Resolving the Unresolved Cosmic X-Ray Background in the Chandra Deep Fields

We present a measurement of the surface brightness of the cosmic X-raybackground (CXB) in the Chandra Deep Fields, after excluding all detectedX-ray, optical and infrared sources. The work is motivated by a recent X-raystacking analysis by Worsley and collaborators, which showed that galaxiesdetected by HST but not by Chandra may account for most of the unresolved CXBat E>1 keV. We find that after excluding HST and Spitzer IRAC sources, some CXBstill remains, but it is marginally significant: (3.4+/-1.4)x10^-13 ergs cm^-2s^-1 deg^2 in the 1-2 keV band and (4+/-9)x10^-13 ergs cm^-2 s^-1 deg^2 in the2-5 keV band, or 7%+/-3% and 4%+/-9% of the total CXB, respectively. Of the 1-2keV signal resolved by the HST sources, 34%+/-2% comes from objects withoptical colors typical of ``normal'' galaxies (which make up 25% of the HSTsources), while the remaining flux comes from objects with colors of starburstand irregular galaxies. In the 0.65-1 keV band (just above the bright GalacticO VII line) the remaining diffuse intensity is (1.0+/-0.2)x10^-12 ergs cm^-2s^-1 deg^2. This flux includes emission from the Galaxy as well as from thehypothetical warm-hot intergalactic medium (WHIM), and provides a conservativeupper limit on the WHIM signal that comes interestingly close to theoreticalpredictions.Comment: 5 emulateapj pages, 4 figures, 1 table, accepted to ApJL (mostly minor improvements in response to referee's comments, in particular further treatment of uncertainties on resolved fluxes