The Milky Way. Photo: Anglo-Australian Observatory

A telltale bulge in the disk of the Milky Way galaxy may be the remnants of a smaller galaxy consumed billions of years ago as our galaxy formed, astronomers announced this week.

The discovery, based on the observations of 1,500 Sun-like stars and announced at a meeting of the American Astronomical Society in San Diego, may provide scientists with new data to support -- or challenge -- existing models of how galaxies are created.

Astronomers from the U.S., U.K., and Australia detected a "puffed up" region of old stars known as the thick disk as part of the Two Degree Field (2dF) Old Stellar Population Survey, an effort to map out the structure of the galaxy. That increased thickness in the outer regions of the galaxy is thought to be caused by a collision between the Milky Way and a smaller satellite galaxy that pumped energy into that region of the Milky Way, increasing its thickness.

"Just as the tidal interactions between the Moon and the Earth cause distortions, there's going to be tidal interactions as a satellite galaxy comes into the Milky Way, and those can be severe enough to actually tear mass off the outer parts of the satellite galaxy," explained Rosemary Wyse of Johns Hopkins University. "Those stars should be left behind on an orbit that is similar to the orbit of the satellite galaxy at that time."

Astronomers believe that large galaxies like the Milky Way were formed by the mergers of a number of smaller galaxies, and the thick disk is the remnant of the last such collision. "What's not clear is what merged when," said Wyse, "and this is very significant in trying to constrain the theories of how galaxies evolve."

The discovery comes at a time when astronomers are struggling with models of galactic formation. "The favorite theory of galaxy formation right now is that based on a universe that's dominated by cold dark matter," said Wyse. Under that theory, large galaxies form from the collision of smaller ones like the remnant seen in the Milky Way. However, Wyse notes, "it also predicts that lots of the small galaxies survived this process. And we do not see nearly as many small ones left as are predicted to survive."

At the same time other models of galaxy formation have been brought forward. A paper published in the journal Nature earlier this month noted the existence of a massive cloud of hydrogen gas around a distant, young galaxy. The cloud has enough gas to form 100 billion stars, enough for a fairly large galaxy without the need to build up gradually through collisions.

Wyse and her colleagues plan to eventually survey 10,000 old Sun-like stars in the galaxy using a wide-field spectrograph mounted on the 4-meter Anglo-Australian Telescope in an effort to look for any additional features that may be remnants of galactic collisions.