Spatially resolved broad-line region in a quasar at z = 4

Dynamical black hole mass and prominent outflow

¹ Max Planck Institute for extraterrestrial Physics Giessenbachstraße 1 85748 Garching, Germany
² LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS 92190 Meudon, France
³ Universiteé Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange Nice, France
⁴ Univ. Grenoble Alpes, CNRS, IPAG 38000 Grenoble, France
⁵ Max Planck Institute for Astronomy Königstuhl 17 69117 Heidelberg, Germany
⁶ 1 st Institute of Physics, University of Cologne Zülpicher Straße 77 50937 Cologne, Germany
⁷ European Southern Observatory Karl-Schwarzschild-Straße 2 85748 Garching, Germany
⁸ School of Physics & Astronomy, University of Southampton Southampton SO17 1BJ, UK
⁹ Institute of Astronomy, KU Leuven Celestijnenlaan 200D 3001 Leuven, Belgium
¹⁰ Faculdade de Engenharia, Universidade do Porto rua Dr. Roberto Frias 4200-465 Porto, Portugal
¹¹ Departments of Physics and Astronomy, Le Conte Hall, University of California Berkeley CA 94720, USA
¹² Department of Physics, Technical University Munich James-Franck-Straße 1 85748 Garching, Germany
¹³ CENTRA – Centro de Astrofísica e Gravitação, IST, Universidade de Lisboa 1049-001 Lisboa, Portugal
¹⁴ Max Planck Institute for Radio Astronomy Auf dem Hügel 69 53121 Bonn, Germany
¹⁵ School of Physics, University College Dublin Belfield Dublin 4, Ireland
¹⁶ Universidad Nacional Autónoma de México. Instituto de Astronomía. A.P. 70-264 Ciudad de México 04510, México
¹⁷ Univ. Lyon, Univ. Lyon 1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574 69230 Saint Genis-Laval, France
¹⁸ Dublin Institute for Advanced Studies 31 Fitzwilliam Place D02 XF86 Dublin, Ireland
¹⁹ Kavli Institute for Astronomy and Astrophysics, Peking University Beijing 10087, People’s Republic of China
²⁰ Department of Astronomy, School of Physics, Peking University Beijing 100871, People’s Republic of China
²¹ Research School of Astronomy and Astrophysics, Australian National University Weston Creek ACT 2611, Australia
²² Centre for Gravitational Astrophysics (CGA), Australian National University Building 38 Science Road Acton ACT 2601, Australia
²³ Space & Astronomy, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Space & Astronomy P. O. Box 1130 Bentley WA 6102, Australia
²⁴ School of Physics, University of Melbourne Parkville VIC 3010, Australia
²⁵ INAF – Osservatorio Astrofisico di Arcetri Largo E. Fermi 50125 Firenze, Italy
²⁶ European Southern Observatory, Alonso de Cordova 3107 Vitacura Casilla 19001 Santiago, Chile

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Received: 17 September 2025
Accepted: 10 December 2025

Abstract

We present the first near-infrared interferometric data of a QSO at z = 4. The K-band observations were performed with GRAVITY+ on the VLTI using all four UTs, detecting a differential phase signal that traces the spatially resolved kinematics for both the Hβ and Hγ lines in the broad-line region (BLR). We fit the two lines simultaneously with an updated model that includes distinct rotating and conical outflowing components. For the best-fit model, more than 80% of the H I line emission from the BLR originates in an outflow with a velocity up to 10⁴ km s⁻¹. This is oriented so that our line of sight is along an edge of the conical structure, which produces the prominent blue wing on the line profile. A combination of anisotropic line emission and mid-plane opacity leads to the single-sided phase signal. The model is able to qualitatively match both the outflowing C IV line profile and the systemic O I fluorescent emission. The black hole mass of 8 × 10⁸ M_⊙ that we derive is the highest redshift black hole mass measurement to date obtained directly from BLR dynamics. It is an order of magnitude lower than that inferred from various single epoch scaling relations, and it implies that the accretion is highly super-Eddington. With reference to recent simulations, the data suggest that this QSO is emitting close to its radiative limit in a regime where strong outflows are expected around a polar conical region.