X-ray Background beyond NGC 891
Credit: NASA/CXC/U.Mich./J.Bregman & J.Irwin

The Great Beyond

Astronomers have a hard time getting distances even to discrete objects. Imagine how hard it is to determine the distance to diffuse emission. The only recourse astronomers have in such cases is to observe a discrete source and to try to determine whether the diffuse emission is in front of or behind the object. X-ray astronomers have a particularly hard job in this regard, since a mysterious X-ray "background" seems to permeate the universe - but where does it come from? The ROSAT X-ray Observatory showed that at least some of the x-ray background came from beyond the moon, and that some of it came from beyond nearby molecular clouds outside the solar system but within our galaxy. The Chandra X-ray Observatory has now pushed the limit beyond our galaxy. Chandra recently obtained an X-ray image of the edge-on galaxy NGC 891. Like all spirals, NGC 891 has a relatively thin disk of gas and dust, and, because we view the galaxy edge-on, this disk shows up as a dark lane splitting the galaxy, as shown in the optical image on the above left. The Chandra X-ray image (above right) shows rather faint X-ray emission from the galaxy - but there's almost no x-rays from the disk of the galaxy where the gas and dust are thickest. This shows that at least some of the background X-rays originate beyond NGC 891 - perhaps in a vast, distant intergalactic cloud of multimillion degree gas.

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Page Author: Dr. Michael F. Corcoran
Last modified August 5, 2002