. Instead they are probably dim, diffuse “classical” dwarfs akin to Sculptor and Fornax, two Milky Way satellites astronomers spotted in 1938. Faint though they are, the classical dwarfs outshine a new breed of even dimmer “ultrafaint” dwarf galaxies that observers have turned up in the past decade.
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Since 2004 astronomers have found about two dozen galaxies orbiting Andromeda and about a dozen orbiting the Milky Way. But whereas 16 of Andromeda's new satellite galaxies are classical dwarfs, emitting more than 100,000 times as much light as the sun, only one of the Milky Way's newfound satellites shines this brightly—the rest are ultrafaint. Nevertheless, Andromeda and the Milky Way possess nearly identical distributions of satellites out to 330,000 light-years, suggesting that our galaxy's missing satellites probably lie farther out.
Finding them will be a challenge. “Our position within the Milky Way hurts us,” Yniguez says. “We're looking at Andromeda from the outside, whereas with the Milky Way we're looking from the inside.” Future searches, Yniguez hopes, will expand the Milky Way's known galactic empire so that it more closely matches that of its partner in Andromeda.