A Pristine Galaxy Heading Our Way

Maps of stars at distance of dwarf galaxy
The color map is a smoothed rendition of stars that show characteristics of being in the pristine dwarf galaxy Andromeda XII, based on imagery by the Canada-France-Hawaii Telescope. White dots mark the positions of individual stars studied in a followup with the DEIMOS spectrograph on the Keck II telescope. Star symbols mark the eight whose spectra showed them to be part of the galaxy rather than unrelated stars in the foreground or background. The frame is 10 arcminutes tall.
Scott Chapman, University of Cambridge

A recently discovered dwarf galaxy known as Andromeda XII is crashing our local party. The diminutive new arrival may be a latecomer, but it’s making up for lost time by approaching our cluster of galaxies — the Local Group — at breakneck speed.

Astronomers only discovered the tiny galaxy in October 2006. Although it doesn’t look like much, it’s turning out to be a treasure — detailed observations by Scott Chapman (Cambridge University, England), Jorge Penarrubia (University of Victoria, British Columbia), and their colleagues have revealed some remarkable properties. Spectra from the 10-meter Keck II telescope in Hawaii show that the galaxy is approaching the Sun at a speed of 556 kilometers per second. That translates to an approach velocity toward the Andromeda Galaxy (M31) of 281 kilometers per second, probably greater than the escape velocity of the whole Local Group.

“Andromeda XII is by far the fastest-moving dwarf galaxy in the Local Group,” said Penarrubia during a press conference at the spring 2007 American Astronomical Society meeting in Hawaii. “It has incredible kinetic energy.” It's about 375,000 light-years from the center of M31, and lies behind it from our perspective.

The high speed implies that the dwarf is falling into the Local Group for the first time. The Local Group consists of the Milky Way, M31, M33, and dozens of dwarfs.

And XII is only about 400 light-years across, with perhaps 100,000 to 1 million stars and a luminosity of only 100,000 Suns. That might make it seem like a mere globular star cluster, but it’s not. “There is probably 50 to 100 times more mass in dark matter than in stars,” says Penarrubia — a much higher ratio than in most galaxies. “It’s a perfect natural laboratory for studying the properties of dark matter.”

Today’s standard cosmic models invoke slow-moving (cold) dark-matter particles to explain the formation of large-scale cosmic structure. They actually predict many more dwarfs galaxies than astronomers have found. But if And XII is typical, maybe that’s just because they’re so dim and hard to recognize.

Penarrubia notes another crucial aspect of And XII: its apparently pristine nature. Since it’s approaching the Local Group from a distant and relatively empty region of space, it has never yet been gravitationally stirred up by close encounters with neighbors like the Milky Way and M31. This gives astronomers a golden opportunity to study the mass and star-formation history of an object that is probably similar to the original, early-universe building blocks of all galaxies.

“It’s hard to study the dark-matter properties of galaxies that have been orbiting for a long time,” says Penarrubia. He notes that dwarf galaxies orbiting the Milky Way or M31 for a long time have been seriously disrupted.

An independent dwarf-galaxy researcher comments that uncertainties remain about whether And XII is really a newcomer to the Local Group, and if so where it came from. But if it is newly arriving in our cluster, it may help answer an outstanding question: How massive were these dwarfs when they originated? The answer has cosmological implications. “At present it would be the only galaxy known with so few stars that we could be sure was undisturbed by tides, letting us get a unique peek at the conditions under which the tiniest dwarf galaxies actually form,” the commenter says.

And XII’s fast speed (which has led some to dub it the “Olympian Galaxy”) may also force astronomers to revise M31’s estimated mass, dark matter included, upward by as much as 30 percent.

More information is in a Keck Observatory press release.