A grand-design spiral galaxy 1.5 billion years after the Big Bang with JWST

Abstract

We report the discovery of Alaknanda, a large ($\sim10$ kpc diameter), massive ($\log(M_\star/M_\odot)\sim10.2$), candidate grand-design spiral galaxy with photometric redshift $z_{phot}\sim4.05$ in the UNCOVER and Medium band, Mega Science surveys with JWST. This is among the highest redshift spiral galaxies discovered with JWST. Our morphological analysis using GALFIT reveals that this galaxy is a well-formed disk, with two symmetric spiral arms that are clearly visible in the GALFIT residual. In the rest-frame near-UV and far-UV, we clearly see the beads-on-a-string pattern of star formation; in the rest-frame visible bands, each string appears as an arm. Spectral energy distribution modeling using the BAGPIPES and Prospector codes is strongly constrained by detections and flux measurements in 21 JWST and HST filters. From the BAGPIPES modeling, the stellar mass-weighted age is $\sim 199$ Myr, implying 50\% of the stars in the galaxy formed after $z\sim4.6$. This is a highly star-forming galaxy with a star formation rate (SFR) of $\sim 63 \, M_\odot \, \text{yr}^{-1}$. We detect flux excesses in the F250M and F335M filters due to the presence of H-$\alpha$+[NII] and [OIII]+H-$\beta$ emission line complexes respectively. Detection of a spiral galaxy at $z \sim 4$ indicates that massive and large spiral galaxies and disks were already in place merely 1.5 billion years after the Big Bang. Future observations with NIRSpec IFU and ALMA will be able to probe the kinematics of the galactic disk, throwing light on the possible origin of the spiral arms in this galaxy.