HYPERION

Interacting companion and outflow in the most luminous z  >  6 quasar

¹ Dipartimento di Fisica, Università di Trieste, Sezione di Astronomia, Via G.B. Tiepolo 11, 34131 Trieste, Italy
² INAF – Osservatorio Astronomico di Trieste, Via G. Tiepolo 11, 34143 Trieste, Italy
e-mail: roberta.tripodi@inaf.it
³ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy
⁴ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁵ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁶ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁷ Department of Astronomy, The University of Texas at Austin, Austin, TX, USA
⁸ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
⁹ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK2200 Copenhagen N, Denmark
¹⁰ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹¹ Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo, Kashiwa 277-8583, Japan
¹² Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
¹³ Dipartimento di Fisica e Astronomia, University of Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino (Firenze), Italy
¹⁴ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Italy
¹⁵ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straüe 1, 85748 Garching bei München, Germany
¹⁶ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK

Received: 2 June 2023
Accepted: 19 October 2023

Abstract

We present ALMA deep observations of the [CII] 158 μm emission line and the continuum at 253 GHz and 99 GHz towards SDSS J0100+2802 at z ≃ 6.3, the most luminous quasi-stellar object (QSO) at z > 6. J0100+2802 belongs to the HYPERION sample of luminous QSOs at z ∼ 6 − 7.5. The observations have a 2.2 arcsec resolution in band 3 and a 0.9 arcsec resolution in band 6, and are optimized to detect extended emission around the QSO. We detect an interacting, tidally disrupted companion both in [CII], peaking at z ∼ 6.332, and in continuum, stretching on scales up to 20 kpc from the quasar, with a knotty morphology. The higher velocity dispersion in the direction of the companion emission and the complex morphology of the tidally stretched galaxy suggest a possible ongoing or future merger. For the newly detected companion, we derive the range of the dust mass, M_dust = (0.3 − 2.6)×10⁷ M_⊙, and of the star formation rate, SFR  =  [35 − 344] M_⊙, obtained from the modelling of its cold dust spectral energy distribution. This shows that both the QSO and its companion are gas-rich and that a major merger may be at the origin of the boosted star formation. This close interacting companion is undetected by deep JWST imaging observations, showing the effectiveness of ALMA in detecting dust-obscured sources, especially in the vicinity of optically bright quasars. We also detect a broad blueshifted component in the [CII] spectrum, which we interpret as a gaseous outflow for which we estimate a mass outflow rate in the range Ṁ_out = (118 − 269) M_⊙ yr⁻¹. J0100+2802 was recently found to reside in a strong overdensity, however this close companion remained undetected by both previous higher resolution ALMA observations and by JWST-NIRCAM imaging. Our results highlight the importance of deep medium-resolution ALMA observations for the study of QSOs and their environment in the Epoch of Reionisation.