The earliest galaxies in the universe have been observed for the first time

Researchers using NASA’s James Webb Space Telescope (JWST) have identified three galaxies that may have been actively forming between 400 to 600 million years after the Big Bang. These galaxies are surrounded by gas primarily composed of hydrogen and helium, the earliest elements in the universe. 

Webb’s sensitive instruments detected dense gas around these galaxies, likely fueling new star formation.

Sparkling islands in a sea of gas

“These galaxies are like sparkling islands in a sea of otherwise neutral, opaque gas,” said lead author Kasper Heintz, an assistant professor of astrophysics at the Cosmic Dawn Center (DAWN) at the University of Copenhagen. 

“Without Webb, we would not be able to observe these very early galaxies, let alone learn so much about their formation.”

“We’re moving away from a picture of galaxies as isolated ecosystems. At this stage in the history of the universe, galaxies are all intimately connected to the intergalactic medium with its filaments and structures of pristine gas,” noted co-author Simone Nielsen, a PhD student at DAWN.

Ancient galaxies formed from neutral hydrogen gas 

In Webb’s images, the galaxies appear as faint red smudges, making additional data, such as spectra, essential for the team’s conclusions. The spectra revealed that light from these galaxies is absorbed by significant amounts of neutral hydrogen gas. 

“The gas must be very widespread and cover a very large fraction of the galaxy. This suggests that we are seeing the assembly of neutral hydrogen gas into galaxies. That gas will go on to cool, clump, and form new stars,” explained co-author Darach Watson, a professor of astrophysics at the same institution.

Several hundred million years after the Big Bang, during the Era of Reionization, the universe was very different. The gas between stars and galaxies was mostly opaque and only became fully transparent around one billion years post-Big Bang. The stars in galaxies contributed to heating and ionizing the surrounding gas, eventually making it transparent.

Many questions remain about early galaxies

By aligning Webb’s data with star formation models, the researchers found that these galaxies mostly consist of young stars. “The fact that we are seeing large gas reservoirs also suggests that the galaxies have not had enough time to form most of their stars yet,” said Watson.

“Images and data of these distant galaxies were impossible to obtain before Webb. Plus, we had a good sense of what we were going to find when we first glimpsed the data – we were almost making discoveries by eye,” noted Gabriel Brammer, an astrophysicist at DAWN.

There are still many questions to address. Researchers aim to determine the specific locations of the gas, whether it is near the galaxy centers or in their outskirts, and if the gas is pristine or contains heavier elements. 

“The next step is to build large statistical samples of galaxies and quantify the prevalence and prominence of their features in detail,” Heintz concluded.

More about the earliest galaxies of our universe 

Early galaxies were much smaller and less structured than the ones we see today, often resembling irregular clumps of stars rather than the majestic spiral or elliptical shapes common now. 

The early universe was filled with hydrogen and helium, the raw materials from which these first stars and galaxies formed.

Formation of the galaxies

As these primordial gases began to collapse under gravity, they formed the first stars and, eventually, galaxies. 

The process was likely influenced by dark matter, an unseen substance that makes up most of the universe’s mass and creates gravitational wells that can pull in normal matter.

Starburst galaxies 

These ancient galaxies were hotbeds of star formation, known as “starburst” galaxies. They were much less metal-rich than later galaxies because they hadn’t yet undergone generations of star birth and death, which produce heavy elements. 

Over time, as stars formed, died, and ejected their material back into space, these galaxies grew and evolved, merging with others to form larger galaxies.

Distant, ancient light

The study of early galaxies helps scientists understand more about the formation and evolution of structures in the universe and the role of dark matter in shaping these processes. 

By observing distant, ancient light with powerful telescopes, astronomers can glimpse these early stages of galaxy formation and development.

The study is published in the journal Science.

Image Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)

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