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Researchers are closing in on ironclad evidence for the black hole believed to lurk at the center of our Milky Way galaxy.
Astronomers used a "virtual" telescope spanning more than 2,800 miles (4,500 km) to home in on Sagittarius A* ("A-star"), the light source believed to mark the location of a black hole four million times as massive as the sun.
They were able to resolve Sagittarius A* to within 37 microarcseconds, the width of a baseball on the moon as seen from Earth. Based on the size of the light-emitting region, they believe it is offset from the exact location of the black hole, which pulls gas and dust into a disk swirling around it that gives off light.
Instead, they speculate that Sagittarius A* is either high-speed gas on one side of the rotating accretion disk or a jet of matter being ejected from around the black hole.
The case for a black hole was already "pretty solid," says study author Shepherd Doeleman, an astrophysicist at the Massachusetts Institute of Technology's Haystack Observatory. "We're now able to get information that is really on the same size scale as where we think all the action is happening in the galactic black hole."
Prior observations of the presumed black hole were obscured by surrounding gas and dust that reflect longer wavelength radio waves.
To pierce the haze, Doeleman and his colleagues used special equipment to link up four radio telescopes—one each in Arizona and California and two in Hawaii—in a technique called very long baseline interferometry. The resolving power of such a virtual telescope grows with the size of the telescope array. The joint telescopes allowed them to scan radio signals as short as 0.05 inch (1.3 millimeters) in wavelength, in the microwave range, capable of penetrating the cloud.
The researchers found that Sagittarius A* measured about 30 million miles (50 million kilometers) across, or one third the average distance between the sun and Earth. Researchers would like to observe light coming from around the black hole event horizon—the boundary past which not even light can escape the concentrated pull of gravity.
Distortion of space and time around the event horizon is believed to make the event horizon appear larger than its true diameter and—in this case—larger than the features the group resolved, Doeleman says. He says the group hopes to increase the power of the telescope array even more to look for a predicted "shadow" in front of the black hole, which would provide concrete proof of its existence.
Another coup would be measuring the black hole's rate of spin, its other basic property besides mass and something that researchers have never observed directly before either. It's exciting, Doeleman says, that "now we can start asking these questions."
