February 4, 1998
Astronomers have known since 1994 that a small galaxy orbiting the Milky Way has actually entered Earth's home galaxy. A team of scientists made the discovery unexpectedly while analyzing stars in the concentrated, elliptical bulge at the center of our own galaxy.
They realized that certain stars, which all had essentially the same velocity, were not moving in the proper manner to be in the center of the Milky Way. They were found to be in a dwarf galaxy located along the line of sight to the center of our galaxy, but on the far side of the Milky Way.
"It's close enough that you can study individual stars in it the same way that you study stars in our galaxy," said Rosemary Wyse, an astrophysicist at Johns Hopkins University. She will present an overview of the galaxy on Feb. 13, during an annual meeting of the American Association for the Advancement of Science, at the Philadelphia Marriott hotel, 1201 Market St. Her paper, entitled, The Invasion of the Milky Way Galaxy, will be delivered as part of the meeting's Galaxies in Collision talks, from 2 to 5 p.m.
Known as the Sagittarius dwarf spheroidal galaxy -- since it is observed in the direction of the constellation Sagittarius -- it is roughly one-tenth the diameter of the Milky Way but weighs less than one-thousandth as much as the Milky Way. It is nearly as close to the center of our own galaxy as is the Earth. The galaxy is one of nine known nearby, or companion, dwarf spheroidal galaxies to the Milky Way.
"The other galaxies are far enough away that you don't really think of them as invading your space," said Wyse, a professor in the Johns Hopkins Department of Physics and Astronomy. "You can more or less ignore them. But Sagittarius has come right in."
Wyse, and four other astronomers, have made new observations of stars inside of the dwarf galaxy and have calculated that it makes a complete orbit around the Milky Way in less than one billion years. As the small galaxy orbits around the center of the Milky Way, it plunges into the central regions of the larger galaxy. The astronomers have inferred that it has orbited our galaxy at least 10 times.
Findings from that work have led to the conclusion that the small galaxy is surprisingly sturdy; after orbiting the Milky Way that many times, the smaller galaxy should have been pulled apart by our galaxy's strong gravitational forces, unless it harbors more matter than indicated by the number of visible stars it contains.
"It's just got a lot of dark matter, so it's able to hold onto its stars," Wyse said.
The astronomers analyzed spectra from observations they made with the Anglo-Australian Telescope and the Cerro Tololo Interamerican Observatory. The other astronomers involved in the work were Nicholas Suntzeff, from the Cerro Tololo Observatory, Rodrigo Ibata, from the European Southern Observatory, both located in Chile, Gerard Gilmore, at the Institute of Astronomy and Mike Irwin at the Royal Greenwich Observatory, both located in the United Kingdom. Ibata, Gilmore and Irwin made the original discovery in 1994.
It is important to study Sagittarius in the overall quest to learn how galaxies form and evolve, Wyse said. Astronomers propose that such small companion galaxies might have merged to make the larger galaxies like the Milky Way.
Astronomers are trying to understand more about a diffuse "halo" of stars that surrounds the central, elliptical bulge and disk of stars in the Milky Way and other galaxies. For example, how does the halo form? Does it represent the shredded bits of small satellites like Sagittarius?
Wyse said her findings indicate that, at most, 10 percent of the stars in the halo came from dwarf galaxies like Sagittarius, which merged with the Milky Way over the past 8 billion years or so.
Studying Sagittarius may help answer other questions, such as: Does the central bulge of our galaxy also come from merging companion galaxies, but from more dense pieces that were capable of migrating to the center? What types of stars make up other galaxies that we can study in great detail?
Sagittarius also gives astronomers an opportunity to study the dark matter of another galaxy up close. Scientists hope to clarify what the nature of the dark matter is. According to theories and observations, the universe contains more matter than is directly observed using current technology. Astronomers think that at least 90 percent of the mass in the universe is yet to be observed directly.
By comparing the number of stars and the luminosity of Sagittarius, astronomers will be able to learn what kinds of dark matter prevail in that galaxy. In the near future, astronomers hope to learn whether other companion galaxies also are invading the Milky Way. Because the Sagittarius discovery was made by chance, it is possible that such galaxies have gone undetected.
Picking out the companion galaxies would be difficult because it would involve analyzing the fine color differences of stars in the most concentrated regions of the Milky Way. "Unless you know it's there, you can't find it," Wyse said, referring to Sagittarius.
Note: An image of the Sagittarius dwarf galaxy entering the Milky Way is available on-line.