Obscured AGN and the X-ray, Optical and Far-Infrared Number Counts of AGN in the GOODS Fields

Abstract

The deep X-ray, optical, and far-infrared fields that constitute GOODS are sensitive to obscured AGN (N_H>10^{22} cm^{-2}) at the quasar epoch (z~2-3), as well as to unobscured AGN as distant as z~7. Luminous X-ray emission is a sign of accretion onto a supermassive black hole and thus reveals all but the most heavily obscured AGN. We combine X-ray luminosity functions with appropriate spectral energy distributions for AGN to model the X-ray, optical and far-infrared flux distributions of the X-ray sources in the GOODS fields. A simple model based on the unified paradigm for AGN, with ~3 times as many obscured AGN as unobscured, successfully reproduces the z-band flux distributions measured in the deep HST ACS observations on the GOODS North and South fields. This model is also consistent with the observed spectroscopic and photometric redshift distributions 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 can be explained by bias against the most heavily obscured AGN generated both by X-ray observations and the identification of sources via optical spectroscopy. We predict the AGN number counts for Spitzer MIPS 24 um and IRAC 3.6-8 um observations in the GOODS fields, which will verify whether most AGN in the early Universe are obscured in the optical. Such AGN should be very bright far-infrared sources and include some obscured AGN missed even by X-ray observations.