A quasar-galaxy merger at z ∼ 6.2: Black hole mass and quasar properties from the NIRSpec spectrum

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
e-mail: federica.loiacono1@inaf.it
² Gemini Observatory, NSF’s NOIRLab, 670 N A’ohoku Place, Hilo, HI 96720, USA
³ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Institute for Theoretical Physics, University of Heidelberg, Philosophenweg 16, 69120 Heidelberg, Germany
⁵ MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Ave., Cambridge, MA 02139, USA
⁶ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁷ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
⁸ The Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark
⁹ Steward Observatory, University of Arizona, 933 N Cherry Avenue, Tucson, AZ 85721, USA
¹⁰ Department of Physics, University of Florida, Gainesville, FL 32611-8440, USA
¹¹ National Radio Astronomy Observatory, PO Box O Socorro, NM 87801, USA
¹² Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
¹³ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹⁴ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching b. München, Germany
¹⁵ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
¹⁶ INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
¹⁷ INAF – Osservatorio Astronomico di Brera, Via Brera 20, 20121 Milano, Italy
¹⁸ Department of Physics, Northwestern College, 101 7th St SW, Orange City, IA 51041, USA
¹⁹ National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre, 5071 West Saanich Road, Victoria BC V9E 2E7, Canada
²⁰ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
²¹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Avenida Ejercito Libertador 441, Santiago, Chile
²² Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
²³ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
²⁴ Leiden Observatory, Leiden University, Niels Bohrweg 2, NL-2333 CA Leiden, The Netherlands
²⁵ I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
²⁶ School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
²⁷ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands

Received: 9 November 2023
Accepted: 15 February 2024

Abstract

We present JWST/NIRSpec integral field data of the quasar PJ308-21 at z = 6.2342. As shown by previous ALMA and HST imaging, the quasar has two companion sources, interacting with the quasar host galaxy. The high-resolution G395H/290LP NIRSpec spectrum covers the 2.87 − 5.27 μm wavelength range and shows the rest-frame optical emission of the quasar with exquisite quality (signal-to-noise ratio ∼100 − 400 per spectral element). Based on the Hβ line from the broad line region, we obtain an estimate of the black hole mass M_BH, Hβ ∼ 2.7 × 10⁹ M_⊙. This value is within a factor ≲1.5 of the Hα-based black hole mass from the same spectrum (M_BH, Hα ∼ 1.93 × 10⁹ M_⊙) and is consistent with a previous estimate relying on the Mg IIλ2799 line (M_BH, MgII ∼ 2.65 × 10⁹ M_⊙). All these M_BH estimates are within the ∼0.5 dex intrinsic scatter of the adopted mass calibrations. The high Eddington ratio of PJ308-21 λ_Edd, Hβ ∼ 0.67 (λ_Edd, Hα ∼ 0.96) is in line with the overall quasar population at z ≳ 6. The relative strengths of the [O III], Fe II, and Hβ lines are consistent with the empirical “Eigenvector 1” correlations as observed for low redshift quasars. We find evidence for blueshifted [O III] λ5007 emission with a velocity offset Δv_{[O III]} = −1922 ± 39 km s⁻¹ from the systemic velocity and a full width at half maximum (FWHM) FWHM([O III]) = 2776₋₇₄⁺⁷⁵ km s⁻¹. This may be the signature of outflowing gas from the nuclear region, despite the true values of Δv_{[O III]} and FWHM([O III]) likely being more uncertain due to the blending with Hβ and Fe II lines. Our study demonstrates the unique capabilities of NIRSpec in capturing quasar spectra at cosmic dawn and studying their properties in unprecedented detail.