A Massive Barred Spiral Galaxy at z = 5.102 Discovered by JWST

We report M1149-BSG-z5, a barred spiral galaxy at z = 5.102, identified in the parallel field of MACS J1149+2223 with JWST and the Hubble Space Telescope. M1149-BSG-z5 is the highest-redshift barred galaxy candidate to date. Both isophote ellipse fitting and structural modeling support a stellar bar of length ≈ 4.5 kpc, and extended spiral arms peaking at r ≈ 5.5 kpc. M1149-BSG-z5 is a massive star-forming main-sequence galaxy, with a stellar mass of 10 10.45 M ⊙ and a star formation rate of 144 M ⊙ yr −1 . A concentrated bulge is embedded in an extended disk with a global Sérsic index n = 2.37. With an effective radius of R e = 2.61 kpc measured from a stacked F277W + F356W + F444W image, M1149-BSG-z5 is larger than typical galaxies at z ∼ 5 and comparable to barred galaxies at 2 < z < 4. M1149-BSG-z5 also hosts a broad-line active galactic nucleus, with a relatively low black hole-to-stellar mass ratio of M BH / M * ∼ 10 −3 . Its metal-enriched emission-line properties indicate that it is already chemically evolved. Taken together, these properties imply that M1149-BSG-z5 assembled early and is already structurally evolved. We also find that M1149-BSG-z5 resides in an overdense region with a nearby companion galaxy, suggesting interaction-driven bar formation. Its concentrated light distribution, early assembly, and location on the star-forming main sequence also suggest baryon-dominated, gas-rich conditions, where gravitational instability may further accelerate bar formation.