A deep X-ray look to the most obscured quasar at z  ∼  3.6 and its environment

¹ Dipartimento di Matematica e Fisica, Universitá degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
² INAF – Osservatorio Astronomico di Roma, Via di Frascati 33, 00078 Monte Porzio Catone, Italy
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, Firenze I-50125, Italy
⁴ Dipartimento di Fisica e Astronomia, Universitá degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
⁵ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85748 Garching, Germany
⁶ Dipartimento di Fisica “G. Occhialini", Universitá degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
⁷ Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy
⁸ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
¹⁰ INAF – Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, I–34131 Trieste, Italy
¹¹ IFPU – Institut for fundamental physics of the Universe, Via Beirut 2, 34014 Trieste, Italy
¹² Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹³ Cavendish Laboratory Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
¹⁴ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹⁵ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
¹⁶ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People’s Republic of China
¹⁷ Dipartimento di Fisica, Universitá di Roma La Sapienza, Piazzale Aldo Moro 2, I-00185 Roma, Italy
¹⁸ INFN – Sezione Roma1, Dipartimento di Fisica, Universitá di Roma La Sapienza, Piazzale Aldo Moro 2, I-00185 Roma, Italy
¹⁹ Sapienza School for Advanced Studies, Viale Regina Elena 291, I-00161 Roma, Italy
²⁰ Dipartimento di Fisica e Astronomia ‘Augusto Righi’, Universitá degli Studi di Bologna, Via P. Gobetti, 93/2, 40129 Bologna, Italy

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Received: 28 July 2025
Accepted: 5 November 2025

Abstract

Context. The most luminous and obscured quasars (QSOs) detected through sensitive infrared all-sky surveys are thought to represent a key co-evolutionary phase from nuclear to circumgalactic (CG) scales in the formation of massive galaxies. In this context, hot dust obscured galaxies (hot DOGs) in the redshift interval z ∼ 2 − 4 (the so-called cosmic noon) provide unique opportunities to investigate the relation between cosmic mass assembly and the nuclear accretion processes of luminous QSOs and galaxies at high-z. W0410−0913 (hereafter W0410−09) is a luminous (L_bol ∼ 6.4 × 10⁴⁷ erg s⁻¹) and obscured QSO at z = 3.631 that is characterized by a 30 kpc CG Lyα nebula (CGLAN), which is rather small when the ∼ 100 kpc Lyα nebulae around the unobscured QSO is compared to the Type I QSO peers, and by an exceptional overdense environment of Lyα emitters (LAEs), with ∼19 of them located in the CG region with a radius of 300 kpc at a distance of ±200 km s⁻¹ from the Hot DOG.

Aims. Our aim is to detect and characterize active nuclear accretion in the Hot DOG W0410−09 and its environment.

Methods. We carried out this study by exploiting a deep proprietary ∼280 ks Chandra X-ray Observatory observation. We employed a set of empirical models suited for obscured sources and physically motivated spectroscopic models to account for a toroidal X-ray obscurer and the reprocessing of the X-ray radiation.

Results. The source W0410−09 consistently exhibits nuclear obscuration levels from mild to high star formation; it is Compton-thick (CT) and has a hydrogen column density of N_H > 10²⁴ cm⁻² (and up to N_H ∼ 10²⁵ cm⁻²) and an intrinsic luminosity of L_2 − 10 > 10⁴⁵ erg s⁻¹. W0410−09 is therefore one of the most luminous and obscured QSO at z > 3.5 discovered so far. This level of obscuration and the highly accreting nature of the Hot DOGs suggest that W0410−09 is undergoing a blow-out phase. This phase is predicted by models of merger-driven QSO formation scenarios, where strong winds begin to clear the dusty obscuring medium from the nuclear surroundings. We speculate that this heavy nuclear obscuration limits the amount of UV disk emission that powers its CGLAN, which in turn likely explains its small nebula size. Except for W0410−09, we detected no X-ray emission from any of the 19 LAEs. We analyzed their combined emission in several bands and only found a significant signal at the 3σ level in the 6−7 keV rest-frame energy band. We interpret this as caused by the Fe Kα line. This strongly suggests heavily obscured but so far undetected active galactic nuclei (AGN) emission in several LAEs. Considering W0410−09, we estimate an AGN fraction of f_AGN^LAE = 5₋₄⁺¹²%. This value can reach ∼35% when we account for unresolved obscured AGN, as suggested by the detection of the Fe Kα line.

Conclusions. W0410−09 is powered by an intrinsically luminous CT quasar. Its high obscuration likely explains the limited extent of its CGLAN. Our analysis suggests that this object is in a crucial transitional blow-out phase, during which powerful QSO-driven outflows will sweep out the nuclear obscuration to pave the way for an unobscured bright quasar.