Looking back into the cosmic dawn – astronomers confirm faintest galaxy ever seen

The universe we live in is a transparent universe where light from stars and galaxies shines on a clear, dark backdrop. But that wasn’t always the case – in its early years the universe was filled with a fog of hydrogen atoms that blocked light from the first stars and galaxies.

Intense ultraviolet light from first-generation stars and galaxies is thought to light up the hydrogen fog, transforming the universe into what we see today. Where previous generations of telescopes lacked the ability to study these early cosmic objects, astronomers are now using the cutting-edge technology of the James Webb Space Telescope to study stars and galaxies that formed just after the Big Bang.

I am an astronomer who studies the most distant galaxies in the universe using the world’s leading ground- and space-based telescopes. Using new observations from the Webb telescope and a phenomenon called gravitational lensing, my team confirmed the existence of the faintest currently known galaxy in the early universe. The galaxy, called JD1, is seen as it was when the universe was only 480 million years old, or 4% of its current age.

A brief history of the early universe

The first billion years of the universe’s life was a crucial period in its evolution. In the first moments after the Big Bang, matter and light were bound together in a hot, dense “soup” of elementary particles.

However, a fraction of a second after the Big Bang, the universe expanded extremely rapidly. This expansion eventually allowed the universe to cool enough to separate from the “soup” of light and matter and – after about 380,000 years – form hydrogen atoms. Hydrogen atoms appeared as an intergalactic mist, and the universe was dark because there was no light from stars and galaxies. This period is known as the cosmic dark ages.

The arrival of the first generation of stars and galaxies, several hundred million years after the Big Bang, bathed the universe in extremely hot UV light, which burned – or ionized – the hydrogen fog. This process revealed the transparent, complex and beautiful universe we see today.

Astronomers like me refer to the first billion years of the universe – as this hydrogen fog burns – as the age of reionization. To fully understand this time period, we examine when the first stars and galaxies formed, what their main characteristics are, and whether they can produce enough UV light to burn off all the hydrogen.

Searching for faint galaxies in the early universe

The first step towards understanding the reionization era is to find and confirm the distances to galaxies that astronomers think may be responsible for this process. Because light moves at a finite speed, it takes time to reach our telescopes, so astronomers see objects as they were in the past.

For example, light from the Milky Way, the center of our galaxy, takes about 27,000 years to reach Earth, so we see it as it was 27,000 years ago. This means that if we want to see the first moments after the Big Bang (the universe is 13.8 billion years old), we have to look for objects at great distances.

Because galaxies living in this time period are so far away, they appear extremely dim and small to our telescopes and emit most of their light in infrared. That means astronomers need powerful infrared telescopes like Webb to find them. Before Webb, nearly all of the distant galaxies found by astronomers were exceptionally bright and large because our telescopes weren’t sensitive enough to spot fainter, smaller galaxies.

However, it is the second population, not the bright ones, that is far more numerous, representative, and likely to be the main drivers of the reionization process. So these faint galaxies are the ones that astronomers need to study in more detail. It’s like trying to understand human evolution by studying entire populations rather than just a few very tall humans. By allowing us to see faint galaxies, Webb opens a new window to study the early universe.

A typical early galaxy

JD1 is such a “typical” faint galaxy. It was discovered by the Hubble Space Telescope in 2014 as a suspicious distant galaxy. But Hubble did not have the ability or the sensitivity to verify its distance; could only make an educated guess.

Small and faint nearby galaxies are sometimes mistaken for distant galaxies, so astronomers need to be sure of their distance before making claims about their properties. Therefore, distant galaxies remain “candidates” until confirmed. The Webb telescope is finally capable of confirming them, and JD1 was one of the first major Webb confirmations of an extremely distant galaxy candidate found by Hubble. This confirmation ranks it as the faintest galaxy ever seen in the early universe.

To validate JD1, an international team of astronomers and I used Webb’s near-infrared spectrograph, NIRSpec, to obtain the galaxy’s infrared spectrum. The spectrum allowed us to pinpoint the distance from Earth and determine its age, the number of young stars it formed, and the amount of dust and heavy elements it produced.

Gravitational lensing, nature’s magnifying glass

Even for Webb, it would have been impossible to see JD1 without nature’s help. JD1 is located behind a large nearby galaxy cluster called Abell 2744, whose combined gravitational force bends and amplifies the light from JD1. This effect, known as gravitational lensing, makes the JD1 appear larger and 13 times brighter than normal.

Without gravitational lensing, astronomers wouldn’t be able to see JD1, even with Webb. The combination of JD1’s gravitational magnification and new images from one of Webb’s near-infrared instruments, the NIRCam, enabled our team to study the galaxy’s structure in unprecedented detail and resolution.

Not only does this mean that as astronomers we can study the interior regions of early galaxies, it also means that we can begin to determine whether such early galaxies are small, compact and isolated sources, or whether they merged and interacted with nearby galaxies. By studying these galaxies, we trace the building blocks that shape the universe and reveal our cosmic home.