Astronomers have discovered two streams of stars in the Southern Galactic hemisphere that were torn off the Sagittarius dwarf galaxy, proving that the Milky Way galaxy continues to gulp down its small neighboring dwarf galaxies.

According to a team of astronomers led by Sergey Koposov and Vasily Belokurov of Cambridge University, the discovery came from analyzing data from the latest Sloan Digital Sky Survey (SDSS-III). The scientists said that the new streams were related to the two previously known streams in the Northern Galactic hemisphere.

"We have long known that when small dwarf galaxies fall into bigger galaxies, elongated streams, or tails, of stars are pulled out of the dwarf by the enormous tidal field," said Sergey Koposov.

The remnants of the Sagittarius dwarf galaxy, which used to be one of the brightest of the Milky Way, was found to be so small that it has lost half of its stars and all its gas over the last billion years. Sagittarius was known to have two tails, one in front of and one behind the remnant.
Prior to the SDSS-III imaging, Sagittarius was shown with its tidal tail in the Northern Galactic sky in 2006, with one of the tails was forked into two.

"That was an amazing discovery," said Vasily Belokurov, from the University's Institute of Astronomy, "but the remaining piece of the puzzle, the structure in the South, was missing until now."

The astronomers analyzed density maps of over 13 million including the Southern Galactic sky. These showed that the Sagittarius stream in the South is split into two, the brighter stream which is more enriched with iron and is said to be the younger one and a dimmer stream which could be the older one.

Three theories were presented by scientists. One is that the Sagittarius dwarf galaxy was perhaps a part of a binary galactic system, similar to the present day Large and Small Magellanic Clouds, and each of these producing a leading and trailing tail. Then falling into the Milky Way Galaxy the tails become four in all.

A co-author, Geraint Lewis of Sydney University, proposes another theory. "Perhaps the Sagittarius dwarf galaxy has suffered an encounter with an object in the game of Galactic billiards. Maybe a collision with a massive clump of dark matter, or even another satellite galaxy, has split each of the streams into two," he said.

A third theory suggests that debris from Sagittarius may have spread into different streams at different points in time and that different epochs may have suffered different amounts of precession in the Galaxy, causing the split streams.

Whatever the explanation, SDSS-III has provided a wealth of new information on the engorgement of the Sagittarius galaxy.