Black hole survey.

Scanning the sky for high-energy X rays, NASA's Swift satellite has completed the first comprehensive census of active supermassive black holes that lie within 400 million light-years of Earth. The study, reported in October at a meeting of the American Astronomical Society in San Francisco, found more than 200 supermassive black holes, including several that had been overlooked in previously studied galaxies.

Each black hole, millions to billions times as massive as the sun, lurks at the center of a galaxy but in visible light may lie hidden behind thick layers of dust. However, because X rays penetrate the dust, astronomers can detect black holes by looking for the energetic X rays emitted by the gas swirling around and into them.

Dormant black holes, like the one at the center of the Milky Way, emit too little radiation to be part of the new census.

The Swift satellite was built to record gamma-ray bursts, the most energetic explosions in the universe. But between bursts, Swift's Burst Alert Telescope searches the sky for X rays, notes study coauthor Jack Tueller of NASA's Goddard Space Flight Center in Greenbelt, Md.

The Swift study builds upon X-ray observations made by the European Space Agency's INTEGRAL satellite and NASA's Chandra X-ray Observatory. INTEGRAL was limited to looking for supermassive black holes in galaxies in the plane of the Milky Way, while Chandra, sensitive to lower-energy X rays than Swift is, could find only more-luminous black holes.--R.C.