Milky Way Changing Shape, Assimilating Other Galaxies When Stars Collide
Peter N. Spotts, writer of The Christian Science Monitor, The Christian Science Monitor
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. …