Astronomers find nine new dwarf galaxy candidates

Scientists from the United States and Britain said Tuesday they have discovered nine ultra- faint objects that are believed to belong to the rare category of dwarf galaxies orbiting our home galaxy, the Milky Way.

The new satellites were found in the southern hemisphere near the Large and Small Magellanic Cloud, the largest and most well- known dwarf galaxies in the Milky Way's orbit, according to two papers describing the findings in the Astrophysical Journal.

They are a billion times dimmer than the Milky Way, and a million times less massive. The closest of these objects is about 95,000 light years away, while the most distant is more than a million light years away.

"The discovery of so many satellites in such a small area of the sky was completely unexpected," said Sergey Koposov of the University of Cambridge, lead author of one of the two papers. "I could not believe my eyes."

Three of the discovered objects are definite dwarf galaxies, while others could be either dwarf galaxies or globular clusters, objects with similar visible properties to dwarf galaxies, but not held together with dark matter, Koposov said.

The findings were jointly announced by the Cambridge team and an independent group of researchers with the Dark Energy Survey, headquartered at the U.S. Department of Energy's Fermi National Accelerator Laboratory. Both teams used the publicly available data taken during the first year of the Dark Energy Survey, a five- year effort to photograph a large portion of the southern sky in unprecedented detail, to carry out their analysis.

Dwarf galaxies are the smallest known galaxies, the faintest of which contain just 5,000 stars. By contrast, the Milky Way, an average-sized galaxy, contains billions of stars. Scientists have previously found more than two dozen of these galaxies around the Milky Way, but about half of them were discovered in 2005 and 2006.

Since they contain up to 99 percent dark matter and just one percent observable matter, dwarf galaxies are ideal for testing whether existing dark matter models are correct. Dark matter, which makes up 25 percent of all matter and energy in our universe, is invisible, and only makes its presence known through its gravitational pull.

"Dwarf satellites are the final frontier for testing our theories of dark matter," said Vasily Belokurov of the Cambridge team. "We need to find them to determine whether our cosmological picture makes sense. Finding such a large group of satellites near the Magellanic Clouds was surprising, though, as earlier surveys of the southern sky found very little, so we were not expecting to stumble on such treasure."

The closest of these pieces of "treasure" is so close to the Milky Way that it's being torn apart by the latter's massive tidal forces. The most distant is right on the fringes of the Milky Way, but is about to get pulled in, the researchers said.