A quasar-galaxy merger at z ∼ 6.2: Rapid host growth via the accretion of two massive satellite galaxies

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, I-40129 Bologna, Italy
² Gemini Observatory, NSF’s NOIRLab, 670 N A’ohoku Place, Hilo, HI, 96720 USA
³ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, I-20126 Milano, Italy
⁴ INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, I-20126 Milano, Italy
⁵ INAF - Osservatorio Astronomico di Brera, via Brera 20, I-20121 Milano, Italy
⁶ Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell’Insubria, via Valleggio 11, I-22100 Como, Italy
⁷ Departement d’Astronomie, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁸ Department of Astrophysical Sciences, Princeton University, Princeton, NJ, 08544 USA
⁹ Leiden Observatory, Leiden University, Niels Bohrweg 2, NL-2333 CA Leiden, The Netherlands
¹⁰ Steward Observatory, University of Arizona, 933 N Cherry Avenue, Tucson, AZ, 85721 US
¹¹ Max Planck Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
¹² Department of Physics, University of Florida, Gainesville, FL, 32611-8440 USA
¹³ Institute for Theoretical Physics, University of Heidelberg, Philosophenweg 16, 69120 Heidelberg, Germany
¹⁴ National Radio Astronomy Observatory, PO Box O Socorro, NM, 87801 USA
¹⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA, 02138 USA
¹⁶ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109 USA
¹⁷ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, D-85748 Garching b. München, Germany
¹⁸ Newcastle University, School of Mathematics, Statistics and Physics, Herschel Building, Newcastle upon Tyne, NE1 7RU UK
¹⁹ MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Ave., Cambridge, MA, 02139 USA
²⁰ Department of Physics, Northwestern College, 101 7th St SW, Orange City, IA, 51041 USA
²¹ National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre, 5071 West Saanich Road, Victoria, BC, V9E 2E7 Canada
²² ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions, Melbourne, Australia
²³ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Avenida Ejercito Libertador 441, Santiago, Chile
²⁴ Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8583 Japan
²⁵ Center for Data-Driven Discovery, Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba, 277-8583 Japan
²⁶ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, 100871 PR China
²⁷ I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
²⁸ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
²⁹ School of Physics and Astronomy, Tel Aviv University, Tel Aviv, 69978 Israel
³⁰ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
³¹ The Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark
³² Institut d’Astrophysique de Paris, Sorbonne Université, CNRS, UMR 7095, 98 bis bd Arago, 75014 Paris, France
³³ Department of Astronomy, Huazhong University of Science and Technology, Wuhan, 430074 PR China
³⁴ Department of Astronomy, Tsinghua University, Beijing, 100084 PR China

Received: 15 January 2024
Accepted: 10 June 2024

Abstract

We present JWST/NIRSpec integral field spectroscopy in the rest-frame optical bands of the system PJ308–21, a quasar at z = 6.2342 caught as its host galaxy interacts with companion galaxies. We detect the spatially extended emission of several emission lines (Hα, Hβ, [O III], [N II], [S II], and He II), which we used to study the properties of the ionized phase of the interstellar medium: the source and hardness of the photoionizing radiation field, metallicity, dust reddening, electron density and temperature, and star formation. We also marginally detected continuum starlight emission associated with the companion sources. We find that at least two independent satellite galaxies are part of the system. While the quasar host appears highly enriched and obscured, with photoionization conditions typical of an Active Galactic Nucleus, the western companion shows minimal dust extinction, low metallicity (Z ∼ 0.4 Z_⊙), and star formation driven photoionization. The eastern companion shows higher extinction and metallicity (Z ∼ 0.8 Z_⊙) compared to the western companion, and it is at least partially photoionized by the nearby quasar. We do not find any indication of AGN in the companion sources. Our study shows that while the quasar host galaxy is already very massive (M_dyn > 10¹¹ M_⊙), it is still rapidly building up by accreting two relatively massive (M_star ∼ 10¹⁰ M_⊙) companion sources. This dataset showcases the power of JWST in exposing the buildup of massive galaxies in the first gigayear of the Universe.