Free Astronomy Magazine September-October 2024

19 SEPTEMBER-OCTOBER 2024 ASTRO PUBLISHING Those spectra show that light from these galaxies is being absorbed by large amounts of neutral hydrogen gas. “The gas must be very wide- spread and cover a very large frac- tion of the galaxy,” said Darach Watson, a co-author who is a pro- fessor at DAWN. “This suggests that we are seeing the assembly of neu- tral hydrogen gas into galaxies. That gas will go on to cool, clump, and form new stars.” The universe was a very different place several hundred million years after the big bang during a period known as the Era of Reionization. Gas between stars and galaxies was largely opaque. Gas throughout the universe only became fully trans- parent around 1 billion years after the big bang. Galaxies’ stars con- tributed to heating and ionizing the gas around them, causing the gas to eventually become com- pletely transparent. By matching Webb’s data to models of star formation, the researchers also found that these galaxies pri- marily have populations of young stars. “The fact that we are seeing T his illustration shows a galaxy forming only a few hundred mil- lion years after the big bang, when gas was a mix of transparent and opaque during the Era of Reioniza- tion. Data from NASA’s James Webb Space Telescope show that there is a lot of cold, neutral gas in the neigh- borhood of these early galaxies – and that the gas may be more dense than anticipated. Webb observed these galaxies as part of its Cosmic Evolution Early Release Science (CEERS) Survey a few months after it began taking observations in 2022. CEERS includes both images and data known as spectra from the mi- croshutters aboard its NIRSpec (Near-Infrared Spectrograph). Data from CEERS were released immedi- ately to support discoveries like this as part of Webb’s Early Release Sci- ence (ERS) program. [NASA, ESA, CSA, Joseph Olmsted (STScI)] large gas reservoirs also suggests that the galaxies have not had enough time to form most of their stars yet,” Watson added. Webb is not only meeting the mis- sion goals that drove its develop- ment and launch – it is exceeding them. “Images and data of these distant galaxies were impossible to obtain before Webb,” explained Gabriel Brammer, a co-author and associate professor at DAWN. “ 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.” There remain many more questions to address. Where, specifically, is the gas? How much is located near the centers of the galaxies – or in their outskirts? Is the gas pristine or al- ready populated by heavier ele- ments? Significant research lies ahead. “The next step is to build large statistical samples of galaxies and quantify the prevalence and prominence of their features in de- tail,” Heintz said. !