Galaxies Had a “Thick Phase” – JWST Just Revealed How They Slimmed Down

The James Webb Space Telescope has given astronomers a stunning new window into the history of galaxies like our own Milky Way.

By analyzing 111 edge-on galaxies, researchers discovered that galaxies form in two main stages: an early, chaotic thick disk followed by a calmer, thin disk that forms inside it. This pattern was observed even in galaxies 10 billion light-years away. The findings suggest that our Milky Way’s layered structure isn’t unique but part of a broader galactic trend, offering new insight into how galaxies evolve, build stars, and possibly even shape life-sustaining elements like oxygen and carbon.

Disk Galaxies and Their Fossil Records

Many galaxies, including our own Milky Way, have a remarkable structure: a flat, spinning disk made of stars. These stellar disks typically come in two main parts. The thin disk is home to younger stars rich in heavy elements like oxygen and carbon, while the thick disk holds older, more primitive stars. These distinct components act like cosmic fossils, preserving clues about how galaxies grow, form stars, and create the elements essential for life.

Until recently, scientists could only see these two-layered disks in the Milky Way and a few nearby galaxies. Previous telescopes simply weren’t powerful enough to spot the thin edge of faraway galaxies when viewed from the side.

That all changed with the launch of the James Webb Space Telescope (JWST) in 2021—the most advanced and sensitive space telescope ever built.

A Galactic Survey Through Time

Now, an international team of astronomers has analyzed 111 edge-on galaxies captured by JWST. These side views allowed researchers to clearly see the vertical structure of distant galaxies and identify both thin and thick disks.

Takafumi Tsukui (formerly of the Australian National University and now based at Tohoku University), who led the research team, says that observing distant galaxies is akin to using a time machine, allowing us to see how galaxies have formed their disks over cosmic history.

“Thanks to the JWST’s sharp vision, we were able to identify thin and thick disks in galaxies beyond our local universe, some going as far back as 10 billion years ago.”

The study revealed a consistent trend: in the earlier universe, more galaxies appear to have had a single thick disk, while in later epochs, more galaxies showed a two-layered structure with an additional thin disk component. This suggests that galaxies first formed a thick disk, followed by the formation of a thin disk within it. In more massive galaxies, this thin disk appears to have formed earlier.

Timeline of Disk Formation

The study estimated the thin disk formation time for Milky Way-sized galaxies to be around 8 billion years ago. This figure aligns with formation timelines for the Milky Way itself, where stellar ages can be measured.

To understand the revealed sequential formation from thick to thin disks and the corresponding formation timelines, the team not only examined the stellar structure but also the motion of gas, direct ingredients of stars obtained from the Atacama Large Millimeter/submillimeter Array (ALMA) and ground-based surveys in the literature. These observations supported a coherent formation scenario:

• In the early universe, galactic disks are rich in gas and highly turbulent
• Intense star formation in the turbulent disks gives rise to thick stellar disks
• As stellar disks develop, they help stabilize the gas disks and reduce the turbulence
• As the disk calms, a thin stellar disk forms inside the pre-developed thick stellar disks
• Whereas larger galaxies can efficiently convert gas into stars, forming thin disks earlier

Are We Typical? What JWST Tells Us

Tsukui emphasizes that the images provided by JWST help answer one of the biggest questions in astronomy: was our galaxy’s formation typical or unique? “The JWST images provided a window into galaxies that resemble the Milky Way’s early state, bringing us valuable insights from galaxies far away.”

The team hopes that their study will help bridge studies of nearby galaxies with far-away ones and refine our understanding of disk formation. The study was published in the journal Monthly Notices of the Royal Astronomical Society on June 26, 2025.

Reference: “The emergence of galactic thin and thick discs across cosmic history” by Takafumi Tsukui, Emily Wisnioski, Joss Bland-Hawthorn and Ken Freeman, 26 June 2025, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/staf604

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