X-ray view of a massive node of the cosmic web at z  ∼  3

II. Discovery of extended X-ray emission around a hyperluminous quasar

¹ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, I-20126 Milano, Italy
² Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747, AD, Groningen, The Netherlands
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50127 Firenze, Italy
⁴ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
⁵ Dipartimento di Fisica – Sezione di Astronomia, Università di Trieste, Via Tiepolo 11, 34131 Trieste, Italy
⁶ IFPU, Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy
⁷ INAF – Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
⁸ INFN, Sezione di Trieste, Via Valerio 2, 34127 Trieste, Italy
⁹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
¹⁰ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
¹¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Rome, Italy
¹² Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc, 135, Roma, Italy
¹³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy

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Received: 28 August 2025
Accepted: 27 May 2026

Abstract

While the warm (T ∼ 10⁴ K), ionized gas in the circumgalactic medium (CGM) at z > 3 is now routinely observed around bright quasars in Lyα emission, little is known about the CGM hot phase (T ∼ 10⁷ K). Here, we report the analysis of 634 ks of Chandra X-ray observations in the MQN01 Cosmic Structure, a region containing one of the brightest Lyα nebulae and the largest galaxy overdensity known at z > 3. We detect ≈66 net counts (≈8σ) of X-ray emission in the 0.5–2 keV band extending to at least ∼30 kpc from the brightest quasar in MQN01. The morphology and spectrum are consistent with thermal emission from hot plasma in collisional ionization equilibrium. Quasi-stellar object (QSO) photoionization is found to have a negligible impact, and alternative scenarios such as inverse Compton scattering are disfavoured. A joint spatial and spectral Markov chain Monte Carlo (MCMC) analysis provides consistency with a β model with a steep density profile (β ≃ 2.0^+1.4_−0.8, r_core = 36⁺¹³₋₁₆ kpc) and an average gas temperature of kT ≃ (1.8 ± 0.4) keV. Assuming that the QSO resides within a virialized halo, whose virial temperature is equal to the observed gas temperature, the implied halo mass and virial radius would be M_vir ∼ 3 × 10¹³ M_⊙ and ∼190 kpc. Further assuming that the observed emission traces the brighter inner region of a distribution extending to the virial radius, the extrapolated hot gas mass would be M_hot ≈ 2.6^+1.7_−0.6 × 10¹² M_⊙, which is ≃ 8.3^+3.0_−9.8% of M_vir, or 56⁺⁶⁵₋₂₀% of the theoretical cosmological baryon budget of the halo. The hot gas also emits an exceptionally high X-ray luminosity, with a measured L_2 − 10 = 2.25_−1.38^+0.77 × 10⁴⁵ erg s⁻¹ within the central 30 kpc. This system is a clear outlier in the L_X–T_X plane, indicating a thermodynamic state distinct from that of evolved lower-redshift hot halos. The cooling time in the inner 15-30 kpc is comparable to the local dynamical time (1 < t_cool/t_ff < 10), suggesting that the gas could become locally unstable in the absence of heating or feedback. Moreover, the thermal pressure associated with the detected CGM hot phase is large enough to confine the cold and dense clumps, which are required to reproduce the high Lyα emission associated with the inner regions of the MQN01 structure. Although limited to a single system, our results provide unique information on the multi-phase properties of the CGM (or proto-intracluster medium) and a view of the nascent thermal hot gas phase observed in local galaxy clusters.