An extremely high-velocity outflow in SMSS J2157-3602, the most luminous quasar in the first 1.3 Gyr

¹ INAF – Istituto di Astrofisica Spaziale e Fisica cosmica Milano, Via Alfonso Corti 12, 20133 Milano, Italy
² Physical Sciences Division, School of STEM, University of Washington Bothell, Bothell, WA 98011, USA
³ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁴ INAF, Osservatorio Astronomico di Roma, Via Frascati 33, I– 00078 Monte Porzio Catone, Italy
⁵ School of Physics, HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
⁶ Department of Physics and Astronomy, York University, 4700 Keele Street, Toronto, ON M3J 1P3, Canada
⁷ Dipartimento di Fisica, Sapienza Universtià di Roma, Piazzale Aldo Moro 5, 00185, Rome
⁸ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11 34143 Trieste, Italy
⁹ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, 34014 Trieste, Italy
¹⁰ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti, 93/3, 40129 Bologna, Italy
¹¹ School of General Education, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan
¹² Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston Creek ACT 2611, Australia
¹³ Centre for Gravitational Astrophysics (CGA), Australian National University, Building 38 Science Road, Acton ACT 2601, Australia
¹⁴ Dipartimento di Fisica ed Astronomia (DIFA), Università di Bologna, Via Gobetti, 93/2, 40129 Bologna, Italy

^⋆ Corresponding author: giustina.vietri@inaf.it

Received: 26 May 2025
Accepted: 8 September 2025

Abstract

We report the discovery of an extremely high-velocity outflow (EHVO) in the most luminous QSO (L_Bol ∼ 2.29 × 10⁴⁸ erg/s), named SMSS J2157-3602, at z = 4.692. Combined XSHOOTER and NIRES observations reveal that the EHVO reaches a maximum velocity of v_max ∼ 0.13c and persists over rest-frame timescales of a few months up to one year. SMSS J2157-3602 also exhibits one of the highest balnicity index values discovered for an EHVO so far. In addition, the blueshifted CIV emission traces a high-velocity (v_CIV⁵⁰ ∼ 4660 km/s) outflow from the broad-line region (BLR). Thanks to an XMM-Newton observation, we were also able to reveal the X-ray weak nature of this QSO, which likely prevents the overionization of the innermost disk atmosphere and facilitates the efficient launch of the detected EHVO and BLR winds. The extraordinary luminosity of SMSS J2157-3602 and the extreme velocity of the EHVO make it a unique laboratory for testing active galactic nucleus (AGN) driven feedback under extreme conditions. Current uncertainties on the outflow’s location and column density strengthen the case for a dedicated follow-up, which will be essential to assess the full feedback potential of this remarkable quasar.