Imagine looking back in time nearly 11 billion years to watch galaxies grow up. That’s exactly what astronomers have done using the incredible power of the James Webb Space Telescope (JWST). By acting as cosmic archaeologists, they’ve studied ancient galaxies to figure out a fundamental mystery: how galaxies like our own Milky Way formed their distinctive double-layered structure, known as dual disks. This groundbreaking work reveals key moments in the life cycle of galaxies, offering a window into our own cosmic past.
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Key takeaways from this discovery include:
- Galaxies similar to the Milky Way developed a thick disk of stars first, followed later by a thin disk.
- The timing of this two-stage formation depends significantly on the galaxy’s mass.
- Turbulent gas in the early universe drove the formation of the thick disk, while stabilizing gas led to the thin disk.
- These findings match theories about when our own Milky Way formed its thin disk.
Unearthing Ancient Galaxies
Our Milky Way galaxy isn’t just a flat pancake of stars; it actually has two main disk components. There’s a thicker, older disk and a thinner, younger disk nestled inside it. Each disk has its own unique population of stellar populations and star movements. Scientists have long wondered how and why galaxies developed this dual-disk structure.
To investigate, the research team turned to the JWST. This telescope is powerful enough to peer back billions of years, observing galaxies as they existed when the universe was just a fraction of its current age – as early as 2.8 billion years after the Big Bang. They focused on a sample of 111 galaxies viewed “edge-on” from Earth. Looking at galaxies from the side makes it much easier to distinguish and measure the thickness of their different disk structures.
Distant edge-on disk galaxies observed by the James Webb Space Telescope, showing their flat structures against space.
Team leader Takafumi Tsukui of the Australian National University highlighted JWST’s crucial role in this study. “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 the JWST,” he said in a statement. JWST’s incredible resolution and ability to see faint, older stars even through cosmic dust were key to identifying these distinct disk layers in such ancient galaxies.
Two Layers of Stars
By examining the 111 galaxies, the team could classify them based on whether they had one disk or two. This separation revealed a compelling pattern: galaxies seem to build their thick stellar disk first, with the thin disk forming later as a subsequent layer.
Examples of different galaxy types studied by JWST, illustrating variations in thick and thin stellar disk structures.
The Timeline Emerges
The study showed that the timing of this transition from a single, thick disk to a dual-disk structure appears linked to the galaxy’s mass. More massive galaxies transformed into dual-disk systems around 8 billion years ago. Lower-mass galaxies, on the other hand, didn’t seem to form their thin disks until they were about 4 billion years old.
“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, another researcher on the team. She added, “To see thin stellar disks already in place 8 billion years ago, or even earlier, was surprising.”
The Role of Cosmic Gas
But what caused these transitions? To answer this, the researchers looked beyond their initial sample and investigated how gas moved within these different types of galaxies. They used data from instruments like the Atacama Large Millimeter/submillimeter Array (ALMA) and other ground-based observatories, which are excellent at observing gas.
The data revealed that in the turbulent early universe, chaotic gas flows within galaxies triggered bursts of intense star formation. This intense activity is what built the initial thick stellar disks. As stars formed, they consumed the gas, and the remaining gas settled down, becoming less turbulent and thinner. This calmer, thinner gas then led to the formation of the embedded thin stellar disk. The reason massive galaxies formed their thin disks earlier is likely because they converted gas into stars more efficiently, reaching the “calm gas” stage faster.
Connecting to Our Milky Way
Crucially, the timing of these observed transitions in ancient galaxies aligns with theories about when our own Milky Way galaxy formed its thin disk of stars. This suggests that the galaxies studied by JWST aren’t just random cosmic objects; they are likely representatives, or proxies, for the kind of galaxy the Milky Way was at different points in its 14-billion-year history.
An artist's illustration depicting the spiral structure of the Milky Way galaxy, our cosmic home.
This research demonstrates the remarkable ability of the JWST to not only show us distant parts of the universe but also help us piece together the history of our own cosmic neighborhood by observing galaxies that serve as historical snapshots.
What’s Next?
The team isn’t finished yet. They plan to add more data to their study to see if these findings hold up. They also want to investigate other properties of these ancient stars, such as their movement, age, and metallicity (the amount of elements heavier than hydrogen and helium).
“By doing so,” Tsukui explained, “we can bridge the insights from galaxies near and far, and refine our understanding of disk formation.” This ongoing work will continue to deepen our understanding of how our galactic home came to be.
The team’s results were published in the July edition of the journal Monthly Notices of the Royal Astronomical Society.
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