The entire sky is bathed in a diffuse glow of X rays that astronomers have never been fully able to explain. Now the orbiting Chandra X-ray Observatory has found the dark secret behind these pervasive rays: They come from millions upon millions of black holes, some of which may date from an era before visible galaxies existed.
Courtesy: The Andromeda galaxy looks calm in visible light (left). An X-ray closeup (right) reveals a black hole, marked in blue.
Astrophysicist Richard Mushotzky of NASA's Goddard Space Flight Center and his colleagues made this discovery after directing Chandra to take a long exposure of a small patch of sky. The observatory's sensitive detectors resolved 80 percent of the X-ray glow into discrete objects. When the Hubble Space Telescope and Keck telescope in Hawaii zeroed in on the objects, they found that about half appear to be distant galaxies. The X rays probably come from hot gas spiraling into massive black holes in the galaxies' centers.
For the other half of the X-ray sources, scientists could find no visible counterpart. Perhaps these are infant galaxies, swathed in thick clouds of dust, or even more distant and primitive objects in which stars have not yet begun to shine, leaving only the black holes. Other studies of closer galaxies, including our neighboring Andromeda galaxy, confirm that supermassive black holes are everywhere. "Is the black hole the seed around which the galaxy grows, or does the galaxy create the environment in which a black hole can grow? Our results provide the first direct information," says Mushotzky.