An x-ray background for star formation.

An X-ray background for star formation

A group of astrophysicists has discovered a type of faint galaxy only a few billion light-years from Earth that may generate much of the bright glow of X-rays that appears to fill the sky. First observed during a rocket experiment in 1962, this X-ray background has long puzzled astronomers. A variety of individual objects, including quasars and hot clouds of inter-galactic gas, produce significant amounts of X-rays, but their total output is insufficient to explain both the brightness and the slight graininess of the X-ray sky. Now, David J. Helfand and Thomas Hamilton of Columbia University in New York City and their colleagues have identified a special group of faint, red galaxies that appear to generate X-rays that may be numerous enough to account for the X-ray background.

To pinpoint these sources, the researchers compared X-ray, radio-wave and visible-light observations of one particular region of the sky. They found more than 100 faint radio sources in a region roughly the size of the full moon as it appears from Earth. A significant number of these coincide with bright points in the X-ray map. "We found to our delight and somewhat to our surprise that the faintest radio sources lined up with the bumps in the X-ray field," Helfand says. Subsequent optical telescope observations revealed an unexpected abundance of large but faint galaxies in the vicinity of the radio sources. "We see only the nearest neighbors, but these are probably representative of even more such galaxies farther away, to make up the full X-ray background," Hamilton says.

A close look at seven of the fainth galaxies shows them to be within a few billion light-years of Earth. Hamilton and Helfand speculate that such galaxies are in a relatively early stage of their evolution, when large numbers of massive stars form and then explode to produce supernovas, neutron stars and possibly black holes--all of which are associated with intense radio-wave and X-ray emissions. The X-ray background "doesn't tell us about the formation of galaxies, but it may be a key link in studies of the evolution of galaxies," Helfand says.