From Scientific American:
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.