Galaxies actively forming in the early universe feed on cold gas

Researchers analyzing data from NASA’s James Webb Space Telescope have identified three galaxies that may have been actively forming when the universe was just 400 to 600 million years old. Webb’s data shows that these galaxies are surrounded by gas that the researchers suspect is composed almost exclusively of hydrogen and helium, the earliest elements that exist in the cosmos. Webb’s instruments are so sensitive that they were able to detect an unusual amount of dense gas around these galaxies. This gas will likely eventually fuel the formation of new stars in the galaxies.

“These galaxies are like sparkling islands in a sea of ​​otherwise neutral, opaque gas,” explains Kasper Heintz, the lead author and assistant professor of astrophysics at the Cosmic Dawn Center (DAWN) at the University of Copenhagen in Denmark. “Without Webb, we wouldn’t be able to observe these very early galaxies, let alone learn so much about their formation.”

“We are moving away from the view 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,” said Simone Nielsen, a co-author and PhD student also at DAWN.

In Webb’s images, the galaxies look like faint red spots. Therefore, additional data, known as spectra, were crucial to the team’s conclusions. These spectra show that light from these galaxies is absorbed by large amounts of neutral hydrogen gas. “The gas must be very widespread and cover a very large part of the Milky Way,” said Darach Watson, co-author and professor at DAWN. “This suggests that we are seeing the assembly of neutral hydrogen gas in galaxies. That gas will cool, clump and form new stars.”

The universe looked very different a few hundred million years after the Big Bang, during a period known as the epoch of reionization. Gas between stars and galaxies was largely opaque. Gas throughout the universe did not become fully transparent until about 1 billion years after the Big Bang. The stars of galaxies helped heat and ionize the gas around them, eventually making the gas completely transparent.

By matching Webb’s data with models of star formation, the researchers also found that these galaxies mainly contain populations of young stars. “The fact that we see large reservoirs of gas also suggests that galaxies haven’t had enough time to form most of their stars,” Watson added.

This is just the beginning

Webb is not only meeting but exceeding the mission goals that guided its development and launch. “Images and data of these distant galaxies were impossible to obtain before Webb,” explains Gabriel Brammer, co-author and associate professor at DAWN. “Besides, we had a good idea of ​​what we were going to find when we first glimpsed the data – we were almost making discoveries by eye.”

There are many more questions that need to be answered. Where is the gas specifically? How much is near the centers of the galaxies – or on their outskirts? Is the gas pristine or already populated by heavier elements? Important research lies ahead. “The next step is to build large statistical samples of galaxies and quantify in detail the prevalence and prominence of their features,” Heintz said.

The researchers’ findings were made possible thanks to Webb’s Cosmic Evolution Early Release Science (CEERS) Survey, which includes spectra of distant galaxies from the telescope’s NIRSpec (Near-Infrared Spectrograph), and was immediately released to support discoveries like this as part from Webb’s Early Release Science (ERS) program.

This work is published in the May 24, 2024 issue of the journal Science.

The James Webb Space Telescope is the world’s premier observatory for space science. Webb solves mysteries in our solar system, looks beyond to distant worlds around other stars and investigates the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners ESA (European Space Agency) and CSA (Canadian Space Agency).

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