Astronomers find Webb’s data inconsistent with reionization model
Reionization is a critical period in which the first stars and galaxies change the physical structure of their surroundings and, ultimately, the entire universe. The established theory is that this era ended about a billion years after the Big Bang. But calculating this milestone using observations from the James Webb Space Telescope (JWST) means that reionization ended at least 350 million years earlier than expected. This is according to a new paper published in the Royal Astronomical Society’s Monthly Notices: Letters.
Throughout its history, the universe has undergone several major changes. For the first 380,000 years after the Big Bang, it was a hot, dense plasma of protons and electrons. Eventually, the object cooled enough that protons and electrons could combine to form neutral hydrogen atoms. Then, about 100 million years after the Big Bang, the first stars and galaxies began to form, beginning the era of reionization.
These first stars were huge and hot, with some predicted to have between 30 and 300 times the mass of the Sun, and emitted large amounts of energy in the form of extreme ultraviolet light. This energy was so powerful that when it hit a nearby hydrogen atom, it split the hydrogen atom into protons and electrons in a process called ionization. After hundreds of millions of years, nearly all the hydrogen in the universe has been ionized, ending the era of reionization.
Considering that approximately 75% of all matter is hydrogen, this represents a tremendous change. “This is the last major change that will occur,” explained Julian Muñoz, assistant professor of astronomy at the University of Texas at Austin and lead author of the paper. “We went from being neutral, cold, and boring to being ionized and hot. And this didn’t just happen to one or two galaxies. It happened throughout the universe.”
“This process heated and ionized the gas in the universe, controlling the rate of galaxy growth and evolution,” added study co-author John Chisholm, assistant professor of astronomy at the University of Texas at Austin. “These early stars established the overall structure of galaxies in the universe.”
Since astronomers cannot directly observe the reionization process, they must use models to predict when it will end. These models are based on indirect evidence, such as measurements of how much light reaches us from the Big Bang’s afterglow, called the cosmic microwave background radiation.
Other evidence is the early abundance of wavelengths associated with energy changes in hydrogen, called the Lyman-alpha forest. Both of these help astronomers calculate how much hydrogen is converted during reionization and, by extension, how much energy is required to do so.
“This is an accounting game,” Munoz said. “We know that all hydrogen is neutral before reionization. From there we need enough extreme ultraviolet light to split each atom. So at the end of the day, the reionization is You can calculate when it ends.
Now, the James Webb Space Telescope is challenging established models. This will allow astronomers to peer deeper into the universe than ever before into this important era. This has led to many unexpected observations in the early universe, one of which is galaxies emitting greater amounts of extreme ultraviolet light than expected. “JWST revealed that bright galaxies alone are enough to ionize the universe,” Chisholm said. “This is contrary to what many people expected.”
Therefore, with these new observations, the accounting was closed. “If we were to blindly trust James Webb, we would say that reionization ended 550 to 650 million years after the Big Bang, rather than the current estimate of 1 billion years,” Muñoz said. explained. “If this were true, the cosmic microwave background would look different, and the Lyman Alpha forest would look different. So there’s a sense of tension.”
In other words, it is unlikely that reionization occurred hundreds of millions of years earlier than predicted. So what’s going on? One explanation could be that established models are missing some important information. For example, ionized protons and electrons can come back together to reform a neutral hydrogen atom. This process is called recombination. If they occur more frequently than current models assume, the amount of extreme ultraviolet light needed to ionize the entire Universe could increase.
“We need to look more closely and deeply at galaxies to better understand the recombination process,” Muñoz said. “Resolving this tension about reionization is an important step toward finally understanding this pivotal period. We look forward to seeing what happens in the coming years.”
Additional study authors are Jordan Mirocha of NASA Jet Propulsion Laboratory and the California Institute of Technology. Stephen Farlanet of the University of California, Los Angeles; Charlotte Mason of the University of Copenhagen.
Further information: Julian B Muñoz et al., Reionization after JWST: A photon budget crisis?, Monthly Notices of the Royal Astronomical Society: Letters (2024). DOI: 10.1093/mnrasl/slae086
Provided by the University of Texas at Austin
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