Talk Title: GOODS Discovery of a Significant Population of Hidden AGN
Abstract: We have analysed the optical and infrared properties of X-ray sources in the Great Observatories Origins Deep Survey (GOODS), a deep, multiwavelength survey covering 0.1 square degrees in two fields. The HST ACS counts down to z-magnitude~28 can be well explained by a unified AGN scheme that postulates roughly 3 times as many obscured as unobscured AGN. This scenario is consistent with the observed spectroscopic and photometric redshift distributions of the GOODS AGN once selection effects are considered. The previously reported discrepancy between observed spectroscopic redshift distributions and the predictions of population synthesis models for the X-ray background (which include a similarly large number of obscured AGN) is explained by bias against the most heavily obscured AGN in optical spectroscopic samples, and even in the X-ray surveys. Starting from hard X-ray luminosity functions and spectral energy distributions appropriate to the unified scheme for AGN, we make predictions for the number counts of AGN in the Spitzer MIPS 24 micron and IRAC 3.6-8 micron bands. A preliminary analysis of the GOODS Spitzer data appear to confirm that indeed large numbers of obscured AGN are present in the early Universe (z>1). Some of the brightest far-infrared sources will be AGN that are too obscured to be detected with Chandra, and that look like ultraluminous infrared galaxies. This scenario is consistent with the observed spectroscopic and photometric redshift distributions of the GOODS AGN once selection effects are considered. The previously reported discrepancy between observed spectroscopic redshift distributions and the predictions of population synthesis models for the X-ray background (which include a similarly large number of obscured AGN) is explained by bias against the most heavily obscured AGN in both X-ray surveys and optical spectroscopic samples. We present the model predictions for the number counts of AGN in the Spitzer MIPS 24 micron and IRAC 3.6-8 micron bands. The GOODS Spitzer observations will verify whether large numbers of obscured AGN are indeed present in the early Universe; these will be very bright far-infrared sources, including some, missed by X-ray observations, that look like ultraluminous infrared galaxies.
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