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JWST Discovery of M1149-BSG-z5: The Most Distant Barred Spiral Galaxy

Researchers using the James Webb Space Telescope (JWST) report the discovery of M1149-BSG-z5, a massive barred spiral galaxy at a spectroscopic redshift of z = 5.102, making it the highest-redshift barred galaxy candidate identified to date . The galaxy was found in the parallel field of the galaxy cluster MACS J1149+2223 using JWST/NIRCam imaging, JWST/NIRSpec spectroscopy, and archival HST data .

Key Physical Properties

• Stellar mass and maturity: Despite being observed only about 1.1 billion years after the Big Bang (the universe was roughly 8% of its current age), M1149-BSG-z5 is a massive galaxy. Best-fit SED modeling yields a stellar mass of approximately 10¹⁰–10¹¹ M☉, indicating remarkably rapid mass assembly .
• Bar structure: Both isophote ellipse fitting and structural modeling confirm a stellar bar with length a_bar ≈ 1.1–1.5 kpc . The bar is clearly detected in the JWST NIRCam images and is statistically significant in the ellipticity and position-angle profiles.
• Spiral structure: The galaxy shows clear multiple spiral arms emanating from the bar ends, consistent with a classical barred spiral morphology .
• Star formation rate: M1149-BSG-z5 lies on or above the main sequence of star-forming galaxies at z ~ 5, with a star formation rate on the order of several tens to ~100 M☉/yr . The galaxy is actively forming stars in both the bar region and the spiral arms.

Evolutionary Implications

The existence of a fully formed barred spiral at z = 5.102 pushes back the epoch of bar formation much earlier than standard cosmological simulations have predicted . This discovery challenges models that suggested bars need several billion years to form and stabilize in disk galaxies. The galaxy appears structurally mature—with a well-defined bulge, bar, and disk—just over a billion years after cosmic dawn, implying that disk settling and bar formation can occur within the first ~1–1.5 Gyr of the universe .

Potential Formation Mechanisms

The study discusses two primary mechanisms for the early bar formation in M1149-BSG-z5 :

1. Gravitational instability in baryon-dominated systems: In gas-rich, turbulent disks at high redshift, the central regions can become baryon-dominated. Rapid gravitational instabilities in these dense, cold-gas-rich disks can drive bar formation on timescales as short as a few hundred million years, without requiring a pre-existing, mature stellar disk .

2. Environment and tidal interactions: M1149-BSG-z5 lies within a galaxy overdensity—the MACS J1149+2223 field is a known cluster environment. Tidal interactions or minor mergers with nearby companion galaxies could have triggered or accelerated bar formation. Spectroscopy shows the galaxy has a close neighbor at a similar redshift, supporting a possible interaction scenario .

The authors conclude that a combination of baryon-dominated instability and mild environmental interactions is the most likely formation pathway, and that the classical picture of bars being a late-forming phenomenon (z < 1–2) is no longer tenable given multiple JWST discoveries of barred galaxies at z > 3 .

Note on the Current Record

M1149-BSG-z5 at z = 5.102 now supersedes previous record-holders like CEERS-2112 (z ≈ 3) and COSMOS-74706 (z ≈ 4) as the most distant barred spiral galaxy confirmed to date . The paper was posted to arXiv on June 23, 2026, and as of this writing is a preprint that has not yet been fully peer-reviewed .