Chandra resolves X-ray glow in the universe

WHILE TAKING a giant leap toward solving one of the greatest mysteries of astronomy, NASA's Chandra X-ray Observatory also may have revealed the most distant objects ever seen in the Universe and discovered two puzzling new types of cosmic objects. Chandra has resolved most of the X-ray background, a pervasive glow of X- rays throughout the universe, which was first discovered in the early days of space exploration.

Before now, scientists have not been able to discern the origin of the hard, or high-energy, X-ray background, because until Chandra no telescope has had the technology to resolve it. "This is a major discovery," said Dr. Alan Bunner, Director of NASA's Structure and Evolution of the Universe science theme. "Since it was first observed 37 years ago, understanding the source of the X-ray background has been a Holy Grail of X-ray astronomy. Now, it is within reach."

An article describing this work has been submitted to the journal Nature by Dr. Richard Mushotzky of NASA's Goddard Space Flight Center, Greenbelt, Md., Drs. Lennox Cowie and Amy Barger at the University of Hawaii, Honolulu, and Dr. Keith Arnaud of the University of Maryland, College Park.

The Chandra team looked at a small section of the sky, a circle about one-fifth the size of a full moon, and resolved about 80% of the X- ray glow in this region into specific light sources. Stretched across the entire sky, this adds up to approximately 70 million sources, most of which are galaxies.

One-third of the sources are galaxies whose cores shine bright in X-rays, yet do not shine in visible light. There may be tens of millions of these "veiled galactic nuclei" in the universe. Each of these galaxies likely harbors a massive black hole at its core that produces X-rays as gas is pulled toward it at nearly the speed of light.

A second new class of objects, comprising approximately one- third of the sources, is assumed to be "ultra-faint galaxies." Mushotzky said that these sources may emit little or no optical light, either because the dust around the galaxy blocks the light totally or because the optical light is eventually absorbed during its long journey across the universe. In the latter scenario, Mushotzky said that these sources would be well over 14 billion light years away and thus the earliest, most distant objects ever identified.

Resolution of the X-ray background relied on a 27.7-hour Chandra observation using the Advanced CCD Imaging Spectrometer in early December 1999, and also utilized data from the Japan-U.S. Advanced Satellite for Cosmology and Astrophysics.

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