The Case of the Cosmic Bends

By Natalie Angler

A spectacular magnetic field may be sculpting the Milky Way

Astronomers have identified fiery quasars and the wispy shadows of supernova explosions at the very edges of the known universe. Yet the core of our own galaxy, the Milky Way, has long been an elusive stranger. Thick clouds of interstellar dust and gas absorb most of the light from the galaxy's central bulge long before it reaches planet earth, a small and distant suburb 30,000 light-years away from the Milky Way's midsection.*

Three radio astronomers have now probed behind that celestial curtain and found a spectacular feature that has never before been closely observed: a band of gas 10 to 20 light-years thick, seemingly composed of lacy filaments, stretching up to 600 light-years above the plane of the Milky Way. The belt is the first hint that an enormous magnetic field may be far more important in shaping the core of the galaxy than had previously been imagined. Scientists have yet to gauge the full impact of the finding, but it could undermine existing theories about star formation at the galactic center and possibly illuminate how the Milky Way evolves.

Indeed, the appearance of the magnetic arc was so unlike anything ever before observed in the universe that for weeks its three discoverers refused to accept their own finding. "We were very frustrated," says Farhad Yusef-Zadeh, a graduate student at Columbia University and one of the trio. "The first thing we thought was to try to get rid of the structure, to fudge the data. But it wouldn't go away."

Astronomers have long been trying to study the center of the galaxy, using the feeble and sporadic X rays, gamma rays and other invisible electromagnetic energy that manages to seep through the celestial dust in the Milky Way. But the resulting computer-derived pictures have been too small and too blurry to provide details.

All that began to change in 1980 when the National Radio Astronomy Observatory inaugurated the Very Large Array (VLA), an extraordinary $78.3 million string of 27 radio telescopes set out like giant Dixie cups across 21 miles of the desert of New Mexico. The VLA is one of a kind: the separate signals received by the 27 instruments can be melded into a single seamless picture, providing scientists with huge, highly detailed portraits of the heavens. Pointing the VLA toward the galactic center, Yusef-Zadeh and his Columbia colleague, Don Chance, along with Astronomer Mark Morris of UCLA, mapped out 200 light-years of the galaxy.

At first glance the arc looked ordinary, like colliding clouds of galactic dust or a swatch of newborn, fuzzy stars. Calculations soon ruled out both possibilities. The astronomers then wondered if the threadlike arc might be the tattered remnant of interstellar material that had been sucked into a black hole at the galaxy's center; that notion too was discarded. Explains Frank Kerr, provost of the sciences at the University of Maryland, who has studied the structure of the Milky Way since 1951: "You'd expect a black hole to be pulling in all directions, not in an isolated arc." Kerr and others are now leaning toward the presence of a powerful and mysterious magnetic field as the most plausible cause.

The magnetic field, if it exists, defies scientists' notions about proper celestial behavior. Perhaps even more baffling to astronomers, the belt of gas rises up at a right angle to the plane of the Milky Way, an extraordinary position for a magnetic field; normally it should lie in the galactic plane, its lines of force trapped like hair around a drain. Attempting to account for the bizarre properties of the field, some scientists are postulating the existence of a "dynamo" at the center of the galaxy, a source of relentless energy. But investigators cannot ascribe the dynamo to any known generating force, like giant stars or a black hole; those phenomena are too small to account for the size of the magnetic field.

So far the strange arc has raised only questions, not answers. Once gravity was thought to be the sole force shaping the large scale of the galaxy. Now it has lost some more of its supremacy to whatever molded the giant ribbon of gas. Listening to the radio waves crackling noisily from the center of the galaxy, astronomers were sure that the bulge of dust was a maternity ward for new stars. Now the strange arc has revealed itself as one mighty source of those waves, raising the startling possibility that the middle of the Milky Way is a barren womb. Seeking to solve the mysteries, astronomers will return to the Very Large Array again and again, tracking the motion of the gas within the filaments and possibly the origins of the dynamo at the core of the galaxy, where the answer to many questions may be revealed .

-- By Natalie Angier.

Reported by Dan Fagin/New York and Carol Foote/San Francisco

* A light-year is the distance that light, traveling at 186,000 miles per sec., covers in one year.

With reporting by Dan Fagin; Carol Foote