Discovery could force theorists to rethink origins of galaxy
THE APPARENT RING might be merely an outer spiral arm of the galaxy, one group of researchers said, but the evidence suggests it is instead a well of several hundred million stars that encircles the entire Milky Way out beyond the main galactic disk.
The findings, made by two separate groups, were announced here today at the 201st meeting of the American Astronomical Society.
PROBLEM FOR THEORY
The Milky Way�s main disk stretches across roughly 100,000 light-years of space. Most of the galaxy�s tens of billions of stars reside within this relatively thin disk and a thicker bulge near the center. Our solar system sits about 30,000 light-years from the middle. In widely separated patches of sky, the researchers found stars beyond the main disk but along the same plane.
One study resulted from the Sloan Digital Sky Survey, an effort to map in detail one-fourth of the entire sky using the Apache Point Observatory in New Mexico.
A four-year examination of Sloan data by an American-led team of astronomers found tens of thousands of stars in a structure outside the galaxy. The structure appears to be part of a ring that study members said might be the result of a collision or collisions billions of years ago between the Milky Way and one or more smaller galaxies, whose stars were redistributed into the newly observed stream.
"When we find large groups of stars formed into rings it�s an indication that at least part of our galaxy was formed by a lot of smaller or dwarf galaxies mixing together," said Heidi Jo Newberg, a researcher at the Rensselaer Polytechnic Institute who co-led the study.
The stars in the ring orbit the galactic center at about half the speed of our sun, said study member Brian Yanny of the Fermi National Accelerator Laboratory. The ring appears to be about 10 times thicker than the disk, Yanny said.
A merger, if it is what created the ring, likely occurred as long ago as 10 billion years, he said.
That galaxies develop through mergers has long been supposed. But another research team, based in Europe, looked into the same apparent ring and found troubling details.
The aged stars in the ring are problematic for current theory, explained Rodrigo Ibata of the Observatoire de Strasbourg in France and a member of the European-led team.
"This ring is unusual in that it appears to consist only of old stars," Ibata told SPACE.com. "Though there are several galaxies known with bright rings of young stars, none are known to have a ring similar to that of the Milky Way."
The ring might be the product of the Milky Way swallowing a smaller "satellite" galaxy, Ibata and his colleagues say. Or perhaps it was created by some disturbance to the galaxy�s main disk, wherein some stars were kicked outward to form the ring. Either way, it presents a puzzle.
"If the ring turns out to be due to a satellite galaxy, it would mean that we are seeing the Milky Way cannibalizing a small galaxy and incorporating it into the galactic disk, which would affect our understanding of how galactic disks grow," Ibata said. "Alternatively, if the ring turns out to be a disturbance of the disk, its very old age will pose a daunting problem to current theoretical models of galaxy formation which predict that galaxy disks grow slowly over time by building up outwards from the central regions, so that the outer regions are relatively young."
HARD TO SEE
Other galaxies might actually have similar rings that simply haven�t been detected.
"From afar the Milky Way would be seen to have a red ring around it," Ibata said of the newfound structure. "However, due to its low surface brightness, the ring would be difficult to detect, which may account for the fact that such structures have not been seen in other galaxies."
Seeing through and across the main galactic plane in our own galaxy is tough work, too. The newly detected stars had previously gone unnoticed because they were obscured by intervening space dust and the light of other stars. Observations were possible only recently because of advances in electronic cameras that record faint light over long periods of time.
The stars weigh, on average, more than our sun. The ring is estimated to contain a mass equal to about 1 billion suns, which would represent only about 1 percent of the galaxy�s total mass, the European team said.
"Our mass estimate is based on the assumption that the structure is a complete ring that encircles the galaxy," said Annette Ferguson, Ibata�s colleague and leader of the European team. "Our observations only allow us to trace the structure around about one-third of the circumference of the galaxy. This means that it is possible that the total mass in the ring may be even less than a billion solar masses."
Closure on the extent of the ring is not likely to come anytime soon.
"It will be difficult to trace the feature much further than our current observations, since that involves looking through the bright inner parts of the galaxy and trying to count the stars on the other side," said Ferguson, a researcher at University of Groningen in the Netherlands.
The European observations were made with the 2.5-meter Isaac Newton Telescope on La Palma in the Canary Islands.