Abstract

Recent work has suggested that the fraction of obscured AGNs declines with increasing luminosity, but it has been difficult to quantify this trend. Here we attempt to measure this fraction as a function of luminosity by studying the ratio of mid-infrared to intrinsic nuclear bolometric luminosity in unobscured AGNs. Because the mid-infrared is created by dust reprocessing of shorter wavelength nuclear light, this ratio is a diagnostic of fobsc, the fraction of solid angle around the nucleus covered by obscuring matter. In order to eliminate possible redshift dependences while also achieving a large dynamic range in luminosity, we have collected archival 24 μm MIPS photometry from objects with z ∼ 1 in the Sloan Digital Sky Survey, the Great Observatories Origins Deep Survey, and the Cosmic Evolution Survey. To measure the bolometric luminosity for each object, we used archival optical data supplemented by GALEX data. We find that the mean ratio of 24 μm to bolometric luminosity decreases by a factor of ∼3 in the Lbol = 1044-3 × 1047 ergs S-1 range, but there is also a large scatter at constant Lbol. Using radiation transfer solutions for model geometries, we show how the IR/bolometric ratio relates to fobsc and compare these values with those obtained from samples of X-ray-selected AGNs. Although we find approximate agreement, our method indicates somewhat higher values of fobsc, particularly in the middle range of luminosities, suggesting that there may be a significant number of heavily obscured AGNs missed by X-ray surveys. © 2008. The American Astronomical Society. All rights reserved.