Chandra resolves cosmic X-ray glow and finds mysterious new sources 13/01/2000
This Chandra X-ray Observatory image of a 27.7 hour observation of a region in the direction of the constellation Canes Venatici, close to the Big Dipper, shows about 3 dozen X-ray sources. Some of the sources were too faint to be seen by optical telescopes such as the Hubble Space Telescope and the Keck 10 meter telescope in Hawaii.
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Image Credit: NASA, Goddard Space Flight Center, (Mushotzky et al.).  
While taking a giant leap towards solving one of the greatest mysteries of X-ray 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. Not bad for being on the job only five months.

Chandra has resolved most of the X-ray background, a pervasive glow of X-rays throughout the Universe, first discovered in the early days of space exploration. Before now, scientists have not been able to discern the background's origin, because no X-ray telescope until Chandra has had both the angular resolution and sensitivity 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 thirty-seven years ago, understanding the source of the X-ray background has been a Holy Grail of X-ray astronomy. Now, it is within reach."

"We are all very excited by this finding," said Mushotzky. "The resolution of most of the hard X-ray background during the first few months of the Chandra mission is a tribute to the power of this observatory and bodes extremely well for its scientific future,"

Scientists have known about the X-ray glow, called the X-ray background, since the dawn of X-ray astronomy in the early 1960s. They have been unable to discern its origin, however, for no X-ray telescope until Chandra has had both the angular resolution and sensitivity to resolve it. The German-led ROSAT mission, now completed, resolved much of the lower-energy X-ray background, showing that it arose in very faraway galaxies with extremely bright cores, called quasars or Active Galactic Nuclei (AGN).

The Chandra team sampled a region of the sky about one-fifth the angular area of a full moon and resolved about 80 percent of the more-energetic X-ray background into discrete sources. Stretched across the entire sky, this would account for approximately 70 million sources, most of which would be identified with galaxies. Their analysis confirms that a significant fraction of the X-ray background cannot be due to diffuse radiation from hot, intergalactic gas.

Combined X-ray and optical observations showed that nearly one third of the sources are galaxies whose cores are very bright in X-rays yet emit virtually no optical light from the core. The observation suggests that these "veiled galactic nuclei" galaxies may number in the tens of millions over the whole sky. They almost certainly harbor a massive black hole at their core that produces X-rays as the gas is pulled toward it at nearly the speed of light.

Their bright X-ray cores place these galaxies in the AGN family. Because these numerous AGN are bright in X-rays, but optically dim, the Chandra observation implies that optical surveys of AGN are very incomplete.

A second new class of objects, comprising approximately one-third of the background, 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 by relatively cool gas during its long journey across the Universe. In the latter scenario, Mushotzky said that these sources would have a redshift of 6 or higher, meaning that they are well over 14 billion light years away and thus the earliest, most distant objects ever identified.

 
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