PANORAMIC: Discovery of an Ultra-Massive Grand-Design Spiral Galaxy at $z\sim 5.2$

Abstract

We report the discovery of an ultra-massive grand-design red spiral galaxy, named Zhúlóng (Torch Dragon), at $z_{\mathrm{phot}}=5.2_{-0.2}^{+0.3}$ in the JWST PANORAMIC survey, identified as 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 $R_{\mathrm{e}}=3.7\pm 0.1\mathrm{kpc}$), with spiral arms extending across 19 kpc; 2) a clear transition from the red, quiescent core ($F150W-F444W=3.1$ mag) with high stellar mass surface density ($\log (\hat{I}\pounds M_{\star }/M_{\odot }{\mathrm{kpc}}^{-2})=9.91_{-0.09}^{+0.11}$) to the star-forming outer regions, as revealed by spatially resolved SED analysis, which indicates significant inside-out galaxy growth; 3) an extremely high stellar mass at its redshift, with $\log (M_{\star }/M_{\odot })=11.03_{-0.08}^{+0.10}$ comparable to the Milky Way, and an implied baryon-to-star conversion efficiency ($\hat{I}\mu \sim 0.3$) that is 1.5 times higher than even the most efficient galaxies at later epochs; 4) despite an active disk, a relatively modest overall star formation rate ($\mathrm{SFR}=66_{-46}^{+89}{M}_{\odot }{\mathrm{yr}}^{-1}$), which is $>$0.5 dex below the star formation main sequence at $z\sim 5.2$ and $>$10 times lower than ultra-massive dusty galaxies at $z=5-6$. Altogether, Zhúlóng shows that mature galaxies emerged much earlier than expected 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.

12 pages, 5 main figures, 3 extended figures, 1 table; accepted in A&A