Galaxies in the Early Universe Preferred their Food Cold

One of the main objectives of the James Webb Space Telescope (JWST) is to study the early Universe by using its powerful infrared optics to spot the first galaxies while they were still forming. Using Webb data, a team led by the Cosmic Dawn Center in Denmark pinpointed three galaxies that appear to have been actively forming just 400 to 600 million years after the Big Bang. This places them within the Era of Reionization, when the Universe was permeated by opaque clouds of neutral hydrogen that were slowly heated and ionized by the first stars and galaxies.

This process caused the Universe to become transparent roughly 1 billion years after the Big Bang and (therefore) visible to astronomers today. When the team consulted the data obtained by Webb, they observed that these galaxies were surrounded by an unusual amount of dense gas composed almost entirely of hydrogen and helium, which likely became fuel for further galactic growth. These findings already reveal valuable information about the formation of early galaxies and show how Webb is exceeding its mission objectives.

The research was led by Kasper E. Heintz, a NASA Hubble Fellow and an assistant professor of astrophysics, and his colleagues at the Cosmic Dawn Center (DAWN) at the Niels Bohr Institute. They were joined by researchers from ETH Zurich, the MIT Kavli Institute for Astrophysics and Space Research, the Space Telescope Science Institute (STScI), the Association of Universities for Research in Astronomy (AURA), the European Space Agency (ESA), the NSF’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab), and multiple universities.

Illustration showing the process of Cosmic Reionization, divided into four periods. Credit: NASA

According to models of galaxy formation, the first galaxies are believed to have resulted from the infall of neutral, pristine gas onto the first protogalactic halos. However, the abundance of neutral atomic hydrogen in galaxies has remained unknown due to the difficulty of observing the earliest cosmological periods. “These galaxies are like sparkling islands in a sea of otherwise neutral, opaque gas,” Heintz explained in a NASA press release. “Without Webb, we would not be able to observe these very early galaxies, let alone learn so much about their formation.”

Since the galaxies appeared as little more than red blobs in the Webb images, the team also relied on data obtained by Webb‘s Near Infrared Camera (NIRCam) through the Cosmic Evolution Early Release Science (CEERS) Survey and shared through the Early Release Science (ERS) program. The spectra revealed that light from these galaxies is absorbed by large amounts of neutral hydrogen gas. They then matched the Webb data to models of star formation, which revealed that these galaxies are primarily populated by young stars. Said co-author Darach Watson, a professor at DAWN:

“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. 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.”

“We’re moving away from a picture of galaxies as isolated ecosystems,” added Simone Nielsen, a co-author and PhD student at DAWN. “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.”

The timeline from the Big Bang on the right towards the present on the left. In the middle is the Reionization Period where the initial bubbles caused the Cosmic Dawn. Credit: NASA SVS

These results illustrate what is now possible for astronomers, thanks to next-generation telescopes like Webb. Of course, many unanswered questions remain, not the least of which has to do with the distribution of the cold gas in these early galaxies. For instance, how much is located near the center of galaxies versus their outskirts? Also, astronomers are still unsure if this gas is pristine or already populated by heavier elements. As Heintz indicated, “The next step is to build large statistical samples of galaxies and quantify the prevalence and prominence of their features in detail.”

Further Reading: NASA, Science

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