Assault on Andromeda: nearby galaxy had recent collision.

If the dinosaurs ever looked skyward, they might have been treated to a rare spectacle. About 210 million years ago, a small galaxy plunged into Andromeda--the spiral galaxy closest to the Milky Way. Streamers of stars created by the collision would have been visible for million of years. Although the minor galaxy moved on, Andromeda still holds signs of the encounter. These include a newly discovered ring of glowing dust surrounding the inner part of the galaxy.

Astronomers had previously found other features that suggested a collision: an outer dust ring, some warping of Andromeda's spiral disk, and loops and ripples in the halo of gas and dust surrounding the galaxy. But the new inner ring clinches the notion that a satellite galaxy recently barreled through Andromeda, David Block of the University of the Witwatersrand in Johannesburg and his colleagues report in the Oct. 19 Nature.

The ring "revolutionizes the history of the Andromeda galaxy," asserts Block. Although now rare, such galactic encounters were common in the cosmos billions of years ago and stimulated galaxy growth. Andromeda provides "an absolutely unique vantage point for studying head-on collisions, right on our doorstep," Block adds.

Block's team used NASA's infrared Spitzer Space Telescope to examine the inner part of the Andromeda galaxy, which lies just 2.5 million light-years from Earth and is visible to the naked eye. The new inner ring, composed of fine dust particles, shows up at some infrared wavelengths, but in visible light, the bright stars at the galaxy's core hide it.

The ring is about 4,900 light-years long and 3,300 light-years wide. Both the inner and outer rings are expanding like ripples in a pond. Such ripples appear whenever a small galaxy- collides nearly head-on with a larger one, says Block.

The only alternative model to account for Andromeda's overall disheveled appearance holds that a rotating bar of gas and dust at the galaxy's center has disturbed the galaxy's structure. Block's team reports that the radius of the inner ring is offset from Andromeda's bright center by roughly 1,600 light-years, or about 40 percent of the ring's average radius. In contrast, the outer ring's diameter is offset by just 10 percent. These offsets can't be explained by the rotating-bar theory.

Team members Frederic Bournaud and Francoise Combes of the Observatory of Paris say that they have identified the alleged hit-and-run galaxy. The astronomers' computer simulation reveals that a collision between Andromeda and its companion dwarf galaxy Ma2 reproduces the rings.

"The new inner ring adds evidence of a recent encounter, specifically a rare, head-on, bull's-eye collision, and [the researchers] add M32 to the top of the list of candidate culprits," comments Kirk Borne of the QSS Group at NASA's Goddard Space Flight Center in Greenbelt, Md.

Andromeda's future holds even more violence. Several billion years from now, scientists predict, the galaxy and the Milky Way will collide to become a single, giant, elliptical galaxy.