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M. Urry (Yale University)
The deepest X-ray surveys have resolved a significant fraction of the X-ray ``background'' into individual AGN. The spectrum of the background, first measured more than 20 years ago with the HEAO-1 A2 experiment, has a peak energy output near 40 keV and is harder than Type 1 AGN spectra locally, suggesting that obscured AGN make a significant contribution. Deep X-ray surveys, combined with population synthesis of AGN to match the integrated all-sky spectrum, allow us to measure the ratio of obscured to unobscured AGN. This in turn is an important constraint on the co-evolution of black holes and galaxies.
We have developed a simple population-synthesis model that matches the multiwavelength X-ray and optical number counts in the ultra-deep GOODS survey, as well as in shallower surveys. We also make predictions for the Spitzer infrared counts and show that the higher redshift, obscured AGN are preferentially missing from spectroscopic follow-up samples. Our model, which assumes a fixed ratio of 3:1 for obscured to unobscured AGN, predicts that AGN are missed in even the deepest Chandra fields. These include the Compton-thick sources needed to produce the hard X-ray spectrum of the background above 10 keV.
This work was supported in part by NASA grant HST-GO-09425.13-A.
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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.