Daniel Ivanov

The formation of stellar bars is an important milestone in the secular evolution of spiral galaxies, which typically indicates the presence of a massive rotationally supported disk. Determining when these structures first appeared in the early Universe is crucial to constraining the timeline of Galactic disk assembly. Here, we report the discovery of COSMOS-74706, a barred spiral galaxy at z _spec = 3.159. Imaging of COSMOS-74706 with JWST/NIRCam indicates a disk-like morphology and spiral structure with an elongated central feature aligned between the spiral arms, most conspicuously visible in the F200W, F277W, and F356W filters. Three independent methods all support the presence of a bar: visual inspection of residuals from Sérsic profile fitting shows a linear structure, isophotal ellipse-fitting displays characteristic profiles of ellipticity and position angle consistent with a bar signature, and Fourier decomposition of the galaxy produces a central bisymmetric mode above a threshold strength calibrated to z = 1−3 barred spirals. Leveraging archival Keck/MOSDEF spectroscopy overlapping with a blue clump on the edge of the galaxy, a robust redshift is inferred, with photometric constraints indicating that this structure lies at the same redshift as the main spiral. This spectroscopic evidence placing an unlensed barred spiral at z > 3 supports the idea that galaxies with rotationally supported disks and disk-halo properties that are conducive to bar formation were already in place within 2 Gyr after the Big Bang.

We present a detailed analysis of a massive barred galaxy at z _spec = 3.1591 using deep multiband imaging from the Hubble Space Telescope (HST) and JWST. For the first time, we resolve its morphology and stellar structures thanks to the JWST/NIRCam near-infrared (NIR) and mid-infrared (MIR) photometry. The galaxy possesses two distinct components with significantly different colors. Through careful image decomposition and masking, we isolate and characterize the flux contribution from each component. The galaxy exhibits a clear spiral morphology, and in a separate companion paper, we present evidence suggesting the presence of a stellar bar. Based on spatially resolved spectral energy distribution modeling with Prospector , we derive the star formation histories and other physical properties of the bar and the surrounding regions. The total stellar mass of the galaxy is constrained as log ( M * / M ⊙ ) = 10.63 ± 0.13 . We find that the bar region contains around 30% of the total stellar mass, but only accounts for around 8% of the recent star formation rate. The region containing the potential bar shows a significantly older mass-weighted stellar age, supporting the inside-out scenario for galaxy formation, and providing tentative evidence for bar quenching in the early stage. The quick onset of a stellar bar at this redshift requires a low dark matter fraction, suggesting the baryon-dominated nature of high- z massive galaxies, and offering rare insight into galaxy evolution at around 2 billion yr after the Big Bang.