Modern disk galaxies frequently display distinct thin and thick disks. The mechanisms driving the formation of these two discs and the timeline of their emergence are still unanswered questions. To investigate these issues, astronomers examined various epochs (statistical samples of 111 edge-on disk galaxies dating back up to 11 billion years, or approximately 2.8 billion years post-Big Bang) utilizing archived data from the NASA/ESA/CSA James Webb Space Telescope.
Webb/nircam composite images of a quarter of the team’s samples were sorted by increasing redshift. Image credit: Tsukui et al., doi: 10.1093/mnras/staf604.
Present-day disk galaxies often comprise extensive, star-rich outer disks alongside thin, star-like disks.
For instance, the thick discs of the Milky Way reach approximately 3,000 light-years in height, while the thin discs are roughly 1,000 light-years thick.
But what mechanisms lead to the formation of this dual disk structure?
“The thickness of high redshift discs, or unique measurements from the early universe, serve as benchmarks for theoretical research that can only be conducted using Webb,” states Takagi, an astronomer at the Australian National University.
“Typically, older, thicker disk stars are dim, while the younger, thinner disk stars dominate the galaxy.”
“However, Webb’s exceptional resolution allows us to observe and highlight faint older stars, enabling us to distinguish between two disk structures in a galaxy and measure their thickness separately.”
Through an analysis of 111 edge-on targets over cosmological time, astronomers studied both single-disc and double-disc galaxies.
The findings indicate that galaxies initially form a thick disk, which is followed by the formation of a thin disk.
The timing of this process is contingent on the galaxy’s mass: high-mass, single-disk galaxies transitioned to two-disk structures around 8 billion years ago.
In contrast, a thin disk emerged about 4 billion years ago within low-mass, single-disk galaxies.
“This is the first time we’ve resolved a thin star disk at such a high redshift,” remarked Dr. Emily Wysnioski from the Australian National University.
“The novelty becomes evident when observing the onset of thin star disks.”
“It was astonishing to witness a thin star disk from 8 billion years ago, and even further back.”
To elucidate the transition from a single thick disk to a dual-disk structure, as well as the timing differences between high-mass and low-mass galaxies, researchers expanded their investigation beyond the initial edge-on-galaxy samples. They examined data showing the movement of gases from large millimeter/sub-millimeter arrays (ALMAs) in Atacama and ground surveys.
By considering the movement of the galaxy’s gas disks, they found their results aligned with the “turbulent gas disk” scenario.
In this framework, the turbulent gas disks of the early universe catalyze intense star formation, leading to the creation of thick star disks.
As stars form, they stabilize the gas disks, diminishing turbulence and consequently resulting in thinner disks.
Larger galaxies can convert gas into stars more efficiently and thus calm down more quickly than their lower-mass counterparts, leading to the formation of the earlier thin disk.
“This study delineates structural differences between thin and thick discs, but we aim to explore further,” Dr. Tsukui mentioned.
“We look to incorporate the types of information typically acquired from nearby galaxies, such as stellar movement, age, and metallicity.”
“By doing so, we can bridge insights from both nearby and distant galaxies, enhancing our understanding of disk formation.”
Survey results were published in Monthly Notices of the Royal Astronomical Society.
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Takagi Tsukui et al. 2025. The emergence of thin and thick discs of galaxies across the history of the universe. mnras 540(4): 3493-3522; doi: 10.1093/mnras/staf604
Source: www.sci.news
