An international team of researchers has analyzed 111 JWST images of distant edge-on galaxies, or ones where the alignments allowed the researchers to detect the galaxies' vertical disk features, according to a study published in the Monthly Notices of the Royal Astronomical Society.
The figures illustrate the revealed sequential disk formation in this study: thick disk only galaxies dominate the early epochs (bottom panel), while galaxies exhibiting both thin and thick disks appear more commonly in later epochs of the universe (top panel). Image Credit: NASA, ESA, CSA, T. Tsukui
Many galaxies, including the Milky Way, have a flat, stretched, rotating stellar disk. These disk galaxies typically have two primary parts: a thin disk and a thick disk. The narrow disk comprises youthful, metal-rich stars, whereas the thick disk contains older, metal-poor stars.
Download your PDF copy now!
These separate components include fossil records that help astronomers understand how galaxies produce stars, accumulate elements like oxygen and carbon, which are necessary for life, and develop into their current forms.
Until recently, thin and thick disks have only been discovered in the Milky Way and surrounding galaxies. Previous telescopes could not identify the thin edge of a distant galaxy when seen from the side.
Takafumi Tsukui (previously of the Australian National University and currently at Tohoku University), who headed the study team, believes that watching distant galaxies is like operating a time machine, allowing us to witness how galaxies have formed their disks throughout 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.
Takafumi Tsukui, Study Team Lead, Tohoku University
A consistent pattern emerged from the study: more galaxies appear to have had a single thick disk in the earlier cosmos, but more galaxies in later epochs displayed a two-layered structure with an extra thin disk component. This implies that galaxies developed a thick disk first, and then a thin disk within it. It seems that this thin disk originated sooner in more massive galaxies.
According to the study, galaxies comparable in size to the Milky Way began forming their thin disks around 8 billion years ago. This estimate aligns with the Milky Way’s formation timeline, supported by measurements of stellar ages.
Alongside their analysis of stellar structure, the team also examined gas dynamics, the raw material for star formation, using data from the Atacama Large Millimeter/submillimeter Array (ALMA) and various ground-based surveys. This helped them better understand the observed transition from thick to thin disk formation and the associated timelines. A logical formation model was supported by these observations:
- In the early cosmos, galaxy disks were rich in gas and intensely chaotic
- Intense star production in turbulent disks produces thick stellar disks
- As stellar disks form, they help stabilize gas disks and minimize turbulence
- As the disk settles, a tiny stellar disk develops inside the pre-existing thick stellar disks
- Larger galaxies may effectively convert gas to stars, generating narrow disks early
Tsukui adds that the images supplied by JWST assist in addressing one of astronomy’s most pressing questions: Was the galaxy’s origin 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,” Tsukui added.
The researchers expect that these findings will help bridge the gap between studies of close and distant galaxies, as well as better the understanding of disk formation.
Journal Reference:
Tsukui, T., et al. (2025) The emergence of galactic thin and thick discs across cosmic history. Monthly Notices of the Royal Astronomical Society. doi.org/10.1093/mnras/staf604.