Invisible Dynamos

Why do galaxies have magnetic fields? It may be because most of them harbor giant black holes at their centers.

By Tim Folger|Friday, July 01, 1994
RELATED TAGS: STARS

Astronomers have known for more than 30 years that galaxies have magnetic fields. And they’ve argued over the origin of those fields for almost as long. At first glance the problem would appear to have a simple solution: a galaxy’s magnetic field is just the combined fields of all its stars, planets, gas, and dust. But galaxy mavens quickly ruled out that option years ago, says Robert Rosner, an astrophysicist at the University of Chicago.

The magnetic field that’s observed in our galaxy is highly organized, he says. Our galaxy is a flat disk, and we’re sitting in this disk. When we look out, what we see is a large-scale field that is parallel to the disk; it lies in the disk. Now think of stars and supernovas and God knows what else spewing out magnetic fields. You’d get a mess. You’d be stunned if you averaged all those fields and got one that was in the plane of the disk. There’s no way of making that work.

Rosner and two University of Chicago colleagues, Sandip Chakrabarti and Samuel Vainshtein, have recently come up with a scheme they believe does work. Galactic magnetic fields, they suggest, are produced by a ring of electrically charged gas rotating around a giant black hole at the center of a galaxy.

Why did Rosner and his cohorts feel compelled to invoke black holes? About three years ago they and another research team working independently at Princeton found a serious flaw in what has been the most widely accepted theory of galactic magnetic fields--a theory that with slight variations has also been used to explain the magnetic fields of individual stars such as the sun.

According to this theory, magnetic fields in galaxies arise from the turbulent motions of interstellar gases. These gases--the expanding detritus of exploded stars, or material expelled in the stellar winds of living stars--fill the galaxy, forming the raw material from which new stars are constantly being made. Much of this star stuff carries traces of magnetic material from the parent stars. As these countless magnetized clouds of gas drift through our rotating galaxy, they behave much like rising or falling masses of air on our rotating Earth: they spin. Meteorologists know that a rising mass of air on Earth will spin around an axis, says Rosner. If the air mass is large enough, you get a hurricane. This is a general property of rising and falling bodies of fluid in a rotating system. When they go up or down, they experience this cyclonic turning.

In the standard theory of galactic magnetism, says Rosner, the rising and spinning of the interstellar clouds twists their magnetic fields out of the plane of the galaxy. Over millions of years the ongoing turbulence stirs the fields together like cream in coffee, forming one giant field that is perpendicular to the plane of the galaxy. But the magnetic field lines are embedded in the rotating galaxy, so they move with it. And since the inner part of the galaxy revolves faster than the outer part--just as Mercury takes less time to orbit the sun than Pluto does-- field lines that pass through both parts get stretched out and pulled down into the galactic disk. Eventually a field that started out with a basically vertical orientation gets converted to a field that lies in the horizontal plane of the galaxy.

Until recently this theory looked pretty solid. But over the last three years Rosner’s group has used computer simulations to study the model in great detail. What they found was that as the budding galactic field grows stronger through the stretching and merging of many components, it becomes strong enough to freeze the cyclonic turbulence that blends the component fields in the first place. This cutoff happens well before the magnetic field reaches the strength that is observed in the galaxy.

That was a staggering result, says Rosner.

Searching for an alternative theory, Rosner and his colleagues were naturally drawn to black holes, which some astronomers now believe may lurk at the heart of many galaxies. In their model, a doughnut-shaped cloud of electrically charged gas surrounds a giant black hole at a galaxy’s center. (The gas is in a stable orbit beyond the point where it would be pulled into the black hole.) As the charged particles orbit the black hole, they generate a magnetic field perpendicular to their motion--and to the plane of the galaxy. At first the field resembles the one produced by that staple of freshman physics, the current loop: the field lines run up through the center of the doughnut and then circle back on themselves, like the field lines of a bar magnet.

But a doughnut of gas around a black hole is not freshman physics. Just as in the case of a rotating galaxy, the inner part of the doughnut revolves more rapidly than the outer part. Here too the field lines get stretched out and wrapped around the black hole into a field that lies within the disk of the galaxy.

But how does this relatively small magnetic field at the galactic center come to encompass the whole galaxy? This is where we appeal to something that’s observed, says Rosner. We don’t have a theory for it, but we know this happens in galaxies that have active centers, and most people believe these active centers involve black holes. What’s known is that there are outflows from them. Some of them are very dramatic, producing jets of material. These outflows basically fill the galaxy with material that’s been expelled from its center. The assumption that we’ve made is that these same outflows will also carry the materials with magnetic fields that were produced in the vicinity of the black hole out into the rest of the galaxy.

Rosner and his colleagues point out that a dense cluster of massive stars at a galactic center would fit their theory just as well as a giant black hole. Since their theory predicts a specific relation between the size of the black hole--or star cluster--and the galaxy’s magnetic field strength, future observations may put their ideas to the test.

But even if their theory proves correct, there is still the problem of explaining the magnetic fields of stars. For the turbulent mixing theory, says Rosner, seems to break down when applied to stars as well as galaxies, and for much the same reason: a growing magnetic field stalls the turbulent mixing that creates the field. In the sun, roiling gases play the same role as interstellar gases in the case of galaxies. If you had asked me five years ago, I would have said by and large we have an idea about how the magnetic cycle works on the sun, says Rosner. But that consensus is now out the window. We don’t really know.

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