Galaxy Invading Milky Way; Apparently Contains Much Unseen Matter

But don't panic; we've got the size advantage.

Astronomers have known since 1994 that a small galaxy orbitingthe Milky Way has actually entered Earth's home galaxy. A team ofscientists made the discovery unexpectedly while analyzing starsin the concentrated, elliptical bulge at the center of our owngalaxy.

They realized that certain stars, which all had essentially thesame velocity, were not moving in the proper manner to be in thecenter of the Milky Way. They were found to be in a dwarf galaxylocated along the line of sight to the center of our galaxy, buton the far side of the Milky Way.

"It's close enough that you can study individual stars in it thesame way that you study stars in our galaxy," said Rosemary Wyse,an astrophysicist at Johns Hopkins University. She will presentan overview of the galaxy on Feb. 13, during an annual meeting ofthe American Association for the Advancement of Science, at thePhiladelphia Marriott hotel, 1201 Market St. Her paper, entitled,The Invasion of the Milky Way Galaxy, will be delivered as partof the meeting's Galaxies in Collision talks, from 2 to 5 p.m.

Known as the Sagittarius dwarf spheroidal galaxy -- since it isobserved in the direction of the constellation Sagittarius -- it isroughly one-tenth the diameter of the Milky Way but weighs lessthan one-thousandth as much as the Milky Way. It is nearly asclose to the center of our own galaxy as is the Earth. The galaxyis one of nine known nearby, or companion, dwarf spheroidalgalaxies to the Milky Way.

"The other galaxies are far enough away that you don't reallythink of them as invading your space," said Wyse, a professor inthe Johns Hopkins Department of Physics and Astronomy. "You canmore or less ignore them. But Sagittarius has come right in."

Wyse, and four other astronomers, have made new observations ofstars inside of the dwarf galaxy and have calculated that itmakes a complete orbit around the Milky Way in less than onebillion years. As the small galaxy orbits around the center ofthe Milky Way, it plunges into the central regions of the largergalaxy. The astronomers have inferred that it has orbited ourgalaxy at least 10 times.

Findings from that work have led to the conclusion that the smallgalaxy is surprisingly sturdy; after orbiting the Milky Way thatmany times, the smaller galaxy should have been pulled apart byour galaxy's strong gravitational forces, unless it harbors morematter 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 ontoits stars," Wyse said.

The astronomers analyzed spectra from observations they made withthe Anglo-Australian Telescope and the Cerro Tololo InteramericanObservatory. The other astronomers involved in the work wereNicholas Suntzeff, from the Cerro Tololo Observatory, RodrigoIbata, from the European Southern Observatory, both located inChile; and Gerard Gilmore, at the Institute of Astronomy and MikeIrwin at the Royal Greenwich Observatory, both located in theUnited Kingdom. Ibata, Gilmore and Irwin made the originaldiscovery in 1994.

It is important to study Sagittarius in the overall quest tolearn how galaxies form and evolve, Wyse said. Astronomerspropose that such small companion galaxies might have merged tomake 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 ofstars in the Milky Way and other galaxies. For example, how doesthe halo form? Does it represent the shredded bits of smallsatellites like Sagittarius?

Wyse said her findings indicate that, at most, 10 percent of thestars in the halo came from dwarf galaxies like Sagittarius,which merged with the Milky Way over the past 8 billion years orso.

Studying Sagittarius may help answer other questions, such as:Does the central bulge of our galaxy also come from mergingcompanion galaxies, but from more dense pieces that were capableof migrating to the center? What types of stars make up othergalaxies that we can study in great detail?

Sagittarius also gives astronomers an opportunity to study thedark matter of another galaxy up close. Scientists hope toclarify what the nature of the dark matter is. According totheories and observations, the universe contains more matter thanis directly observed using current technology. Astronomers thinkthat at least 90 percent of the mass in the universe is yet to beobserved directly.

By comparing the number of stars and the luminosity ofSagittarius, astronomers will be able to learn what kinds of darkmatter prevail in that galaxy. In the near future, astronomershope to learn whether other companion galaxies also are invadingthe Milky Way. Because the Sagittarius discovery was made bychance, it is possible that such galaxies have gone undetected.

Picking out the companion galaxies would be difficult because itwould involve analyzing the fine color differences of stars inthe most concentrated regions of the Milky Way. "Unless you knowit's there, you can't find it," Wyse said, referring toSagittarius.

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Note: An image of the Sagittarius dwarf galaxy entering the MilkyWay is available on-line at the following Web address:http://tarkus.pha.jhu.edu/~wyse/araa97_fig1.gif