JWST Early Galaxies

To understand how the James Webb Space Telescope (JWST) finds the most distant galaxies in the universe, we have to look at what happens to the Lyman break when a galaxy is pushed to extreme, record-breaking distances.

A galaxy at redshift $z = 3$ had its Lyman break shifted into the visible light spectrum. But what happens when we want to look at Cosmic Dawn—the era when the very first galaxies were forming, roughly 300 million years after the Big Bang?

These galaxies sit at staggering redshifts of $z > 10$. Let’s plug that into our redshift equation:

$$\lambda_{\text{obs}} = 912 \mathring{A} \times (1 + 10) \approx 10,032 \mathring{A}$$

$10,000 \mathring{A}$ is exactly $1 \mu m$ . At this extreme distance, the Lyman break is stretched completely out of the visible light spectrum and deep into the near-infrared.

This is exactly why the Hubble Space Telescope eventually hit a “wall” in its ability to find the oldest galaxies. Hubble is primarily an optical and ultraviolet telescope, with limited infrared capabilities. To see the Lyman break of the first galaxies, astronomy needed a telescope specifically designed to see in the infrared. Enter JWST.

JWST’s primary imager is the Near-Infrared Camera (NIRCam). Instead of the traditional U, B, V, and R optical filters, NIRCam uses a suite of highly sensitive infrared filters named after their central wavelengths. For example:
F090W: Centered at $0.9 \mu m$
F115W: Centered at $1.15 \mu m$
F150W: Centered at $1.50 \mu m$
F200W: Centered at $2.00 \mu m$

Astronomers use the exact same logic as optical dropouts, but scaled up to JWST’s infrared filters:

Finding a $z \approx 10$ Galaxy: The Lyman break is shifted to about $1.0 \mu m$. The galaxy will be completely invisible in the F090W filter, but will suddenly appear in the F115W filter and beyond. This is an F090W-dropout.
* Finding a $z \approx 13$ Galaxy: The break shifts to roughly $1.27 \mu m$. The galaxy now drops out of both the F090W and F115W filters, but lights up in the F150W filter. This is an F115W-dropout.

Fig 1: Location of Lyman-breaks in various JWST bands.

This specific technique has allowed JWST to shatter distance records almost immediately after it began operations.

One of the largest survey programs on JWST is the JWST Advanced Deep Extragalactic Survey (JADES). Astronomers aiming JWST at a tiny patch of sky (the GOODS-South field) used NIRCam to take deep images across multiple infrared filters.

They hunted for these extreme dropouts and found targets that simply did not exist in the bluer infrared filters but glowed brightly in the redder ones. Once they identified these candidates, they used JWST’s Near-Infrared Spectrograph (NIRSpec) to stare exactly at those coordinates and measure the precise chemical spectrum, proving beyond a doubt where the Lyman break occurred.

Using this exact method, the JADES team recently discovered JADES-GS-z14-0, which currently holds the record for the most distant known, spectroscopically confirmed galaxy.
* It sits at a redshift of $z = 14.32$.
* We are seeing this galaxy as it existed less than 300 million years after the Big Bang.
* Its Lyman-break is pushed so far into the infrared that it doesn’t even begin to appear until JWST’s F150W and F200W filters!

By combining the simple but brilliant physics of the Lyman-break with the sheer infrared power of JWST, astronomers are finally able to map the very edge of the observable Universe.

Fig 2: NIRCAM image of JADES-GS-z14-0.

Fig 3: Spectrum of JADES-GS-z14-0.

Looking at galaxies in the early universe—like those in the $z > 10$ range found by JWST—is not just about looking farther away; it is about looking back in time to when the universe was less than 500 million years old.

Because we are seeing them in their infancy, these primordial galaxies look and behave fundamentally differently from mature, modern galaxies like our Milky Way.

Here is how the “toddlers” of the universe compare to the adults of today.

Size and Structure: From “Blobs” to Grand Spirals
If you look at the Milky Way, it is a majestic, highly organized structure: a central bulge, a flat disk, and grand spiral arms spanning roughly 100,000 light-years across.

Early galaxies look nothing like this:
Tiny and Compact: High-redshift galaxies are incredibly small. While the Milky Way is massive, these early galaxies are often only a few hundred to a few thousand light-years across—sometimes smaller than a single star cluster in our own galaxy.
Chaotic and Irregular: They do not have neat spiral arms or smooth elliptical shapes. They are often described as “clumpy,” “irregular,” or looking like little bright blobs, pickles, or surfboards. They haven’t had the time (or the gravitational settling) to form into smooth, rotating disks.

Chemical Composition: Pristine Gas vs. Heavy Elements
Astronomers refer to any element heavier than hydrogen and helium as a “metal.” Modern Galaxies (High Metallicity): The Milky Way has been forming stars for over 13 billion years. Generations of stars have lived, forged heavy elements (like carbon, oxygen, and iron) in their cores, and exploded, seeding the galaxy with these materials. Our solar system formed from this highly enriched, dusty material.
Early Galaxies (Low Metallicity): Galaxies at $z > 10$ formed from the nearly pristine gas left over from the Big Bang. They contain almost no heavy elements or dust. The stars burning inside them are incredibly hot, massive, and shine with a harsh, intense ultraviolet light that is rarely seen in the modern universe.

Star Formation: Bursty vs. Steady
The rate at which galaxies build new stars changes drastically over cosmic time. Steady or Dead (Today): The Milky Way is relatively quiet, forming only about 1 to 2 new stars per year. Many massive modern galaxies are completely “quiescent” or dead, forming almost no new stars at all.
Furious and Bursty (Early Universe): Early galaxies were extreme star factories. Because the universe was much smaller and denser, these galaxies were constantly colliding, merging, and being fed by massive streams of cold hydrogen gas. This triggered furious “starbursts,” where galaxies formed stars at rates dozens or hundreds of times faster than the Milky Way does today.

The Big JWST Surprise: “Too Bright, Too Early”
Before JWST launched, standard cosmological models predicted that galaxies slowly assembled piece by piece. Astronomers expected galaxies at $z > 10$ to be incredibly faint, fragile, and low-mass.

However, JWST observations have flipped this expectation on its head. Many of these early galaxies (like JADES-GS-z14-0) are surprisingly bright and appear much more massive than models predicted they could be so soon after the Big Bang.

Astrophysicists are currently debating why this is:
* Are they actually massive, meaning black holes and galaxies grew at impossible speeds?
* Or are they just temporarily ultra-bright because of extreme bursts of star formation or different types of exotic early stars?

Ultimately, to get from the tiny, clumpy, hyperactive blobs of the early universe to the majestic Milky Way, these early galaxies had to undergo billions of years of violent mergers, cannibalizing their neighbors to slowly build up the mass and structure we see today.