GA-NIFS: A massive black hole in a low-metallicity AGN at z ∼ 5.55 revealed by JWST/NIRSpec IFS

¹ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA UK
e-mail: hu215@cam.ac.uk
² Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge, CB3 0HE UK
³ Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT UK
⁴ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield, AL10 9AB UK
⁵ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir km 4, Torrejón de Ardoz, 28850 Madrid, Spain
⁶ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁷ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
⁸ National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre, 5071 West Saanich Road, Victoria, BC, V9E 2E7 Canada
⁹ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Mount Stromlo, Australia
¹⁰ University of Oxford, Department of Physics, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH UK
¹¹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹² European Space Agency, ESAC, Villanueva de la Cañada, 28692 Madrid, Spain
¹³ Cosmic Dawn Center (DAWN), Rådmandsgade 62, 2200 Copenhagen, Denmark
¹⁴ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark
¹⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁶ European Space Agency, c/o STScI, 3700 San Martin Drive, Baltimore, MD, 21218 USA
¹⁷ INAF – Osservatorio Astrofisco di Arcetri, largo E. Fermi 5, 50127 Firenze, Italy
¹⁸ AURA for the European Space Agency, Space Telescope Science Institute, Baltimore, MD, USA

Received: 13 February 2023
Accepted: 21 June 2023

Abstract

We present rest-frame optical data of the compact z = 5.55 galaxy GS_3073 obtained using the integral field spectroscopy mode of the Near-InfraRed Spectrograph on board the James Webb Space Telescope. The galaxy’s prominent broad components in several hydrogen and helium lines (though absent in the forbidden lines) and v detection of a large equivalent width of He IIλ4686, EW(He II) ∼20 Å, unambiguously identify it as an active galactic nucleus (AGN). We measured a gas phase metallicity of Z_gas/Z_⊙∼0.21_−0.04^+0.08 , which is lower than what has been inferred for both more luminous AGN at a similar redshift and lower redshift AGN. We empirically show that classical emission line ratio diagnostic diagrams cannot be used to distinguish between the primary ionisation source (AGN or star formation) for systems with such low metallicity, though different diagnostic diagrams involving He IIλ4686 prove very useful, independent of metallicity. We measured the central black hole mass to be log(M_BH/M_⊙)∼8.2 ± 0.4 based on the luminosity and width of the broad line region of the Hα emission. While this places GS_3073 at the lower end of known high-redshift black hole masses, it still appears to be overly massive when compared to its host galaxy’s mass properties. We detected an outflow with a projected velocity ≳700 km s⁻¹ and inferred an ionised gas mass outflow rate of about 100 M_⊙ yr⁻¹, suggesting that one billion years after the Big Bang, GS_3073 is able to enrich the intergalactic medium with metals.