Deep behind the constellation Sagittarius, a titanic struggle is under way. It is an intergalactic contest that even the Borg, Star Trek's evil assimilators, would envy: The Milky Way is slowly absorbing an invading galaxy.
The invader, one of the Milky Way's nine known small companion galaxies, moves through our galaxy once every billion years. This time around, it's giving astronomers a rare opportunity to look in their own cosmic backyard for answers to questions about the formation of galaxies.
The invasion "has some very important implications for the formation and evolution of galaxies in general," says Rosemary Wyse, an astrophysics professor at The Johns Hopkins University in Baltimore, Md.
It may yield clues about how spiral galaxies like the Milky Way develop a vast bulge of stars at their hubs and it could help pin down the nature of dark matter, which may be 90 percent of the matter in the universe.
The Milky Way's invader, known as the Sagittarius dwarf spheroidal galaxy, was discovered in 1994 by a team of astronomers interested in measuring the motions of stars in our galaxy.
Recent work by Dr. Wyse and colleagues shows that the invader's star population is only 1/10,000th the size of the Milky Way's. Its mass is roughly 1/1,000th that of our galaxy. As seen from Earth, the dwarf galaxy is moving up from underneath the Milky Way and passing close behind its center.
A tenacious galaxy
What puzzles astronomers is the dwarf galaxy's tenacity. Faced with the immense pull of the Milky Way's gravity, "it should have been torn apart," Wyse says. Instead, after an estimated 10 orbits, it has lost stars but retains a recognizable - if stretched - form.
Wyse suggests that a dense halo of dark matter, whose presence is inferred from the galaxy's motions, surrounds the Sagittarius galaxy, holding its remains intact.
During the past 20 years, astronomers have come to appreciate the role such mergers play in forming galaxies. Initially, the two major varieties of galaxies - pinwheel-like spirals, such as the Milky Way, and vast blobs of stars known as elliptical galaxies - were thought to exist in "splendid isolation," says Brad Whitmore, an astronomer at the Space Telescope Science Institute in Baltimore.
Although astronomers had observed colliding galaxies, collisions were thought to be rare. Ellipticals, which constitute roughly 10 percent of all observed galaxies, were considered to be galactic graybeards that were much older than spirals.
In 1972, however, two researchers using a crude computer program mimicked mergers between four known colliding galaxies. Images the program generated looked like the colliding galaxies, complete with tails of stars and gas streaming into space. When the program continued running, the colliding galaxies merged to form elliptical galaxies. Five years later, the team estimated that 10 percent of all galaxies could be the remnants of mergers.
A harmless collision
When galaxies collide, their stars move past each other unscathed, because much of a galaxy's volume is open space. Dr. Whitmore points out, for example, that if the sun were the size of a pea, its nearest neighbor would be 100 miles away. That open space is not empty, however. It contains gas and dust that can interact with gas and dust from an incoming galaxy.
Studies using the Infrared Astronomy Satellite show that such collisions trigger bursts of star formation, as gases in the colliding galaxies combine to form dense regions that become the gravitational seeds for stars. And the Hubble Space Telescope, looking deep into the universe's past, has uncovered a nursery of young, odd-looking star groups - some colliding - that appear to be among the earliest galaxy wannabes.
Interest in galaxy collisions has prompted astronomers to look at the Milky Way's nearest neighbor, the Andromeda galaxy, 2 million light-years away. The Milky Way is headed in its direction. "We'll know within 10 years if Andromeda and the Milky Way will eventually collide," Whitmore told scientists here at the American Association for the Advancement of Science annual meeting, looking toward a new generation of telescopes that will help pin down Andromeda's speed and direction
In the meantime, if the dwarf galaxy in Sagittarius is any indication, the Milky Way is still a work in progress.