When looking up at the sky, you might think galaxies have always looked the way they do today. But a new study from Princeton University and published in the journal, Monthly Notices of the Royal Astronomical Society, reveals something surprising.
Using the James Webb Space Telescope (JWST), scientists found that many galaxies started as thick discs before forming the thin discs we see now. This discovery helps answer a long-standing mystery about how galaxies, like our own Milky Way, were born.
Seeing Back 10 Billion Years
Galaxies often have two layers: a thick disc filled with older stars and a thin disc with younger stars. Until now, scientists did not know when these two discs formed. The new research used JWST to look at galaxies more than 10 billion years old. It is the first time anyone has clearly seen both thin and thick discs at such a far distance.
Researchers studied edge-on galaxies. These are galaxies seen from the side, which makes it easier to see the discs stacked on top of each other. They used special infrared filters to peek past dust and see the real structure. In total, they studied 111 galaxies with redshifts up to 3, which means the light took over 10 billion years to reach us.
The team classified galaxies into two types: those with a single disc and those with both thin and thick discs. They found that the bigger the galaxy, the earlier it formed a thin disc. High-mass galaxies formed their thin discs around 8 billion years ago, while low-mass galaxies did not form them until about 4 billion years ago. This pattern is called “downsizing.”
“Thick discs are like the foundation of a house,” said the team’s lead researcher. “They form first and then the thin disc builds on top, giving the galaxy its current shape.”
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Two Theories Of Disc Formation
Scientists have long debated how thick and thin discs form. One theory is called the “born hot” scenario. In this idea, thick discs form when a young galaxy is still turbulent and messy. High amounts of gas and fast star formation create chaos, heating the galaxy and puffing up the thick disc. Later, when things calm down, a thin disc forms quietly on top.
Observations support this theory. High-redshift galaxies show more turbulence than modern galaxies. “Higher gas fractions and star formation rates in the early universe likely caused this turbulence,” explained the researchers. In other words, young galaxies were wild places with lots of action.
Another theory is called “progressive thickening.” This idea says all stars form in a thin disc. Over time, gravity from giant molecular clouds, spiral arms, or even small galaxy mergers push stars away from the mid-plane. This thickens the disc as stars get scattered.
However, scientists think this process alone is not enough. “Scattering from giant molecular clouds can heat thin-disc stars, but it does not create an entire thick disc,” the paper explained. Mergers with smaller galaxies can help thicken discs, but they also risk disrupting the galaxy’s shape.
A Third Theory: Merging With Other Galaxies
A third idea is the “ex situ” scenario. In this view, thick discs form when a galaxy merges with a smaller one or captures its stars. This can leave behind stars rotating in opposite directions. However, scientists have found fewer counter-rotating stars than this theory predicts. This suggests that while mergers may add to thick discs, they are not the only cause.
All three ideas might work together. The challenge is figuring out which process matters most and when. To do this, scientists need to study many galaxies over billions of years. That is why JWST is so important. Its powerful infrared vision sees ancient galaxies with incredible detail.
How The Study Was Done
The team used data from JWST programs like JADES, CEERS, COSMOS-Web, and PRIMER. They chose galaxies that appeared edge-on in the images. Out of 213 possible galaxies, only 132 passed strict visual inspections. They removed any galaxies with spiral features or warping that made them unsuitable.
For their final analysis, 111 galaxies were used. These included a mix of spectroscopic, grism, and photometric redshifts. Researchers measured each galaxy’s height and width using infrared bands that trace mass, not just visible light. This helped them see past dust and capture the true shape.
They found that the radial sizes and vertical heights of discs relate strongly to the galaxy’s mass. In simple words, the bigger the galaxy, the bigger and taller its discs. But the most surprising result was that thick discs looked similar to single discs in other galaxies. This suggests galaxies form thick discs first, then thin discs later.
What This Means For The Milky Way
Our Milky Way also has both thick and thin discs. The thick disc contains older, metal-poor stars. These stars formed quickly when the galaxy was young and wild. The thin disc has younger, metal-rich stars that formed over longer times. They make up the spiral arms you see in telescope photos.
Thick discs rotate more slowly than thin discs. This is due to something called asymmetric drift, which happens because thick discs have hotter, more random star motions. In contrast, thin discs rotate smoothly like a well-oiled wheel.
Looking To The Future
The study found that thin disc formation was delayed in smaller galaxies. This is explained by the Toomre Q-regulated disc formation theory. In this model, high gas turbulence keeps a thin disc from forming until the galaxy has enough mass to stabilize itself.
Researchers also saw that thick discs keep building up mass even as thin discs form. This means galaxy building is not a simple two-step process. Both discs continue evolving together over time.
For now, one thing is clear. Galaxies were not born in perfect spirals. They grew in layers, starting with hot, thick discs before settling down to form the thin discs that light up the sky today.
Note: The article above provided above by The Brighter Side of News.
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