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

TL;DR

This study confronts why JWST-discovered high-redshift AGN are X-ray weak by stacking Chandra data in rest-frame bands designed to reveal heavy obscuration. It analyzes 38 Type 2 and 50 Type 1 JWST-selected AGN, finding a significant detection only in the hardest rest-frame band for Type 2, consistent with Compton-thick obscuration with log$N_H$ ≈ 24.2 cm$^{-2}$. The Type 1 stack remains undetected, reinforcing the notion that X-ray faintness at high redshift is driven by absorption rather than intrinsic X-ray weakness. Bolometric corrections for the Type 2 population align with local AGN relations, suggesting JWST Type 2 AGN are the obscured extension of the known AGN population and contributing to the hard X-ray background within current uncertainties.

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.

Figures (6)

• Figure 1: Left: Redshift distribution for the Type 2s AGN analysed in this work, all from CDF-S. The median of the distribution is reported, as well as the 16th and 84th percentiles, as labeled. Right: Redshift distribution for the Type 2s AGN analysed in this work, from the CDF-S (blue) and CDF-N (green). The median of the distribution is reported, as well as the 16th and 84th percentiles, as labeled.
• Figure 2: Left: Type 2 stacked image 1-4 keV rest-frame. Center: Type 2 stacked image 4-7.25 keV rest-frame. Right: Type 2 stacked image 10-30 keV rest-frame.
• Figure 3: Total (CDFS+CDFN) stacked images for the Type 1 sample. Left: 1-4 keV SB. Center: 4-7.25 keV MB. Right: 10-30 keV UHB.
• Figure 4: Distribution of stacked net-counts in the UHB for 1000 realizations of random positions (cyan histogram). The number of stacked net counts obtained for the type-2 AGN sample is marked with the dashed red line. All of the random realizations return a lower number of net counts, implying a significance of $>0.999$ of the stacked emission from the type-2 AGN sample.
• Figure 5: Spectral models computed with the uxclumpy code described in Buchner19 rescaled to the observed 10--30 keV flux from the stacking analysis. We refer to the text for details on the model assumptions and geometry. The column density increases from 5 $\times$ 10$^{23}$ cm$^{-2}$ (red curve), to 2 $\times$ 10$^{24}$ cm$^{-2}$ (blue curve) and 10$^{25}$ cm$^{-2}$ (cyan curve). The UHB detection and the upper limits in the SB and MB are also reported.