WASHINGTON, D.C.--A black hole more than 2 million times the mass of the sun almost certainly lurks at the heart of our Milky Way galaxy, according to two new lines of evidence presented here today at a meeting of the American Astronomical Society. Telescopes have spied stars whipping around the galaxy's center as quickly as once every 50 years, while radio waves from the Milky Way's core appear to emanate from an unwavering anchor around which everything in the galaxy swivels.
Strengthening the case for a Milky Way black hole has required astronomers to resolve ever-finer details of star motion around Sagittarius A, a compact radio source at the galaxy's center where the black hole is thought to hide. Previous studies at radio and infrared wavelengths--which penetrate the galactic core's shroud of gas and dust--suggested that Sagittarius A harbors an extremely dense concentration of mass. A black hole seemed the best candidate, but some researchers maintained that a compact cluster of stars could explain those observations.
Hoping to resolve the issue, a team led by Andreas Eckart of the Max Planck Institut für Extraterrestrische Physik in Garching, Germany, charted the paths of stars within 100th of a light-year of the core--the closest to the center yet tracked. Five years of sensitive infrared observations at the European Southern Observatory in Chile revealed stars darting around the core as fast as 1000 kilometers per second. These breakneck speeds, the team calculates, require the gravitational pull of a central mass about 2.6 million times greater than the sun--far more than could be packed into a small star cluster.
A second set of observations indicates that this central mass, unlike the stars whizzing around it, is fixed precisely at the galaxy's center. An international team led by Mark Reid of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, used the VLBA, an array of radio telescopes spanning the United States, to track the motion of Sagittarius A against a fixed background of bright objects in the distant universe, called quasars. The radio source, they deduced, lumbers at less than 20 kilometers per second--slower than Earth orbits the sun. "The simplest explanation, by far, is that this is an absolutely stationary black hole," Reid says.
Astronomer Farhad Yusef-Zadeh of Northwestern University in Evanston, Illinois agrees that it's virtually impossible to explain the new data with anything other than a supermassive black hole. Indeed, his own team has observed a stream of ionized gas hurtling by the galactic center at more than 3 million kilometers per hour. Further studies of the flow of this gas, Yusef-Zadeh says, may give more precise estimates of the black hole's mass and a better picture of its tumultuous neighborhood.