The mystery of the missing gas.

The mystery of the missing gas

The shapes of galaxies vary widely -- from compact balls of stars known as elliptical galasxies to spirals with sweeping arms extending from a central bulge, and irregulars, which have no bulge. Spiral galaxies, like our own, contain large amounts of interstellar gas -- mainly hydrogen and helium -- from which new stars form. Until recently, elliptical galaxies appeared to contain no detectable gas. That difference seemed consistent with the supposition that elliptical galaxies contain only old stars while spiral galaxies contain stars of all ages.

But the picture wasn't complete. What happens to the gas shed by dying stars in elliptical galaxies? What fuels the enormous amount of radio-wave activity observed coming from the centers of such galaxies? These questions made it worthwhile for researchers, using improved instruments, to continue their search for traces of interstellar gas, and they started finding it in isolated cases. Now, Gillian R. Knapp of Princeton (N.J.) University and her colleagues have discovered that almost all elliptical galaxies contain interstellar gas.

Knap and her colleagues came to their conclusion after carefully analyzing data collected in 1983 by the Infrared Astronomy Satellite (IRAS). They put together thousands of individual observations covering several hundred galaxies, averaging the data to increase sensitivity and thereby detect fainter sources of infrared radiation, which would signal the presence of interstellar gas.

"The results amazed me," Knapp says. "These findings are in disagreement with the conventional wisdom that ellipticals contain no gas. It gives us a whole new way of looking at the evolution of elliptical galaxies." For example, it's likely that, like spirals, elliptical galaxies contain stars of all ages, though probably with a different mix. This calls into question the assumption that elliptical galaxies look much the same now as they did billions of years ago. It means that astronomers can no longer safely use the brightness of elliptical galaxies as a way of mapping the expansion of the universe.

In addition, the results indicate that enough gas is present to power the radio-wave sources found at the centers of elliptical galaxies. One possibility is that the emissions occur as gas atoms are pulled into black holes sitting at the galactic centers.

"There remain many areas to investigate," says Knapp. For example, how do the stars and gas in elliptical galaxies evolve, and what did ellipticals look like in the past? "We hope to tackle these and other questions in the coming few years."