Obscured AGN in Multiwavelength Surveys
  Using a simple unification model for AGN, with a fixed ratio of obscured to unobscured AGN of 3:1, we explain the X-ray, optical and infrared properties of the X-ray sources in the GOODS fields. That is, the GOODS data are consistent with the existence of a large population of obscured AGN out to high redshifts. About half of these are so obscured that they are missed even by hard X-ray observations, and are detected only in deep infrared observations. The previously reported decreasing trend of obscured to total AGN ratio with increasing X-ray luminosity can be explained entirely as a selection effect. 
Ezequiel Treister (Yale/U. de Chile), C.M. Urry (Yale), Jeff Van Duyne   (Yale) and the GOODS AGN Team