Resolving the unresolved cosmic X-ray background in the Chandra Deep Fields
Abstract
We present a measurement of the surface brightness of the cosmic X-ray background (CXB) in the Chandra Deep Fields North and South (CDF-N and CDF-S), 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 barely significant: (3.4+/-1.4)x10^-13 ergs cm^-2 s^-1 deg^2 in the 1-2 keV band and (4+/-9)x10^-13 ergs cm^-2 s^-1 deg^2 in the 2-5 keV band, or 7%+/-3% and 4%+/-9%, respectively, of the total CXB. Of the 1-2 keV signal resolved by the HST sources, ~=66% comes from objects with colors typical of starburst and irregular galaxies, while objects with ``normal'' galaxy colors contribute \~=34%. In the 0.65-1 keV band (just above the bright Galactic O VII line and including the Fe XVII lines) the remaining intensity is (12+/-2)x10^-13 ergs cm^-2 s^-1 deg^2. This provides a conservative upper limit on the brightness of the warm-hot intergalactic medium (WHIM) that comes interestingly close to predictions for WHIM emission. A WHIM simulation that accounts for the particular selection of the CDF pointings may constrain the WHIM metallicity.Keywords
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