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.

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.