Cloud links quasars to Seyfert galaxies.

Cloud links quasars to Seyfert galaxies

Quasars invite superlatives -- brightest, farthest, strangest. Though no larger than a single solar system, they shine with the energy of trillions of stars. Now, the quasar known as 3C273 has contributed another distinction: the first indication that a hydrogen cloud exists near a radio-loud quasar.

This discovery, reported in the Sept. 21 NATURE, strengthens the link between quasars and Seyfert galaxies, their more mundane cousins. Less active than quasars. Seyfert galaxies emit X-rays, radio signals and characteristic light spectra. Scientists classify Seyferts as Type 1 or Type 2 by their radiation and spectra.

But in recent years, astronomers have speculated that Types 1 and 2 represent the same thing viewed from different angles (SN: 6/25/88, p.404). They found evidence that a thick torus, or doughnut, of gas surrounds a Seyfert's center and obscures its activity when viewed from the side. It may also fuel the galaxy's activity.

Quasars share many characteristics with Seyfert galaxies, but until now scientists had no evidence that gas clouds accompany or fuel quasars.

In a general survey of hydrogen in the universe, three astronomers have found infrared emissions showing that 3C273 contains molecular hydrogen. Kimiaki Kawara and Minoru Nishida of Japan's Kyoto University, working with Brooke Gregory of the Cerro Tololo Inter-American Observatory in Chile, used a novel eight-channel detector for the study

"It shows that the molecular emission observed in 3C273 is the same type that seems to be emanating from the central engines of Type 2 Seyferts," says astrophysicist Mitchell C. Begelman of the University of Colorado at Boulder. "It's the first direct evidence that there is molecular gas subject to intense heating."

The find indicates not only that quasars may have the same type of structure as Seyfert galaxies but also that the galaxies surrounding quasars may differ from expectations. Like most quasars that give off strong radio signals, 3C273 is thought to lie at the center of an elliptical galaxy, which should contain little hydrogen, says astronomer Joseph S. Miller of the University of California, Santa Cruz.

"This is new information about an object we don't know much about," he says. "It shows there is a large amount of neutral hydrogen in this radio-loud quasar, and that indicates that in a giant elliptical, things are very different than we thought."