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

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_ea =a 3.7 ± 0.1 kpc) with spiral arms extending across 19 kpc; (2) a clear transition from the red, quiescent core (F150W-F444Wa =a 3.1 mag) with a high stellar mass surface density (log(S 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 (ϵa ~a 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 za ~a 5.2 and > 10 times lower than ultra-massive dusty galaxies at za =a 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.