Webb Spots a Fully Formed Spiral 'Twin' of the Milky Way When the Universe Was Just a Toddler

Astronomers using the James Webb Space Telescope have identified what may be the most distant barred spiral galaxy ever seen — a startlingly mature, Milky Way-like disk that already existed less than 1.2 billion years after the Big Bang, a time when such orderly structures were thought to be all but impossible.

The galaxy, cataloged as M1149-BSG-z5, sits at a redshift of 5.102, meaning its light has traveled to us for well over 12 billion years. What makes it remarkable is not just its distance but its shape: running through its center is a stellar bar, a straight cigar-shaped concentration of stars roughly 14,700 light-years long. Bars are a hallmark of settled, rotating disk galaxies like our own, and astronomers had long expected them to be rare in the chaotic, collision-prone conditions of the early cosmos.

By other measures, too, the galaxy looks improbably grown-up. It has an effective radius of about 8,500 light-years — larger than typical galaxies at that epoch — a stellar mass of roughly 28 billion suns, and it is churning out new stars at a rate of about 145 solar masses per year. Its light also suggests a chemically enriched interior; as the team put it, "the emission-line ratios are difficult to reconcile with a very metal-poor interpretation, and instead suggest that the gas is already chemically enriched," another sign of surprising maturity.

The discovery was made by an international team led by Xiaohan Wang of Tsinghua University, using Webb's Near-Infrared Imager and Slitless Spectrograph, or NIRISS, during the telescope's second cycle of observations. The researchers posted their analysis to the arXiv preprint server on June 23. "The discovery of M1149-BSG-z5 and its structural and global properties suggests that bars emerge as early as z > 5," they concluded — pushing back the clock on when galaxies first organized themselves into the elegant, barred spirals that dominate the universe today.

The team also spotted a companion galaxy some 70,000 light-years away and cautioned that more work is needed to nail down the object's nature. Follow-up measurements of how its stars and gas move, they said, "would be the key to confirm its baryon dominance." If it holds up, the find joins a growing body of Webb observations showing that the infant universe assembled complex galaxies far faster than theory predicted.