Have you ever seen the bulge of the Milky Way stretching across the night sky?
When you see the Milky Way, you’re not just seeing a band of stars, you’re peering into a disk galaxy that's structured like a layered pancake.
Astronomers know that many galaxies, including our own, are made of two stellar disks: a thick, older disk and a thin, younger one.
Our own Milky Way galaxy’s thick disk is 3,000 lightyears in height, and its thin disk is roughly 1,000 lightyears thick, for example.
But when did this dual-layered structure form? And how?
The James Webb Space Telescope is giving astronomers new insights that could help them solve the mystery behind the structures of galaxies like our own.
They've been using Webb to analyse 111 edge-on disk galaxies spanning nearly 11 billion years of cosmic history, tracing the evolution of these galactic pancakes back to the early Universe, just 2.8 billion years after the Big Bang.
A tale of two disks
"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," says Takafumi Tsukui, lead author of the study 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."
The team’s sample shows a pattern: galaxies tend to form their thick disk first, followed by a thin disk later on.
But not all galaxies evolve at the same pace.
Bigger, more massive galaxies started growing their thin disks around 8 billion years ago, while smaller galaxies didn't begin until about 4 billion years ago.
This timing difference could be related to the galaxies’ star-making efficiency.
Massive galaxies can convert swirling clouds of gas into stars much faster, calming the turbulent early conditions that created thick disks and allowing thinner, more stable structures to emerge sooner.
Probing deeper
To better understand this transformation, the researchers examined the movement of gas within galaxies using data from the Atacama Large Millimeter/submillimeter Array (ALMA) and other ground-based telescopes.
The findings support the 'turbulent gas disk' scenario of galactic evolution.
This states that early galaxies were chaotic, with turbulent gas sloshing around and collapsing into stars in a frenzy.
This violent starburst phase formed the thick disk. But as more stars formed, they began to stabilise the disk.
The gas settled into a thinner, flatter configuration, and so the thin disk was born.
But the thick disk keeps growing, just more slowly, while the thin disk takes over as the dominant star-forming region.
What it means for our Milk Way
Astronomers say the timing of the thick-to-thin transition lines up with the formation of the Milky Way’s own thin disk.
Webb is able to peer so deep into space it's effectively looking back in time, enabling astronomers to see galaxies that resemble the early Milky Way
"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,” says 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."
Now the team plan to add more detail to their galactic census.
"While this study structurally distinguishes thin and thick disks, there is still much more we would like to explore," says 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."
