SagDEG: A Galaxy Invading Milky Way

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EMBARGOED FOR RELEASE: 13 FEBRUARY 1998 AT 14:00:00 ET US

A Galaxy Invading Milky Way

Apparently Contains Much Unseen Matter

(Release coincides with talks during a meeting of the

American Association for the Advancement of Science)

The Milky Way is being invaded by another galaxy. But don’t panic; we’ve

got the size advantage.

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; and 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 at the following Web address:

http://tarkus.pha.jhu.edu/~wyse/araa97_fig1.gif (our image)

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