JWST-discovered AGN: evidence for heavy obscuration in the type-2 sample from the first stacked X-ray detection

Andrea Comastri, Giorgio Lanzuisi, Fabio Vito, Stefano Marchesi, Marcella Brusa, Roberto Gilli, Ignas Juodzbalis, Roberto Maiolino, Giovanni Mazzolari, Guido Risaliti, Jan Scholtz, Cristian Vignali

First listed 2025-09-30 | Last updated 2025-09-30

Abstract

One of the most puzzling properties of the high-redshift AGN population recently discovered by JWST, including both broad-line and narrow-line sources, is their X-ray weakness. With very few exceptions, and regardless of the optical classification, they are undetected at the limits of the deepest Chandra fields, even when stacking signals from tens of sources in standard observed-frame energy intervals (soft, hard, and full bands). It has been proposed that their elusive nature in the X-ray band is due to heavy absorption by dust-free gas or intrinsic weakness, possibly due to high, super-Eddington accretion. In this work, we perform X-ray stacking in three customized rest-frame energy ranges (1-4, 4-7.25, and 10-30 keV) of a sample of 50 Type 1 and 38 Type 2 AGN identified by JWST in the CDFS and CDFN fields. For the Type 2 sub-sample, we reach a total of about 210 Ms exposure, and we report a significant ($\sim 3σ$) detection in the hardest (10-30 keV rest frame) band, along with relatively tight upper limits in the rest frame softer energy bands. The most straightforward interpretation is in terms of heavy obscuration due to gas column densities well within the Compton thick regime ($> 2 \times 10^{24} $cm$^{-2}$) with a large covering factor, approaching 4$π$. The same procedure applied to the Type 1 sub-sample returns no evidence for a significant signal in about 140 Ms stacked data in any of the adopted bands, confirming their surprisingly elusive nature in the X-ray band obtained with previous stacking experiments. A brief comparison with the current observations and the implications for the evolution of AGN are discussed.

Short digest

Stacks of deep Chandra data in rest-frame 1–4, 4–7.25, and 10–30 keV for 50 Type 1 and 38 Type 2 JWST-selected AGN in the CDFS/CDFN yield the first significant (~3σ) stacked X-ray detection for the Type 2 sample in the hardest band after ~210 Ms exposure. Softer rest-frame bands give only tight upper limits, while the Type 1 stack (~140 Ms) remains undetected in all bands. The hard-only signal implies heavy, Compton-thick obscuration (NH > 2×10^24 cm^-2) with a large covering factor approaching 4π. This supports obscuration-dominated scenarios for early AGN and clarifies why JWST-selected AGN are X-ray weak in standard observed bands.

Key figures to inspect

• Figure 1 (redshift distributions): Check median z for Type 1 vs Type 2 to see how rest-frame 10–30 keV maps onto the observed Chandra bands and why rest-frame stacking is essential.
• Stacked counts/SNR by rest-frame band: Verify the ~3σ detection for Type 2 in 10–30 keV and the non-detections (upper limits) in 1–4 and 4–7.25 keV; contrast with the across-the-board null result for Type 1.
• Stacked X-ray SED and obscuration constraints: Inspect how the band flux ratios translate to NH > 2×10^24 cm^-2 and a near-4π covering factor, and whether reflection-dominated models fit the stacked spectrum.
• [O III]-based bolometric luminosity distribution (Type 2): See the span of log Lbol ≈ 41.5–45 and assess whether luminosity subsets drive the hard-band detection or the soft-band non-detections.