The most ancient Milky Way-like galaxy yet observed

Berlin, Germany (SPX) Apr 22, 2025
An international research collaboration led by the University of Geneva (UNIGE) has identified the most remote spiral galaxy candidate ever recorded. Existing just one billion years after the Big Bang, this immense galactic system already displays a well-developed structure including a central bulge of aged stars, an expansive star-forming disk, and pronounced spiral arms. The findings, based on James Webb Space Telescope (JWST) data, provide critical insight into rapid galaxy formation in the early cosmos. Results are detailed in Astronomy and Astrophysics.

Astrophysicists have long believed that large spiral galaxies such as the Milky Way require several billion years to evolve. Early galaxies, by contrast, were assumed to be compact, disorderly, and irregular. Yet, JWST's deep-space infrared imaging is now uncovering unexpectedly massive, well-ordered galaxies that formed far earlier than models had predicted, pushing researchers to reconsider prevailing theories of galactic evolution.

One standout among these findings is a galaxy named Zhulong, a candidate spiral structure seen at redshift 5.2, corresponding to just 1 billion years after the Big Bang. Despite its ancient epoch, Zhulong shows surprising maturity with a central bulge of old stars, a large stellar disk, and sweeping spiral arms-traits usually associated with galaxies in the present-day universe.

"We named this galaxy Zhulong, meaning 'Torch Dragon' in Chinese mythology. In the myth, Zhulong is a powerful red solar dragon that creates day and night by opening and closing its eyes, symbolizing light and cosmic time," says Dr. Mengyuan Xiao, postdoctoral researcher at the Department of Astronomy at UNIGE and lead author of the study.

"What makes Zhulong stand out is just how much it resembles the Milky Way-both in shape, size, and stellar mass," she adds. The galactic disk extends more than 60,000 light-years and hosts upwards of 100 billion solar masses worth of stars, making it one of the earliest and most complete Milky Way analogues ever discovered.

Zhulong was spotted serendipitously through the PANORAMIC survey (GO-2514), a JWST campaign designed to collect wide-field extragalactic images in parallel with other instrument observations. Led by Christina Williams (NOIRLab) and Pascal Oesch (UNIGE), the program utilizes JWST's "pure parallel" mode to efficiently capture background imagery. "This allows JWST to map large areas of the sky, which is essential for discovering massive galaxies, as they are incredibly rare," says Dr. Christina Williams, assistant astronomer at NOIRLab and principal investigator of PANORAMIC. "This discovery highlights the potential of pure parallel programs for uncovering rare, distant objects that stress-test galaxy formation models."

Conventional cosmology holds that spiral structures emerge only after extensive periods of galactic mergers and settling. The identification of a massive, well-formed spiral galaxy at such an early time challenges that assumption. "This discovery shows how JWST is fundamentally changing our view of the early Universe," comments Prof. Pascal Oesch, associate professor at UNIGE and co-principal investigator of PANORAMIC.

Upcoming observations using both JWST and the Atacama Large Millimeter/submillimeter Array (ALMA) are expected to validate Zhulong's properties and further explore its origins. As more extensive JWST surveys proceed, astronomers anticipate uncovering additional early spirals, providing critical data on the dynamics of galactic formation in the nascent Universe.

Research Report:PANORAMIC: Discovery of an ultra-massive grand-design spiral galaxy at z~5.2