Watching the birth of the universe's earliest galaxies

Thursday, 30 May, 2024

Watching the birth of the universe's earliest galaxies

For the first time in the history of astronomy, researchers have witnessed the birth of some of the universe’s earliest galaxies, somewhere between 13.3 and 13.4 billion years ago. Their discovery was described in the journal Science and made using the James Webb Space Telescope, which brought these ‘live observations’ of three formative galaxies down to Earth.

Through the telescope, researchers at the University of Copenhagen’s Niels Bohr Institute were able to see signals from large amounts of gas that accumulate and accrete onto a mini-galaxy in the process of being built. While this is how galaxies are formed according to theories and computer simulations, it had never actually been witnessed.

“You could say that these are the first ‘direct’ images of galaxy formation that we’ve ever seen,” said Assistant Professor Kasper Elm Heintz, who led the new study. “Whereas the James Webb has previously shown us early galaxies at later stages of evolution, here we witness their very birth, and thus the construction of the first star systems in the universe.”

How they did it

The birth of galaxies took place at a time in the history of the universe known as the Epoch of Reionisation, when the energy and light of some of the first galaxies broke through the mists of hydrogen gas. The researchers measured the formation of these galaxies by using sophisticated models of how their light was absorbed by the neutral hydrogen gas located in and around them, known as the Lyman-alpha transition.

By measuring the light, the researchers were able to distinguish gas from the newly formed galaxies from other gas. This is the most distant measurement of neutral hydrogen gas, which is the building block of the stars and galaxies, discovered by to date — and it was only possible thanks to the Webb Telescope’s incredibly sensitive infrared spectrograph capabilities.

“These galaxies are like sparkling islands in a sea of otherwise neutral, opaque gas,” Heintz said. “Without Webb, we would not be able to observe these very early galaxies, let alone learn so much about their formation.”

The researchers estimate the birth of the three galaxies to have occurred roughly 400–600 million years after the Big Bang, the explosion that began it all. While this sounds like a long time, it equates to the first 3–4% of the universe’s 13.8-billion-year lifetime, when the universe was an enormous opaque gas of hydrogen atoms.

“During the few hundred million years after the Big Bang, the first stars formed, before stars and gas began to coalesce into galaxies,” said Associate Professor Darach Watson. “This is the process that we see the beginning of in our observations.”

Furthermore, by matching Webb’s data to models of star formation, the researchers found that the three galaxies primarily have populations of young stars. According to Watson, “The fact that we are seeing large gas reservoirs … suggests that the galaxies have not had enough time to form most of their stars yet.”

Understanding our origins

Many of the researchers are based at the Niels Bohr Institute’s Cosmic Dawn Center, whose stated goal is to investigate and understand the dawn of the universe. The research team has already applied for more observation time with the Webb Telescope, with hopes of expanding upon their result and learning more about the earliest epoch in the formation of galaxies.

“For now, this is about mapping our new observations of galaxies being formed in even greater detail than before,” said PhD student Simone Vejlgaard. “At the same time, we are constantly trying to push the limit of how far out into the universe we can see. So perhaps we’ll reach even further.”

The team’s discovery also contributes to answering one of humanity’s most basic questions: where do we come from?

“Here, we piece together a bit more of the answer by shedding light on the moment that some of the universe’s first structures were created,” said Associate Professor Gabriel Brammer. “It is a process that we’ll investigate further, until hopefully, we are able to fit even more pieces of the puzzle together.”

Illustration shows a galaxy forming only a few hundred million years after the Big Bang, when gas was a mix of transparent and opaque during the Era of Reionisation. Data from NASA’s James Webb Space Telescope show that there is a lot of cold, neutral gas in the neighbourhood of these early galaxies — and that the gas may be more dense than anticipated. Image credit: NASA, ESA, CSA, Joseph Olmsted (STScI).

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