Milky Way’s black hole seen in new detail

A closer look at a unique feature in the galaxy’s center suggests outlines of the black hole’s event horizon

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New radio wave observations are giving astronomers their closest look yet at the supermassive black hole believed to be lurking at the center of our galaxy.

This illustration shows the Milky Way’s central, 4-million-solar-mass black hole, along with the structures around it, such as a swirling disk of gas and dust and a possible radio-emitting jet. Researchers using an array of radio telescopes have examined the region surrounding the black hole’s event horizon in unprecedented detail. S. Doeleman, M. Weiss/CXC, S. Noble, C. Gammie, NASA

GALAXY PORTRAIT GALAXY PORTRAIT. An array of radio telescopes allowed the closest look yet at the Milky Way’s center, which may appear as it does in this illustration. Yellow and red depict radio emissions from Sagittarius A*, which appears to be located off-center from the black hole that is thought to reside at the galaxy’s center. S. Doeleman, M. Weiss/CXC, S. Noble, C. Gammie, NASA

Reporting in the Sept. 4 Nature, a team has, for the first time, resolved features as small as the black hole’s event horizon — the gravitationally warped region from which nothing, not even light, can escape. “We have now entered a new era, one in which we can directly image structure at the event horizon of a black hole,” asserts Christopher Reynolds of the University of Maryland in College Park in a commentary accompanying the Nature report.

The findings also provide more evidence that the Milky Way’s center truly houses a black hole, estimated to be 4 million times the mass of the sun.

To study the gravitational monster, researchers homed in on Sagittarius A*, the bright radio-emitting body thought to mark the position of the black hole. Because Sagittarius A* is likely fueled by the black hole’s activity, a better look at the radio-emitting body can provide more details about the black hole.

By combining the signals from three radio dishes in California, Hawaii and Arizona, Sheperd Doeleman of MIT and his colleagues effectively created a single radio telescope nearly as wide as the continental United States. Using this strategy, known as very long baseline interferometry, the team examined Sagittarius A* in unprecedented detail.

For three decades, notes Doeleman, astronomers have struggled to observe Sagittarius A* in such fine detail. Now the team has succeeded, resolving features one-third the size of the separation between Earth and the sun. Such fine-scale observations, he says, are the only way that astronomers can study how black holes are fueled and to test whether Einstein’s theory of general relativity holds at the edge of a black hole.

“The new observations we made confirm, for the first time, that there is structure on these scales in Sagittarius A*,” Doeleman says. The structure not only confirms the presence of a black hole, but also reveals more about the relationship between Sagittarius A* and the black hole.

Close to the black hole, gravity acts like a highly distorted lens, bending and magnifying light. As a result of this magnification, he says, radiation from a supermassive black hole appears to come from a region larger than it really originated from. In fact, the radiating region will always appear to have a minimum size.

Yet the radio observations by Doeleman’s team reveal that Sagittarius A* itself is smaller than this minimum size. That suggests that Sagittarius A* “is not centered on the black hole, but rather is offset to one side where one could observe a smaller source,” he says.

One explanation for the offset and smaller size is that Sagittarius A* could be part of a rotating disk of material surrounding the black hole. As the radio-wave–emitting region rotates into view “we only see a small spot of bright emission,” says Doeleman.

Alternatively, he notes, Sagittarius A* “could be the nozzle of a high-speed jet of matter blasting out from the black hole,” which would also offset the radio source.

“All the sophisticated models that exist for [gas and dust flowing] onto the black hole, and for possible jets erupting from the black hole, have been unconstrained by any observations at this high resolution,” he notes. Now, Doeleman adds, “we have the means to test these theories and to find out exactly what’s going on near the black hole.”

As larger numbers of radio telescopes are linked and their sensitivity increased, “the distorted world at the edge of a black hole will literally come into focus,” Reynolds says.

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