As we look back toward cosmic dawn, astronomers confirm the faintest galaxy ever seen

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The universe we live in is a transparent one, where light from stars and galaxies shine brightly against a clear, dark background. But this was not always the case – in the early years the universe was filled with a fog of hydrogen atoms that obscured the light from the earliest stars and galaxies.

Clouds interrupted by bright spots

The early universe was filled with a fog made up of hydrogen atoms until the first stars and galaxies burned it away.

The intense ultraviolet light from the first generations of stars and galaxies is believed to have burned through the hydrogen fog and transformed the universe into what we see today. While previous generations of telescopes lacked the ability to study these early cosmic objects, astronomers now use them James Webb Space Telescopes superior technology to study the stars and galaxies that formed immediately after the Big Bang.

I’m a 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 galaxy currently known 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 billions of the life of the universe were one decisive period in its development. In the first moments after the Big Bang, matter and light were bound together in a hot, dense “soup” of fundamental 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 for light and matter to separate out of their “soup” and—about 380,000 years later—form hydrogen atoms. The hydrogen atoms appeared as an intergalactic fog, and without the light of stars and galaxies, the universe was dark. This period is known as cosmic dark ages.

The arrival of the first generations 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 gas mist. This process gave the transparent, complex and beautiful universe we see today.

Astronomers I call the universe’s first billion years—when this hydrogen fog burned away—for epoch of reionization. To fully understand this time period, we study when the first stars and galaxies formed, what their main properties were, and whether they could produce enough UV light to burn through all the hydrogen.

A visual model showing the burning of hydrogen fog by UV light during the “reionization time”. Ionized or burned areas are blue and transparent. Ionization fronts are red and white, and neutral regions are dark and opaque. Via djxatlanta on Youtube.

The search for faint galaxies in the early universe

The first step toward understanding the epoch of reionization is to find and confirm the distances to galaxies that astronomers believe may be responsible for this process. Because light travels at a finite speed, it takes time to reach our telescopes, so astronomers see objects as they were before.

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

Because galaxies that live in this time period are so far away, they appear extreme weak and small to our telescopes and emits most of its light in the infrared. That means astronomers need powerful infrared telescopes like Webb to find them. Before Webb, virtually all distant galaxies found by astronomers were exceptionally bright and large, simply because our telescopes were not sensitive enough to see the fainter, smaller galaxies.

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

A typical early galaxy

JD1 is such a “typical” faint galaxy. It was discovered in 2014 with the Hubble Space Telescope like a suspiciously distant galaxy. But Hubble didn’t have the capability or sensitivity to confirm its distance—it could only make an educated guess.

Small and weak nearby galaxies can sometimes be mistaken for distant ones, so astronomers need to be certain of their distances before we can make claims about their properties. Distant galaxies therefore remain “candidates” until they are confirmed. The Webb telescope finally has the ability to confirm these, and JD1 was one of the first major Webb confirmations of an extremely distant galaxy candidate found by Hubble. This endorsement ranks it as the faintest galaxy yet seen in the early universe.

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

Bright lights (galaxies and some stars) against a dark background of the sky.  A faint galaxy is shown in a magnified box as a dim spot.

A sky full of galaxies and some stars. JD1, depicted in a zoomed-in box, is the faintest galaxy yet found in the early universe.
Guido Roberts-Borsani/UCLA; original images: NASA, ESA, CSA, Swinburne University of Technology, University of Pittsburgh, STScI

Gravitational lenses, nature’s magnifying glass

Even for Webb, JD1 would be impossible to see without a helping hand from nature. JD1 lies behind a large cluster of nearby galaxies, called Abell 2744, whose combined gravitational force bends and amplifies the light from JD1. This effect, known as gravitational lensing, makes JD1 appear larger and 13 times brighter than it normally would.

Large galaxies can warp and distort light traveling around them. This video shows how this process, called gravitational lensing, works.

Without gravitational lensing, astronomers would not have seen JD1, even with Webb. The combination of JD1’s gravitational magnification and new images from another of Webb’s near-infrared instruments, NIRCamenabled our team to study the structure of the galaxy in unprecedented detail and resolution.

Not only does this mean that we as astronomers can study the inner regions of early galaxies, it also means that we can begin to determine whether such early galaxies were small, compact and isolated sources, or whether they are merging and interacting with nearby galaxies. By studying these galaxies, we trace back to the building blocks that shaped the universe and gave rise to our cosmic home.

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