Chandra reveals a luminous Compton-thick QSO powering a Lyα blob in a z = 4 starbursting protocluster

¹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
e-mail: fvito.astro@gmail.com
² Instituto de Astrofísica and Centro de Astroingenieria, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
³ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100012, PR China
⁴ Department of Astronomy & Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
⁵ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
⁶ Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA
⁷ Department of Physics, University of Arkansas, 226 Physics Building, 825 West Dickson Street, Fayetteville, AR 72701, USA
⁸ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
⁹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
¹⁰ Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile
¹¹ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
¹² School of Physics, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
¹³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
¹⁴ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy

Received: 5 July 2020
Accepted: 10 August 2020

Abstract

Context. Galaxy clusters in the local universe descend from high-redshift overdense regions known as protoclusters. The large gas reservoirs and high rate of galaxy interaction in protoclusters are expected to enhance star-formation activity and trigger luminous supermassive black-hole accretion in the nuclear regions of the host galaxies.

Aims. We investigated the active galactic nucleus (AGN) content of a gas-rich and starbursting protocluster at z = 4.002, known as the Distant Red Core (DRC). In particular, we search for luminous and possibly obscured AGN in 13 identified members of the structure, and compare the results with protoclusters at lower redshifts. We also test whether a hidden AGN can power the Lyα blob (LAB) detected with VLT/MUSE in the DRC.

Methods. We observed all of the identified members of the structure with 139 ks of Chandra ACIS-S imaging. Being less affected by absorption than optical and IR bands, even in the presence of large column densities of obscuring material, X-ray observations are the best tools to detect ongoing nuclear activity in the DRC galaxies.

Results. We detect obscured X-ray emission from the two most gas-rich members of the DRC, named DRC-1 and DRC-2. Both of them are resolved into multiple interacting clumps in high-resolution Atacama Large Millimeter Array and Hubble Space Telescope observations. In particular, DRC-2 is found to host a luminous (L_2−10 keV ≈ 3 × 10⁴⁵ erg s⁻¹ ) Compton-thick (N_H ≳ 10²⁴ cm⁻²) quasar (QSO) candidate, comparable to the most luminous QSOs known at all cosmic times. The AGN fraction among DRC members is consistent with results found for lower redshift protoclusters. However, X-ray stacking analysis reveals that supermassive black hole (SMBH) accretion is likely also taking place in other DRC galaxies that are not detected individually by Chandra.

Conclusions. The luminous AGN detected in the most gas-rich galaxies in the DRC and the widespread SMBH accretion in the other members, which is suggested by stacking analysis, point toward the presence of a strong link between large gas reservoirs, galaxy interactions, and luminous and obscured nuclear activity in protocluster members. The powerful and obscured QSO detected in DRC-2 is likely powering the nearby LAB detected with VLT/MUSE, possibly through photoionization; however, we propose that the diffuse Lyα emission may be due to gas shocked by a massive outflow launched by DRC-2 over a ≈10 kpc scale.