Free Astronomy Magazine September-October 2025

29 SEPTEMBER-OCTOBER 2025 ASTRO PUBLISHING University in Canberra. “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 these 111 targets over cosmological time, the team was able to study single-disk galaxies and double-disk galaxies. Their re- sults 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 con- trast, 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 pos- sible to resolve thin stellar disks at higher redshift. What’s really novel is uncovering when thin stellar disks start to emerge,” said Emily Wis- nioski, a co-author of the paper at the Australian National University in Canberra. “To see thin stellar disks already in place 8 billion years ago, or even earlier, was surprising.” To explain this transition from a sin- gle, thick disk to a thick and thin disk, and the difference in timing for high- and low-mass galaxies, the team looked beyond their initial edge-on galaxy sample and exam- ined data showing gas in motion from the Atacama Large Millime- ter/submillimeter Array (ALMA) and other ground-based surveys. By taking into consideration the mo- tion of the galaxies’ gas disks, the team finds 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 for- mation. 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 be- comes 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. The team notes that thick- and thin- disk formation are not siloed events: The thick disk continues to grow as the galaxy develops, though it’s slower than the thin disk’s rate of growth.Webb’s sensitivity is en- abling astronomers to observe smaller and fainter galaxies, analo- gous to our own, at early times and with unprecedented clarity for the first time. In this study, the team noted that the transition period from thick disk to a thick and thin disk roughly coincides with the for- mation of the Milky Way galaxy’s thin disk. With Webb, astronomers will be able to fur- ther investigate Milky Way-like pro- genitors — galaxies that would have preceded the Milky Way — which could help explain our galaxy’s formation history. In the future, the team intends to in- corporate other da- ta points into their edge-on galaxy sam- ple. “While this study structurally distin- guishes thin and thick disks, there is still much more we would like to ex- plore,” said Tsukui. “ 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 in- sights from galaxies near and far, and refine our understanding of disk formation.” ! [NASA, ESA, CSA, STScI, Takafumi Tsukui (ANU)]