A Spinning Black Hole at a Galaxy's Center

Like all invisible things that are only partly understood, black holes evoke a sense of mystery. Astronomers know that the tremendous gravitational pull of a black hole sucks matter in. They also know that the material falling in causes powerful jets of particles to shoot out of the black hole at nearly the speed of light. But how exactly this phenomenon occurs remains a matter of conjecture, because astronomers have never quite managed to observe the details.

Well, now they have. Sheperd Doeleman, an astrophysicist at the Massachusetts Institute of Technology Haystack Observatory in Westford, and his colleagues have taken the closest look to date at the region where matter swirls around a black hole. By measuring the size of the base of a jet shooting out of the supermassive black hole at the center of the M87 galaxy, the researchers conclude that the black hole must be spinning and that the material orbiting must also be swirling in the same direction. Some of the material from this orbiting "accretion disk" is also falling into the black hole, like water swirling down a drain. The finding appears online today in Science.

For the past few years, Doeleman and his colleagues have been working to link up radio dishes around the world into a virtual telescope with unprecedented magnifying power, which would enable researchers to observe the immediate vicinity of the black hole in the heart of M87—a favorite target for astronomers, as it is one of the brightest objects in the sky. So far, the researchers have linked radio dishes at three sites. That hasn't provided enough resolution to see all the way to the edge of the black hole. But it enabled the researchers to measure the area through which the jet is being emitted.

The size of this emission region fits with only one particular theoretical model of how these jets form. The base of the jet "reduces to the size we measured only when the black hole is spinning and the accretion disk is orbiting in the same direction," Doeleman says. "What we find so exciting is that we are now finally able to measure structures so close to the black hole." He and his colleagues hope to use the Event Horizon Telescope—the instrument being created by linking the radio dishes—to test "whether Einstein's theory of general relativity is valid at the one place in the universe where it might break down: the event horizon of a black hole."

The paper "is very interesting," says Meg Urry, an astrophysicist at Yale University who was not involved in the study. "Measuring the launch point for the jet is absolutely critical for understanding how jets form, and indeed how jet energy is extracted from the black hole-disk system." However, Urry points out, the conclusions rest on a number of assumptions that are "difficult to confirm"—such as whether the measured area does lie directly on top of the black hole rather than off to the side or elsewhere.