Present-day disk galaxies often exhibit distinct thin and thick disks. The formation mechanisms of the two disks and the timing of their onset remain open questions. To address these questions, astronomers analyzed a statistical sample of 111 edge-on disk galaxies at various periods — up to 11 billion years ago, or approximately 2.8 billion years after the Big Bang — using archival data from the NASA/ESA/CSA James Webb Space Telescope.
Webb/NIRCam composite images of a quarter of the team’s sample sorted by increasing redshift. Image credit: Tsukui et al., doi: 10.1093/mnras/staf604.
Present-day disk galaxies often contain a thick, star-filled outer disk and an embedded thin disk of stars.
For instance, Milky Way’s thick disk is approximately 3,000 light-years in height, and its thin disk is roughly 1,000 light-years thick.
But how and why do such dual disk structures form?
“Our unique measurement of the thickness of the disks at high redshift, or at times in the early Universe, is a benchmark for theoretical study that was only possible with Webb,” said Dr. Takafumi Tsukui, an astronomer at the Australian National University.
“Usually, the older, thick disk stars are faint, and the young, thin disk stars outshine the entire galaxy.”
“But with Webb’s resolution and unique ability to see through dust and highlight faint old stars, we can identify the two-disk structure of galaxies and measure their thickness separately.”
By analyzing 111 edge-on targets over cosmological time, the astronomers were able to study single-disk galaxies and double-disk galaxies.
Their results indicate that galaxies form a thick disk first, followed by a thin disk.
The timing of when this takes place is dependent on the galaxy’s mass: high-mass, single-disk galaxies transitioned to two-disk structures around 8 billion years ago.
In contrast, low-mass, single-disk galaxies formed their embedded thin disks later on, about 4 billion years ago.
“This is the first time it has been possible to resolve thin stellar disks at higher redshift,” said Dr. Emily Wisnioski, also from the Australian National University.
“What’s really novel is uncovering when thin stellar disks start to emerge.”
“To see thin stellar disks already in place 8 billion years ago, or even earlier, was surprising.”
To explain this transition from a single, thick disk to a thick and thin disk, and the difference in timing for high- and low-mass galaxies, the researchers looked beyond their initial edge-on galaxy sample and examined data showing gas in motion from the Atacama Large Millimeter/submillimeter Array (ALMA) and ground-based surveys.
By taking into consideration the motion of the galaxies’ gas disks, they found that their results align with the ‘turbulent gas disk’ scenario — one of three major hypotheses that has been proposed to explain the process of thick- and thin-disk formation.
In this scenario, a turbulent gas disk in the early Universe sparks intense star formation, forming a thick stellar disk.
As stars form, they stabilize the gas disk, which becomes less turbulent and, as a result, thinner.
Since massive galaxies can more efficiently convert gas into stars, they settle sooner than their low-mass counterparts, resulting in the earlier formation of thin disks.
“While this study structurally distinguishes thin and thick disks, there is still much more we would like to explore,” Dr. Tsukui said.
“We want to add the type of information people usually get for nearby galaxies, like stellar motion, age, and metallicity.”
“By doing so, we can bridge the insights from galaxies near and far, and refine our understanding of disk formation.”
The findings were published in the Monthly Notices of the Royal Astronomical Society.
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Takafumi Tsukui et al. 2025. The emergence of galactic thin and thick discs across cosmic history. MNRAS 540 (4): 3493-3522; doi: 10.1093/mnras/staf604
