WASHINGTON, D.C.--Not all black holes grow into behemoths, astronomers have learned by probing the centers of galaxies in a new way. Swirling glints of light from the core of a nearby galaxy suggest that its black hole is just one-tenth as hefty as the massive object at the heart of our Milky Way. Astronomers suspect that the galaxy somehow kept the black hole on a starvation diet and that the new technique should offer further insights.
The finding, announced today at the annual meeting of the American Association for the Advancement of Science, relies on a new high-resolution method called "reverberation mapping.” The reverb consists of reflections of light. As matter plunges toward a black hole, it forms a spiraling disk of gas that blazes to millions of degrees. This pulsing radiation bounces off more distant blobs of gas and dust in the rotating disk. Some of these reflections head to Earth. By carefully tracking the timing, astronomers can gauge how quickly the disk is swirling. That reveals the mass of the central black hole, because the stronger the gravity of a huge black hole, the faster its disk whirls.
Using this method, a team led by astronomer Ari Laor of Technion-Israel Institute of Technology, Haifa, mapped light echoes from the core of NGC 4395, a nearby galaxy with a core bright enough to produce the light reflections. The team used the Hubble Space Telescope in April and July 2004 to track the disk's rotation. The results point to a black hole with about 300,000 times the mass of our sun, reports coauthor Bradley Peterson of Ohio State University in Columbus. A 2003 study by Peterson's colleagues of star motions in NGC 4395 had suggested that the hole was several times smaller than that. But Peterson thinks that reverberation mapping is more accurate because it examines gas next to the hole instead of the movement of more distant stars.
Even at 300,000 solar masses, the black hole is shrimpy for a galaxy with such a bright core. "I think this is the runt-of-the-litter syndrome," says Peterson. "It probably started out small and never got a lot to eat, so it didn't grow very much." NGC 4395 lacks the large spherical bulge of stars that most other galaxies possess, he notes. The gravity of such bulges may force gas inward more efficiently, giving other black holes proper meals for billions of years.
"It's a wonderful application of this technique," says astronomer Saeqa Vrtilek of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Other puny black holes probably await discovery, she adds.