THE X-RAY BACKGROUND, the glow of x rays seen in all directions in space, has now largely been resolved into emissions from discrete sources by the Chandra X-Ray Telescope, ending the notion that the x rays come from distant hot gas. Previously only about 20-30% of the x-ray background had been ascribed to point sources (by such telescopes as ASCA). Chandra was launched in July 1999 and put in an elliptical orbit. With its high angular resolution and acute sensitivity it could tell apart x-ray objects (many of them thought to be accretion disks around black holes) that before had been blurred into a continuous x-ray curtain. (Of course, now that the background has been resolved into points it ceases to be a background.)
Richard Mushotzky of Goddard Space Flight Center reported these Chandra results at last week's meeting in Atlanta of the American Astronomical Society (AAS). Resolving the x-ray background was not all. Mushotzky added that the Chandra survey had revealed the existence of two categories of energetic galaxies that had been imaged only poorly or not at all by optical telescopes. He referred to one category as "veiled galactic nuclei," objects (with a redshift of about 1) bright in x rays but obscured by dust at optical wavelengths. The other category was "ultra-faint galaxies."
One interpretation of these galaxies is that optical emission is suppressed owing to absorption over what could be a very long pathway to Earth. Mushotzky speculated that such high redshift (z greater than 5) galaxies could be the most distant, and hence earliest, objects ever identified. The XMM x-ray telescope, just launched, should provide complementary information in the form of high-precision spectra (from which redshifts are derived) of the distant objects.
