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, some CXB still remains, but it is marginally significant: (3.4± 1.4)× 10−13 ergs cm−2 s−1 deg−2 in the 1–2 keV band and (4± 9)× 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)× 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. Subject headings: methods: data analysis — X-rays: diffuse background — X-rays: galaxies