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

TL;DR

This study investigates how the fraction of obscured active galactic nuclei (AGN) varies with luminosity by analyzing infrared and optical data of unobscured quasars, revealing a decline in obscuration with increasing luminosity and potential undercounting in X-ray surveys.

Contribution

The paper introduces a novel method to estimate obscured AGN fractions using infrared to bolometric luminosity ratios in unobscured quasars, accounting for redshift effects and comparing with X-ray data.

Findings

The IR/bolometric ratio decreases with increasing luminosity.

Estimated obscured fraction is higher than X-ray surveys suggest.

Heavily obscured AGN may be underrepresented in X-ray samples.

Abstract

Recent work has suggested that the fraction of obscured AGN 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 AGN. 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 micron MIPS photometry from objects with z~1 in the Sloan Digital Sky Survey (SDSS), the Great Observatories Origins Deep Survey (GOODS) and the Cosmic Evolution Survey (COSMOS). 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 microns to bolometric luminosity decreases by a factor of ~3 in the L_bol=10^44-3x10^47 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 obtained from samples of X-ray selected AGN. 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 AGN missed by X-ray surveys.