In a cosmic revelation that’s rewriting our understanding of galaxies, the James Webb Space Telescope has unveiled a stunning truth about the structure of disk galaxies like our own Milky Way. These galaxies, sprawling across the cosmos, often boast a dual-disk architecture: a thick, star-filled outer disk enveloping a slimmer, vibrant inner disk. For instance, the Milky Way’s thick disk stretches roughly 3,000 light-years in height, while its embedded thin disk measures a mere 1,000 light-years. But how do these layered structures come to be, and why do they form? Thanks to groundbreaking research led by astronomers at the Australian National University, we’re closer than ever to answering these questions.
Using the unparalleled power of the James Webb Space Telescope, researchers have peered deep into the early Universe, capturing glimpses of galaxies as they were billions of years ago. “Our unique measurement of disk thickness at high redshift is a benchmark for theoretical study that was only possible with Webb,” said Dr. Takafumi Tsukui, an astronomer at the Australian National University. Unlike traditional telescopes, Webb’s extraordinary resolution and ability to pierce through cosmic dust allow it to spotlight the faint, ancient stars of thick disks, which are often outshone by the brighter, younger stars in thin disks.
By analyzing 111 edge-on galaxies across cosmic time, the team discovered a clear evolutionary pattern: galaxies form a thick disk first, followed by the emergence of a thinner, embedded disk. The timing of this transformation, however, hinges on a galaxy’s mass. High-mass galaxies transitioned from single-disk to dual-disk structures around 8 billion years ago, while their low-mass counterparts took longer, developing thin disks about 4 billion years ago. “To see thin stellar disks already in place 8 billion years ago, or even earlier, was surprising,” said Dr. Emily Wisnioski, also from the Australian National University. “What’s really novel is uncovering when these thin disks start to emerge.”
To unravel the mystery of how these dual-disk structures form, the researchers turned to additional data from the Atacama Large Millimeter/submillimeter Array (ALMA) and ground-based surveys, focusing on the motion of gas within galaxies. Their findings align with the “turbulent gas disk” scenario, one of the leading theories explaining disk formation. In this model, the early Universe hosts chaotic, turbulent gas disks that fuel intense star formation, giving rise to a thick stellar disk. As stars form, they stabilize the gas disk, reducing its turbulence and allowing it to flatten into a thinner structure.
Massive galaxies, with their ability to convert gas into stars more efficiently, settle into this stable, thin-disk phase earlier than low-mass galaxies. This explains why high-mass galaxies developed their thin disks billions of years before their smaller counterparts. The result is a cosmic timeline where the most massive galaxies lead the way in sculpting the elegant dual-disk structures we observe today.
This groundbreaking study, published in the Monthly Notices of the Royal Astronomical Society, marks a pivotal moment in our understanding of galaxy formation. Yet, the researchers are far from finished. “While this study structurally distinguishes thin and thick disks, there is still much more we would like to explore,” Dr. Tsukui noted. Future investigations aim to delve deeper, examining stellar motion, age, and metallicity—key properties that could bridge our understanding of galaxies both near and far.
The James Webb Space Telescope has not only illuminated the ancient architecture of galaxies but also reshaped our view of their evolutionary journey. By revealing the origins of the Milky Way’s dual-disk structure and those of countless other galaxies, Webb is helping astronomers piece together the grand story of the cosmos—one disk at a time.