Measuring the fraction of obscured quasars by the infrared luminosity of unobscured quasars

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 f(obsc), 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 mu m MIPS photometry from objects with z similar to 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 mu m to bolometric luminosity decreases by a factor of similar to 3 in the L-bol 10(44)-3 x 10(47) ergs s(-1) range, but there is also a large scatter at constant L-bol. Using radiation transfer solutions for model geometries, we show how the IR/ bolometric ratio relates to f(obsc) 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 f(obsc), particularly in the middle range of luminosities, suggesting that there may be a significant number of heavily obscured AGNs missed by X- ray surveys.