By comparing computer simulations of a galaxy collision with actual observations, astronomers at the University of Illinois have found discrete star-formation episodes that may help explain the prodigious star-formation rates that occurred in the early universe.

"Studying galaxy interactions is crucial to understanding the large-scale star formation that occurred when the universe was young -- about one third of its present age -- when it is thought that star formation was at its height," said Susan Lamb, a UI professor of physics and of astronomy. Considered rare events today, galaxy collisions and mergers were much more common in the early universe.

To investigate the sequence of star formation triggered by a galaxy collision, Lamb and graduate student Nathan Hearn used a three-dimensional numerical simulation of a collision between a gas-rich disk galaxy and a gas-free elliptical galaxy. The simulation parameters closely matched the physical characteristics of a pair of galaxies, called Arp 119, which collided a few hundred million years ago.

"By matching our model at different stages of the simulation with observations of Arp 119 taken at different wavelengths, we can explore the history and physical conditions of star-forming regions created in the collision process," Lamb said.

Regions of very strong radio emission, for example, indicate where the earliest bursts of star formation occurred, Lamb said. Regions of intense hydrogen-alpha emission show where stars have formed more recently, while observations in the near infrared provide information about the older stellar population that was present before the collision took place.

The collision process -- which lasted about one hundred million years -- generated a strong density wave that rippled through the disk galaxy like a wave from a pebble tossed in a pond, Lamb said. The passing density wave triggered large-scale star formation in the highly disturbed gas disk.

"Stars were not formed continuously as the density wave spread through the galaxy, however," said Hearn, who presented the researchers' latest findings at a meeting of the American Physical Society, held April 28-May 1 in Washington, D.C.

"We have identified three major episodes of star formation, visible in luminous rings and arcs in the spiral galaxy," Hearn said. "An initial burst took place soon after the collision, another occurred about 22 million years later, and the latest burst is ongoing."

Such episodic star formation could be due to gravitational or hydrodynamic instabilities in the disturbed gas, and may be indicative of processes that occurred on a grand scale in the early universe.

"By making detailed comparisons between simulations and observations, we can build bridges that go backwards in time," Lamb said. "This allows us to investigate conditions similar to those that were present in the early universe."

A paper describing the results appeared in the April 20 issue of The Astrophysical Journal.