GA-NIFS and EIGER: A merging quasar host at z = 7 with an overmassive black hole

¹ Los Alamos National Laboratory, Los Alamos, NM, 87545, USA
² MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Ave., Cambridge, MA, 02139, USA
³ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
⁴ Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge, CB3 0HE, UK
⁵ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir km 4, Torrejón de Ardoz, E-28850 Madrid, Spain
⁶ National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada
⁷ Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
⁸ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
⁹ University of Oxford, Department of Physics, Denys Wilkinson Building, Keble Road, Oxford, OX13RH, United Kingdom
¹⁰ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
¹¹ European Space Agency, c/o STScI, 3700 San Martin Drive, Baltimore, MD, 21218, USA
¹² Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy
¹³ Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38400 Saint-Martin-d’Hères, France
¹⁴ INAF – Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50127 Firenze, Italy
¹⁵ Department of Physics, North Carolina State University, Raleigh, NC, 27695-8202, USA
¹⁶ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo, 181-8588, Japan
¹⁷ Laboratoire d’astrophysique, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire, CH-1290 Versoix, Switzerland
¹⁸ Institute of Science and Technology Austria (ISTA), Am Campus 1, 3400 Klosterneuburg, Austria

^⋆ Corresponding author: mmarshall@lanl.gov

Received: 17 October 2024
Accepted: 3 August 2025

Abstract

The James Webb Space Telescope is revolutionising our ability to understand the host galaxies and local environments of high-z quasars. Here we obtain a comprehensive understanding of the host galaxy of the z = 7.08 quasar J1120+0641 by combining NIRSpec integral field spectroscopy with NIRCam photometry of the host continuum emission. Our emission-line maps reveal that this quasar host is undergoing a merger with a bright companion galaxy. The quasar host and the companion have similar dynamical masses of ∼10¹⁰ M_⊙, suggesting that this is a major galaxy interaction. Through detailed quasar subtraction and SED fitting using the NIRCam data, we obtained an estimate of the host stellar mass of M_* = (3.0_−1.4^+2.5) × 10⁹ M_⊙, with M_∗ = (2.7_−0.5^+0.5) × 10⁹ M_⊙ for the companion galaxy. Using the Hβ Balmer line, we estimated a virial black hole mass of M_BH = (1.9_−1.1^+2.9) × 10⁹ M_⊙. Thus, J1120+0641 has an extreme black hole–stellar mass ratio of M_BH/M_* = 0.63_−0.31^+0.54, which is ∼3 dex larger than expected by the local scaling relations between black hole and stellar mass. J1120+0641 is powered by an overmassive black hole with the highest reported black hole–stellar mass ratio in a quasar host that is currently undergoing a major merger. These new insights highlight the power of JWST for measuring and understanding these extreme first quasars.