Eight New Neighboring Galaxies Found, Scientists Announce

John Roach in Seattle, Washington
for National Geographic News
January 10, 2007
Earth's home galaxy, the Milky Way, has at least eight more galactic neighbors than previously known, scientists announced yesterday—and dozens more finds are expected in the coming years.

The discoveries, made over the past two years, nearly double the number of Milky Way neighbors found in the prior 70 years.

"Seven of them are new dwarf galaxies [bound to] the Milky Way, ranging in distance from roughly 100,000 to 700,000 light-years from us," Daniel Zucker, an astronomer at the University of Cambridge in England, said at a meeting of the American Astronomical Society (AAS) in Seattle, Washington.

The new dwarfs are extremely faint and diffuse and contain at most a few million stars each, Zucker noted. In fact, they are so small that he suggested calling them "hobbit galaxies."

In contrast, the Milky Way, around which the newly discovered dwarfs orbit, contains at least 200 billion stars.

Prevailing theories of galaxy formation and the mysterious substance known as dark matter predict that the Milky Way should have a hundred or more surrounding dwarfs, but until the past few years only 12 were known, Zucker said.

The newly detected galaxies were found as part of the Sloan Digital Sky Survey, a project to create a high-resolution map of more than a quarter of the sky.

To date, the survey has completed about 80 percent of its goal. (Related: "Digital Sky Survey Detects New Stars in the Milky Way" [January 31, 2003].)

"If you make a very simple assumption that [dwarf galaxies] are distributed uniformly across the sky, then you get dozens of new satellites that should be out there," Zucker said.

Eighth Dwarf

The eighth dwarf galaxy—located about 1.4 million light-years away—is even more exciting, Zucker said.

"It is far enough from the Milky Way that it has probably not really been affected much by the Milky Way's gravity," he said. "It's actually free floating."

The other seven dwarfs are essentially crumbs left over from the galactic mergers that made up the Milky Way about a billion years ago, Zucker explained. All of their stars are relatively old.

But the eighth dwarf, named Leo T, has formed new stars in the past few million years—very recently in cosmological terms.

In addition, Leo T contains large pockets of gas, giving it the potential to still form stars.

"This is basically the smallest, faintest, star-forming galaxy known by orders of magnitude," Zucker said.

Structure of the Universe

According to theoretical models, however, many more galaxies resembling Leo T may exist in the galactic neighborhood.

"Is this the tip of an iceberg?" Zucker asked.

The first of the faint new dwarfs came as a surprise, he noted, but it was soon proved to be one of many, perhaps dozens.

Additional dwarf galaxy finds could help scientists unravel the mystery of dark matter, a theoretical form of matter that does not give off or reflect light yet accounts for the vast majority of mass in the universe. (Related: "Dark Matter Mapped in 3-D, Scientists Report" [January 8, 2007].)

Nitya Kallivayalil, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, presented separate findings on the velocity of the Large and Small Magellanic Clouds at the AAS meeting.

The clouds are two of the Milky Way's closest—and previously known—dwarf galactic neighbors.

Commenting on the significance of both studies, she said: "Dwarf galaxies might really be the keys to understanding how structure forms in the universe."

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