Deep X-ray View of a Distant Protocluster Anchored by a Quasar at z=6.63: No Evidence for Enhanced X-ray AGN Activity
Abstract
Whether dense environments accelerate SMBH growth during cosmic reionization remains uncertain because X-ray constraints on z>6 protoclusters are rare. We analyze ∼865 ks of Chandra observations of J0910-0414 quasar field, a massive protocluster at z=6.63. We construct a catalog of 251 X-ray sources and search for AGN among 21 spectroscopically confirmed members, excluding the central quasar. This is the first systematic X-ray AGN census in a z>6 protocluster; none of the members is individually detected. Stacking all members (∼17 Ms total exposure) yields no significant signal and limits the average rest-frame 2-10 keV luminosity to ⟨ L2−10 keV⟩≤9.54×1042 erg s−1. The null result gives a central 90% Jeffreys upper bound of fAGN<0.09. After accounting for the spatially varying sensitivity, the field Compton-thin X-ray luminosity function predicts Nexp,field=0.0356 AGN in the surveyed volume. Assuming the overdensity profile of AGNs in this protocluster is the same as that of galaxies determined by Wang et al. (2024), the expected AGN count becomes Nexp=0.131. Zero detections then imply a one-sided 90% upper limit of ηrel<17.5 on any additional X-ray AGN enhancement beyond that associated with the galaxy overdensity. Although ηrel<17.5 permits more than an order-of-magnitude enhancement, the data show no significant excess of X-ray-detectable AGN in J0910-0414 protocluster. The present X-ray data may miss heavily obscured, intrinsically X-ray-weak, or intermittently accreting SMBHs; the non-detection therefore does not rule out AGNs among the member galaxies.
Related
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BibTeX
@misc{wang2026deep,
title = {{Deep X-ray View of a Distant Protocluster Anchored by a Quasar at $z=6.63$: No Evidence for Enhanced X-ray AGN Activity}},
author = {Shouyi Wang and Feige Wang and Jinyi Yang and Bin Luo and Xiangyu Jin and Fan Zou and Jaclyn B. Champagne and Xiaohui Fan and Elaina Malczewski and Fabio Vito and Xue-Bing Wu and Eduardo Bañados and Fuyan Bian and Thomas Connor and Tiago Costa and Anna-Christina Eilers and Emanuele Paolo Farina and Joseph F. Hennawi and Hyunsung D. Jun and Jiang-Tao Li and Chiara Mazzucchelli and Klaudia Protušová and Fabian Walter and Zhibo Yu and Minghao Yue},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.19317},
eprint = {2610.19317},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.19317},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Deep X-ray View of a Distant Protocluster Anchored by a Quasar at z=6.63: No Evidence for Enhanced X-ray AGN Activity
AU - Shouyi Wang
AU - Feige Wang
AU - Jinyi Yang
AU - Bin Luo
AU - Xiangyu Jin
AU - Fan Zou
AU - Jaclyn B. Champagne
AU - Xiaohui Fan
AU - Elaina Malczewski
AU - Fabio Vito
AU - Xue-Bing Wu
AU - Eduardo Bañados
AU - Fuyan Bian
AU - Thomas Connor
AU - Tiago Costa
AU - Anna-Christina Eilers
AU - Emanuele Paolo Farina
AU - Joseph F. Hennawi
AU - Hyunsung D. Jun
AU - Jiang-Tao Li
AU - Chiara Mazzucchelli
AU - Klaudia Protušová
AU - Fabian Walter
AU - Zhibo Yu
AU - Minghao Yue
PY - 2026
DA - 2026/10/13
PB - arXiv
DO - 10.48550/arxiv.2610.19317
UR - https://arxiv.org/abs/2610.19317
AB - Whether dense environments accelerate SMBH growth during cosmic reionization remains uncertain because X-ray constraints on z>6 protoclusters are rare. We analyze ∼865 ks of Chandra observations of J0910-0414 quasar field, a massive protocluster at z=6.63. We construct a catalog of 251 X-ray sources and search for AGN among 21 spectroscopically confirmed members, excluding the central quasar. This is the first systematic X-ray AGN census in a z>6 protocluster; none of the members is individually detected. Stacking all members (∼17 Ms total exposure) yields no significant signal and limits the average rest-frame 2-10 keV luminosity to ⟨ L_2−10 keV⟩≤9.54×10^42 erg s^−1. The null result gives a central 90% Jeffreys upper bound of f_AGN<0.09. After accounting for the spatially varying sensitivity, the field Compton-thin X-ray luminosity function predicts N_exp,field=0.0356 AGN in the surveyed volume. Assuming the overdensity profile of AGNs in this protocluster is the same as that of galaxies determined by Wang et al. (2024), the expected AGN count becomes N_exp=0.131. Zero detections then imply a one-sided 90% upper limit of η_rel<17.5 on any additional X-ray AGN enhancement beyond that associated with the galaxy overdensity. Although η_rel<17.5 permits more than an order-of-magnitude enhancement, the data show no significant excess of X-ray-detectable AGN in J0910-0414 protocluster. The present X-ray data may miss heavily obscured, intrinsically X-ray-weak, or intermittently accreting SMBHs; the non-detection therefore does not rule out AGNs among the member galaxies.
ER -