The properties of X-ray-selected active galactic nuclei in protoclusters pinpointed by enormous Lyman alpha nebulae

¹ Istituto Nazionale di Astrofisica (INAF) – Osservatorio di Astrofisica e Scienza dello Spazio (OAS), Via Gobetti 101, I-40129 Bologna, Italy
² Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85748 Garching, Germany
³ Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), 11F of Astronomy-Mathematics Building, AS/NTU, No. 1, Section 4, 12 Roosevelt Road, Taipei 106319, Taiwan
⁴ Dipartimento di Fisica e Astronomia (DIFA), Università di Bologna, Via Gobetti 93/2, I-40129 Bologna, Italy
⁵ Department of Astronomy and Astrophysics, UCO/Lick Observatory, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA
⁶ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
⁷ INAF – Osservatorio Astrofisco di Arcetri, Largo E. Fermi 5, 50127 Firenze, Italy
⁸ Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634-0978, USA
⁹ Max-Planck-Institut für extraterrestrische Physik (MPE), Gießenbachstraße 1, 85748 Garching, Germany

^⋆ Corresponding author.

Received: 8 May 2025
Accepted: 2 July 2025

Abstract

Context. Protoclusters of galaxies are overdense regions of the Universe characterized by large gas reservoirs. These environments make them ideal laboratories for investigating galaxy-AGN coevolution and the growth of SMBHs. Galaxies residing in such dense regions are expected to grow their SMBHs efficiently, resulting in a higher incidence of AGN than in the field. Some protoclusters exhibit extended Lyα nebulae in their central regions, indicating the presence of massive gas reservoirs, although their main powering mechanism remains debated.

Aims. We aim to investigate the AGN population, and AGN enhancement, in three protoclusters at 2.3 ≲ z ≲ 3.2, namely the Slug, Fabulous, and J0819, which host enormous (≈200 − 400 kpc) Lyα nebulae (ELANe). Additionally, we search for the presence of diffuse X-ray emission in the same regions of the Lyα nebulae to reveal multiphase gas in these protoclusters.

Methods. To identify AGN among the protocluster members, we used deep (t_exp ∼ 190 − 270 ks) Chandra observations and performed X-ray spectral analysis to derive the properties of those sources. We compared the AGN fraction and space density with those observed in other known protoclusters and from the field environment.

Results. Overall, we find 11 X-ray detected AGN in the three protoclusters (2, 5, and 4 in the Slug, Fabulous, and J0819, respectively). Each structure hosts a central, X-ray-powerful (log(L_X/erg s⁻¹)∼45 − 46) QSO, while the other X-ray sources are mostly moderately luminous (log(L_X/erg s⁻¹)∼44) and obscured, Compton-thin AGN. The fraction of AGN in our targets is comparable with estimates for other protoclusters and significantly higher than what is found for low-redshift clusters. We also find a significant enhancement (2–4 dex) in AGN density, relative to both the field and non-active galaxies in the protoclusters. Finally, we find no significant soft X-ray diffuse emission from the nebulae, thus ruling out gravitational heating as the main powering mechanism of the ELANe.