The University Record, January 18, 1993

Astronomers confirm ‘galactic fountain theory’

By Sally Pobojewski
News and Information Services

U-M astronomers analyzing X-ray emissions from NGC 891, a spiral galaxy similar to our own Milky Way, have found the first direct evidence for the existence of active “weather” patterns or giant galactic storms.

These violent storms take place in the galactic halo—a diffuse cloud of gas molecules and interstellar matter that surrounds the disk of a galaxy like a bun around a hamburger patty, according to Joel N. Bregman, associate professor of astronomy.

Storms appear to be triggered when huge bubbles of superheated gas burst open, shooting gaseous plumes 20,000 light-years out from the galaxy’s disk. (One light-year equals about 5.8 trillion miles.) As the gas cools, it loses buoyancy and crashes back onto the galaxy itself at speeds exceeding 100,000 miles per hour.

U-M astronomers Bregman and Rachel A. Pildis, with James M. Schombert of the Infrared Processing and Analysis Center at the California Institute of Technology, presented their galactic “weather report” at the American Astronomical Society (AAS) meeting in Phoenix, Ariz., earlier this month.

The astronomers detected evidence for galactic storms in X-ray images of NGC 891, located 30 million light-years from Earth in the direction of the constellation Andromeda. X-ray images were obtained from instruments on ROSAT, the Roentgen Satellite—an orbiting observatory developed jointly by Germany, the United Kingdom and the United States.

“NGC 891 is special because we view its galactic disk edge-on, which lets us see gaseous material ejected from the disk,” said Pildis, who holds a National Science Foundation graduate fellowship. “The hot gas emits X-rays, which are detected by ROSAT.”

“Astronomers believed hot gas was being expelled from the disks of many galaxies, including the Milky Way, but this is the first direct evidence confirming the ‘galactic fountain theory,’ ” Bregman said.

According to Bregman, bubbles of superheated gas that trigger galactic storms are only produced in galaxies with a high concentration of exploding stars or supernovae. “It probably requires about 40 supernovae exploding within 10 million years in one small area of a galaxy to start the process,” Bregman said.

The force of shock waves from these stellar explosions creates bubbles of gas with temperatures as high as 10 million degrees C, Bregman explained. The larger the bubble, the greater the likelihood that it will reach the edge of the galaxy’s disk and burst open, triggering the galactic storm cycle.

During their AAS presentation, Bregman, Pildis and Schombert also displayed optical images taken with the 94-inch Hiltner telescope at the Michigan-Dartmouth-MIT Observatory on Kitt Peak near Tucson, Ariz. These images reveal cooler skin surrounding the hot bubbles detected by ROSAT in NGC 891.

“These images show several bubbles, 1,000 to 5,000 light-years across, which are about to or have already broken out, expelling hot gas far above the disk,” Pildis said. “We’ve seen bubbles of hot gas in the Milky Way galaxy, but here we see evidence for bubbles and bubble breakout in other galaxies.”

Bregman explained that galactic storms may serve as a “pressure relief valve” limiting the degree of new star formation and playing a fundamental role in the evolution of a galaxy.

“Stars form when very cold gas, dust and other material get denser and denser until it finally collapses to produce a star,” Bregman explained. “Massive stars live briefly, but generate a great deal of heat and pressure. These galactic storms may relieve built-up pressure and heat by re-distributing mass and energy throughout the galaxy and limiting the level of star formation in any one area.”

Research on galactic weather was supported by the National Science Foundation and NASA.