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Recent work has suggested that the fraction of obscured AGN declines withincreasing luminosity, but it has been difficult to quantify this trend. Here,we attempt to measure this fraction as a function of luminosity by studying theratio of mid-infrared to intrinsic nuclear bolometric luminosity in unobscuredAGN. Because the mid-infrared is created by dust reprocessing of shorterwavelength nuclear light, this ratio is a diagnostic of f_obsc, the fraction ofsolid angle around the nucleus covered by obscuring matter. In order toeliminate possible redshift-dependences while also achieving a large dynamicrange in luminosity, we have collected archival 24 micron MIPS photometry fromobjects with z~1 in the Sloan Digital Sky Survey (SDSS), the GreatObservatories Origins Deep Survey (GOODS) and the Cosmic Evolution Survey(COSMOS). To measure the bolometric luminosity for each object, we usedarchival optical data supplemented by GALEX data. We find that the mean ratioof 24 microns to bolometric luminosity decreases by a factor of ~3 in theL_bol=10^44-3x10^47 ergs s^-1 range, but there is also a large scatter atconstant L_bol. Using radiation transfer solutions for model geometries, weshow how the IR/bolometric ratio relates to f_obsc and compare these valueswith those obtained obtained from samples of X-ray selected AGN. Although wefind approximate agreement, our method indicates somewhat higher values off_obsc, particularly in the middle range of luminosities, suggesting that theremay be a significant number of heavily obscured AGN missed by X-ray surveys.

Measuring the Fraction of Obscured Quasars by the Infrared Luminosity of Unobscured Quasars