A Large Population of Luminous Active Galactic Nuclei Lacking X-ray Detections: Evidence for Heavy Obscuration?

TL;DR

This study identifies a large population of infrared-bright, heavily obscured active galactic nuclei (AGNs) that are not detected in X-ray surveys, highlighting the importance of multiwavelength observations for understanding AGN demographics.

Contribution

The paper introduces a method combining spectral energy distribution modeling and multiwavelength data to uncover heavily obscured AGNs lacking X-ray detections, revealing a significant hidden population.

Findings

Thousands of obscured AGNs identified without X-ray counterparts.

Many sources have column densities exceeding 10^24 cm^-2, indicating Compton-thick obscuration.

X-ray stacking confirms AGN activity in the non-detected sources.

Abstract

We present a large sample of infrared-luminous candidate active galactic nuclei (AGNs) that lack X-ray detections in Chandra, XMM-Newton, and NuSTAR fields. We selected all optically detected SDSS sources with redshift measurements, combined additional broadband photometry from WISE, UKIDSS, 2MASS, and GALEX, and modeled the spectral energy distributions (SEDs) of our sample sources. We parameterize nuclear obscuration in our SEDs with $E(B\!-\!V)_{\text{AGN}}$ and uncover thousands of powerful obscured AGNs that lack X-ray counterparts, many of which are identified as AGN candidates based on straightforward WISE photometric criteria. Using the observed luminosity correlation between restframe 2-10 keV ($L_{\text{X}}$) and restframe AGN 6 $\mu{\text{m}}$ ($L_{\text{MIR}}$), we estimate the intrinsic X-ray luminosities of our sample sources and combine these data with flux limits from X-ray catalogs to determine lower limits on nuclear obscuration. Using the ratio of intrinsic-to-observed X-ray luminosity ($R_{L_{\text{X}}}$), we find a significant fraction of sources with column densities approaching $N_{\text{H}}>$ 10$^{\text{24}}$ cm$^{-{\text{2}}}$, suggesting that multiwavelength observations are necessary to account for the population of heavily obscured AGNs. We simulate the underlying $N_{\text{H}}$ distribution for the X-ray non-detected sources in our sample through survival analysis, and confirm the presence of AGN activity via X-ray stacking. Our results point to a considerable population of extremely obscured AGNs undetected by current X-ray observatories.