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Disk galaxies, like our own Milky Way galaxy, commonly consist of both a thick and thin disk of stars—each with different features, including stellar population and movement. Three major theoretical scenarios have been proposed to explain the formation mechanisms and timing of thick and thin disks.

A team of astronomers recently investigated the structure of disk galaxies by sifting through multiple surveys from NASA's James Webb Space Telescope. This extensive sample includes over 100 edge-on disk galaxies up to roughly 11 billion years ago. in Monthly Notices of the Royal Astronomical Society, the team's analysis aligns with one of the three scenarios, suggesting that thick stellar disk formation occurs first, and thin stellar disk formation follows. When this happens in a galaxy's formation, history depends on the galaxy's mass.

Present-day disk galaxies often contain a thick, star-filled outer disk and an embedded thin disk of stars. For instance, our own Milky Way galaxy's thick disk is approximately 3,000 light-years in height, and its thin disk is roughly 1,000 light-years thick.

How and why does this dual disk structure form? By analyzing archival data from multiple observational programs by NASA's James Webb Space Telescope, a team of astronomers is closer to answers, as well as understanding the origins of disk galaxies in general.

The team carefully identified, visually verified, and 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). This is the first time scientists have investigated thick- and thin-disk structures spanning such vast distances, bridging the gap between observers probing the early universe and galactic archaeologists seeking to understand our own galaxy's history.

"This 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 Takafumi Tsukui, lead author of the paper and a researcher at the Australian National 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."

Data through thick and thin

By analyzing these 111 targets over cosmological time, the team was 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. What's really novel is uncovering when thin stellar disks start to emerge," said Emily Wisnioski, 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."

A turbulent time for galaxies

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 team 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, 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 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. 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.

How this applies to home

Webb's sensitivity is enabling astronomers to observe smaller and fainter galaxies, analogous 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 a thick disk to a thick and thin disk roughly coincides with the formation of the Milky Way galaxy's thin disk. With Webb, astronomers will be able to further investigate Milky Way-like progenitors—galaxies that would have preceded the Milky Way—which could help explain our galaxy's formation history.

In the future, the team intends to incorporate other data points into their edge-on galaxy sample.

"While this study structurally distinguishes thin and thick disks, there is still much more we would like to explore," 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 insights from galaxies near and far, and refine our understanding of disk formation."