PANORAMIC: Discovery of an ultra-massive grand-design spiral galaxy at z ∼ 5.2

¹ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
² NSF National Optical-Infrared Astronomy Research Laboratory, 950 North Cherry Avenue, Tucson, AZ, 85719 USA
³ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ, 85721 USA
⁴ Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128, København N, DK-2200 Denmark
⁵ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁶ Astronomy Centre, University of Sussex, Falmer, Brighton, BN1 9QH UK
⁷ Department of Physics and Astronomy and PITT PACC, University of Pittsburgh, Pittsburgh, PA, 15260 USA
⁸ Department of Physics, University of California, Santa Barbara, CA, 93106 USA
⁹ Department of Astronomy, University of Texas at Austin, 2515 Speedway Blvd. Stop C1400, Austin, TX, 78712 USA
¹⁰ Department of Astronomy, University of Massachusetts Amherst, Amherst, MA, 01003 USA
¹¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
¹² Caltech/IPAC, 1200 E. California Blvd., Pasadena, CA, 91125 USA
¹³ Leiden Observatory, Leiden University, PO Box 9513 NL-2300 AA Leiden, The Netherlands
¹⁴ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218 Hawthorn, 3122 VIC, Australia
¹⁵ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY, 14623 USA
¹⁶ Aix Marseille Univ., CNRS, CNES, LAM, Marseille, France
¹⁷ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA UK
¹⁸ Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge, CB3 0HE UK
¹⁹ Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI, 53706 USA

^⋆ Corresponding author; mengyuan.xiao@unige.ch

Received: 17 December 2024
Accepted: 27 February 2025

Abstract

We report the discovery of an ultra-massive grand-design red spiral galaxy, named Zhúlóng (Torch Dragon), at z_phot = 5.2_−0.2^+0.3 in the JWST PANORAMIC survey; it is the most distant bulge+disk galaxy candidate with spiral arms known to date. Zhúlóng displays an extraordinary combination of properties: (1) a classical bulge centered in a large, face-on exponential stellar disk (half-light radius of R_e = 3.7 ± 0.1 kpc) with spiral arms extending across 19 kpc; (2) a clear transition from the red, quiescent core (F150W − F444W = 3.1 mag) with a high stellar mass surface density (log(Σ M_⋆/M_⊙ kpc⁻²) = 9.91_−0.09^+0.11) to the star-forming outer regions, as revealed by spatially resolved spectral energy distribution analysis, which indicates significant inside-out galaxy growth; (3) an extremely high stellar mass for its redshift, with log(M_⋆/M_⊙) = 11.03_−0.08^+0.10, which is comparable to the Milky Way’s mass, and an implied baryon-to-star conversion efficiency (ϵ ∼ 0.3) that is 1.5 times higher than even the most efficient galaxies at later epochs; and (4) despite an active disk, a relatively modest overall star formation rate (SFR = 66₋₄₆⁺⁸⁹ M_⊙ yr⁻¹), which is > 0.5 dex below the star formation main sequence at z ∼ 5.2 and > 10 times lower than ultra-massive dusty galaxies at z = 5 − 6. Zhúlóng shows that mature galaxies emerged much earlier than previously believed, in the first billion years after the Big Bang, through rapid galaxy formation and morphological evolution. Our finding offers key constraints for models of massive galaxy formation and the origin of spiral structures in the early Universe.