XMM/Lockman Hole
Credit: Hasinger et al., A&A, in press; ESA

X-ray Olbers' Paradox Resolved?

The darkness of the night sky is a bit of a puzzle, since if there are an infinite number of stars stretching to infinity, then in each direction in space you should see a star; though more distant stars are fainter, there are more of them in each area of sky, and the net effect is that the sky is filled with unresolved stars. This puzzle is generally called Olbers' paradox after the physicist who popularized it in the 19th century, though record of it stretches back to the time of the astronomer Johannes Kepler in the 17th century. The night sky looks dark since the universe is not infinite in time, and light from more distant stars has not had time to reach us yet.

At X-ray wavelengths, there is no "night sky"; the X-ray sky is illuminated by a diffuse background in nearly all directions. This diffuse X-ray background was discovered in the early 1960's, but its origin was largely a puzzle until very recently. It's now recognized that at least the high energy part of this background is produced by very distant galaxies which harbor supermassive black holes at their centers. The emission from these galaxies was unresolved by early X-ray observatories. Newer observatories, in particular ROSAT, Chandra and XMM-Newton, have sufficient spatial power to resolve this diffuse glow into individual objects. The image above is a new picture taken with the XMM-Newton observatory of a region of the sky called the Lockman Hole, a region in the Milky Way in which there is little gas or dust to absorb the distant light from faraway galaxies. This image shows that the diffuse glow seen by other X-ray observatories is actually composed of individual emission from distant active galaxies. The object colors represent the average X-ray energy of the emission - red for relatively low-energy X-ray emission and blue for high energy emission. If the Universe was infinite in time and uniformly filled with galaxies, then there would be a true X-ray Olber's paradox. In fact we find that, near us, there are very few active galaxies. Since X-ray emission from these galaxies is powered by supermassive black holes swallowing up gas and stars, this suggests that, as galaxies age, their black holes have less material to devour, and thus produce less X-ray emission.

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Page Author: Dr. Michael F. Corcoran
Last modified February 12, 2002