Galactic Collision Spawns Megacomets

The Hubble Space Telescope has glimpsed massive balls of gas reeling like comets around a galaxy's center. These whirling dervishes may help scientists better understand the strange physics of galactic collisions.

The Hubble's dramatic images reveal in sharp detail the apparent aftermath of a head-on collision between two galaxies that left the larger one shaped like a wagon wheel. The images and their interpretation appear in the latest issue of the Astronomical Journal.

The galaxy, popularly called the Cartwheel, has two bright concentric rings connected by wispy spokes. Scientists think such shapes are formed when a smaller galaxy smashes into the center of a regular spiral galaxy. "It's like a pebble thrown into a pond," says Curt Struck, an Iowa State University astronomer and a member of the team that took the image. In this case gravity waves fan outward, forming the rings of gas, dust, and stars, Struck says. The outer ring appears blue from the light of young stars, but the inner ring's yellow color has long been a puzzle. Yellow light suggests that this ring contains few young stars, although scientists had presumed that turbulence after the collision would spur star formation. Although the mystery is not completely solved, Struck says, the detailed image reveals some interesting clues.

Surrounding the yellow center is a ring of gas clouds that look like giant comets. Their "heads" are a few hundred light-years across, and their tails are between 1000 and 5000 light-years long. The scientists think the structures might have arisen from blobs of gas splashed out of the galaxy during the collision that are now falling back toward the center faster than the oscillating gravity waves. The motion creates a shock wave, says Struck: "It's the equivalent of an interstellar sonic boom" leaving a wake. The comet tails are blue and the heads are bright white, which to Struck indicates they might contain the missing young stars.

But at this point such explanations are only educated guesses, Struck says. The scientists must analyze the spectral signatures of the megacomets to determine exactly what kinds of gas and dust they contain, he says. François Schweizer, an astronomer at the Carnegie Institution of Washington, agrees. "It's an interesting hypothesis," he says, "but there's more work to be done."