Resolving the unresolved cosmic X-ray background in the Chandra deep fields

We present a measurement of the surface brightness of the cosmic X-ray background (CXB) in the Chandra Deep Fields, after excluding all detected X-ray, optical, and infrared sources. The work is motivated by a recent X-ray stacking analysis by Worsley and collaborators, which showed that galaxies detected by HST but not by Chandra may account for most of the unresolved CXB at E > 1 keV. We find that after excluding HST and Spitzer IRAC sources, E 1 1 some CXB still remains, but it is marginally significant: (3.4 +/- 1.4) x 10(-13) ergs cm(-2) s(-1) deg(-2) in the 1-2 keV band and (4 +/- 9) x 10(-13) ergs cm(-2) s(-1) deg(-2) in the 2-5 keV band, or 7% +/- 3% and 4% +/- 9% of the total CXB, respectively. Of the 1-2 keV signal resolved by the HST sources, 34% +/- 2% comes from objects with optical colors typical of "normal" galaxies (which make up 25% of the HST sources), while the remaining flux comes from objects with colors of starburst and irregular galaxies. In the 0.65-1 keV band (just above the bright Galactic O VII line), the remaining diffuse intensity is (1.0 +/- 0.2) x 10(-12) ergs cm(-2) s(-1) deg(-2). This flux includes emission from the Galaxy as well as from the hypothetical warm-hot intergalactic medium (WHIM), and provides a conservative upper limit on the WHIM signal that comes interestingly close to theoretical predictions.