PANORAMIC: Discovery of an Ultra-Massive Grand-Design Spiral Galaxy at $z\sim5.2$

Abstract: We report the discovery of an ultra-massive grand-design red spiral galaxy, named Zh\'ul\'ong (Torch Dragon), at $z_{\rm phot} = 5.2^{+0.3}_{-0.2}$ in the JWST PANORAMIC survey, identified as the most distant bulge+disk galaxy candidate with spiral arms known to date. Zh\'ul\'ong displays an extraordinary combination of properties: 1) a classical bulge centered in a large, face-on exponential stellar disk (half-light radius of $R_{\rm e} = 3.7 \pm 0.1 \, \mathrm{kpc}$), 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 ($\log(\Sigma M_{\star}/M_{\odot} \, \mathrm{kpc}^{-2}) = 9.91_{-0.09}^{+0.11}$) 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 ($\epsilon \sim 0.3$) 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, Zh\'ul\'ong 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.

• The paper identifies Zhúlóng as the most distant ultra-massive spiral galaxy with a classical bulge and an extended stellar disk at z∼5.2.
• The paper employs spatially resolved SED analysis to reveal inside-out growth, with a red, quiescent core and star-forming outer regions.
• The paper underscores that Zhúlóng’s high baryon-to-star conversion efficiency challenges prevailing models of early galaxy formation.

Insights into the Discovery of Zhúlóng: An Ultra-Massive Spiral Galaxy at High Redshift

The paper "PANORAMIC: Discovery of an Ultra-Massive Grand-Design Spiral Galaxy at $z\sim5.2$" reports an intriguing observational finding: the ultra-massive grand-design spiral galaxy Zhúlóng, discovered at a redshift of approximately 5.2. Identified within the JWST PANORAMIC survey, this galaxy emerges as the most distant known example of a bulge+disk system with spiral arms.

Zhúlóng exhibits several compelling properties:

• The galaxy possesses a classical bulge embedded within an expansive stellar disk, characterized by a half-light radius of 3.7 kpc and spiral arms extending across 19 kpc.
• A spatially resolved spectral energy distribution (SED) analysis reveals a transition from a red, quiescent core with high stellar mass surface density to star-forming outer regions, indicative of inside-out galaxy growth, a hallmark of morphological evolution.
• Zhúlóng’s stellar mass, $\log (M_{\star}/M_{\odot})=11.03$, rivals that of the Milky Way. This mass at such a high redshift implies a baryon-to-star conversion efficiency of $\sim 0.3$, which is significantly higher than what is typically observed in galaxies at later epochs.
• Despite an actively developing disk, Zhúlóng has a relatively modest star formation rate (SFR) of 66 $M_{\odot} \, \mathrm{yr}^{-1}$, residing more than 0.5 dex below the star formation main sequence for $z \sim 5.2$.

This discovery offers crucial insights into the formation and evolution of massive galaxies in the early universe and challenges prevailing models of massive galaxy formation. The existence of such a mature galaxy less than a billion years after the Big Bang suggests that processes governing galaxy evolution can occur on much faster timescales than previously estimated.

Morphological and Evolutionary Implications

The morphology of Zhúlóng, characterized by a massive bulge and extensive spiral arms, raises important questions about the formation mechanisms of such structures in the early universe. Traditional views suggest that large stellar disks are a late emergent phenomenon, appearing prominently only by $z\sim2$. However, with $>$50% of discs observed at $z<6$, and the discovery of Zhúlóng, it is evident that substantial morphological features can form earlier than previously thought.

The efficient mass build-up of Zhúlóng, implied by its high baryon-to-star conversion efficiency, may involve rapid star formation episodes and potentially merger-driven growth. The presence of stellar clumps, resembling the clump-driven evolution model, may also contribute to its rapid assembly.

Future Research Directions

Zhúlóng's unexpected properties necessitate further theoretical and observational investigations into early massive galaxy formation. Future JWST and ALMA observations could explore the gas content and dynamics of such galaxies, shedding light on the processes leading to the rapid early formation of spiral structures and dense bulges. Additionally, the discovery highlights the importance of cosmic variance in interpreting high-z galaxy data, encouraging larger survey areas to statistically constrain the rarity and potential pathways of similar massive, morphologically mature galaxies in the early universe.

In summary, the discovery of Zhúlóng represents a significant contribution to the understanding of galactic morphology and mass assembly in the early universe. Its unique combination of properties makes it an important benchmark for models of galaxy formation, raising pivotal questions about the timelines and processes underpinning the emergence of complex structures in the high redshift universe.