Oral Presentation

Statistical Properties of X-ray Detected and Undetected AGN Revealed by Stacking Analysis of the eFEDS Survey

Author(s): Sorami Soga (Nara Women's University), Yoshiki Toba (Ritsumeikan University), Naomi Ota (Nara Women's University)

Presenter: Sorami Soga (Nara Women's University)

A large population of X-ray–faint AGN is predicted in wide-field surveys and key to understanding AGN evolution and their co-evolution with host galaxies.
However, individual X-ray detections are often too weak to constrain intrinsic properties, making stacking analysis essential. Traditional stacking approaches typically assume fixed intrinsic absorption, resulting in large uncertainties in deriving intrinsic X-ray luminosities.

To overcome this limitation, we developed a stacking method that directly estimates intrinsic hydrogen column density (NH) from stacked hardness ratios and uses it to derive intrinsic 2–10 keV luminosities. After validating this approach with simulations reproducing eFEDS instrumental conditions, we applied it to redshift-binned groups of X-ray–detected and X-ray–undetected infrared-selected sources in the eFEDS field, which were selected by Toba et al. (2022).

For X-ray–detected AGN, we find a tight correlation between 6 μm luminosity (L6) and intrinsic X-ray luminosity (Lx), consistent with previous studies and showing no high-luminosity flattening. This correlation exhibits a strong redshift dependence but no significant NH dependence. In contrast, X-ray–undetected infrared sources show much lower stacked X-ray luminosities and no elevated star formation rates, with positions in L6–Lx and SFR–Lx planes resembling normal star-forming galaxies rather than heavily obscured AGN.

Across all samples, the average estimated NH remained below 1e22 cm^-2, and even for the X-ray–undetected sources, the photon-statistics–driven uncertainty in NH stayed within approximately two orders of magnitude. These results indicate that hardness-ratio–based stacking effectively characterizes faint populations and distinguishes weak AGN activity from star formation in wide-field X-ray surveys.