Early galaxy looked like lumpy ‘cosmic grapes’

A cluster of “Cosmic Grapes” is challenging current theoretical models on how galaxies formed during the universe’s earliest eons. After combining observations from the Atacam Large Millimeter/submillimeter Array (ALMA) and the James Webb Space Telescope, astronomers now possess an unprecedented look at about 15 gargantuan “star-forming clumps” inside a single rotating disk that formed only 900 million years after the big bang.

“Early galaxies form through dark matter and gas assembly, evolving into dynamically hot, chaotic structures driven by mergers and feedback,” the international research team explained in their Nature Astronomy study published on August 7. “By contrast, remarkably smooth, rotating disks are observed in massive galaxies only 1.4 billion years after the Big Bang, implying rapid dynamical evolution.”

The Cosmic Grapes initially appeared in past HST data as a typical galaxy with a smooth stellar disk (left). However, deep, high-resolution follow-up observations by JWST (middle) and ALMA (right) revealed that it actually consists of numerous compact stellar clumps embedded within a smooth, rotating gas disk. The red and blue colors in the right panel represent redshifted and blueshifted gas motions, respectively, tracing the rotation of the disk. Credit: NSF/AUI/NSF NRAO/B. Saxton — The Cosmic Grapes initially appeared in past HST data as a typical galaxy with a smooth stellar disk (left). However, deep, high-resolution follow-up observations by JWST (middle) and ALMA (right) revealed that it actually consists of numerous compact stellar clumps embedded within a smooth, rotating gas disk. The red and blue colors in the right panel represent redshifted and blueshifted gas motions, respectively, tracing the rotation of the disk. Credit: NSF / AUI/ NSF NRAO / B. Saxton

Understanding how this cosmic evolution unfolded requires the ability to study young galaxies—something previously made difficult by limitations in observational tools’ sensitivity and spatial resolution. Even with the Hubble Space Telescope’s groundbreaking abilities, the “Cosmic Grapes” galaxy only appeared as a smooth, singular disk-like formation. Using more recent and advanced projects like JWST and ALMA, astronomers were able to refocus on the mystery target with some help from a cosmic neighbor.

According to a study announcement from the National Radio Astronomy Observatory, the Cosmic Grapes structure “happened to be perfectly magnified by a foreground galaxy cluster through gravitational lensing.” The opportunity allowed researchers to devote over 100 hours of quality telescope time to the individual system, making it one of the early universe’s most extensively analyzed galaxies.

Instead of the individual disk seen in Hubble’s images, the exponentially greater resolution from JWST and ALMA showcased a wholly different situation—a rotating galaxy stuffed with giant, lumpy stellar formations resembling the vineyard fruit. The reveal is also the first time that astronomers successfully linked an early galaxy’s smaller internal structures to their larger, collective rotation. The data was so detailed that they even managed to achieve a spatial resolution of 10 parsecs, or about 30 light-years.

What’s particularly striking is that Cosmic Grapes isn’t an oddball or extreme example given what astronomers understand of galactic evolution. Instead, it exists on the standard, main sequence of galaxies when it comes to attributes like star formations, mass, chemical composition, and size. This suggests many of the era’s galaxies that have been previously documented as smooth may more resemble the clumpy, dynamic structure seen through JWST and ALMA.

“Because existing simulations fail to reproduce such a large number of clumps in rotating galaxies at early times, this discovery raises key questions about how galaxies form and evolve,” the National Radio Astronomy Observatory’s announcement explained. “It suggests that our understanding of feedback processes and structure formation in young galaxies may need significant revision.”