Scientists were mystified by images of some of the universe’s earliest galaxies when it appeared they were much larger and brighter than expected—until now.
Images of these six ancient galaxies, taken by the James Webb Space Telescope (JWST), revealed that the galaxies were too big and too mature for their young age.
“It’s bananas,” said Erica Nelson, assistant professor of astrophysics at the University of Colorado Boulder and co-author of the research paper in the journal Nature announcing the find, in a statement.
“You just don’t expect the early universe to be able to organize itself that quickly. These galaxies should not have had time to form,” Nelson said.
The galaxies were found to be around the same size as our Milky Way, which contains around 100,000 million stars—but existed only 500 million to 700 million years after the Big Bang. According to current theories about the universe’s formation, that is not enough time for the galaxies to grow to this size.
“The discovery of these galaxies was a big surprise because they were substantially brighter than anticipated,” said Claude-André Faucher-Giguère, an associate professor of physics and astronomy at Northwestern University’s Weinberg College of Arts and Sciences and a study co-author, in a statement.
“Typically, a galaxy is bright because it’s big. But because these galaxies formed at cosmic dawn, not enough time has passed since the Big Bang. How could these massive galaxies assemble so quickly? Our simulations show that galaxies have no problem forming this brightness by cosmic dawn,” said Faucher-Giguère, who is also a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).
However, the galaxies are actually less big than first assumed, according to new research published in the online journal Astrophysical Journal Letters. They only appeared so large because they are unusually bright as a result of something called “bursty star formation.”
The researchers found that these early galaxies would have had time to form between the Big Bang and the era they existed in, appearing larger thanks to the presence of bursty stars. Bursty star formation is when stars in a galaxy all form at once, creating a period of immense brightness, followed by a lull.
“Bursty star formation is especially common in low-mass galaxies,” Faucher-Giguère said. “The details of why this happens are still the subject of ongoing research. But what we think happens is that a burst of stars form, then a few million years later those stars explode as supernovae. The gas gets kicked out and then falls back in to form new stars, driving the cycle of star formation.
“But when galaxies get massive enough, they have much stronger gravity,” he continued. “When supernovae explode, they are not strong enough to eject gas from the system. The gravity holds the galaxy together and brings it into a steady state.”
Most of the light in a galaxy comes from the most massive stars, Faucher-Giguère said. “Because more massive stars burn at a higher speed, they are shorter lived. They rapidly use up their fuel in nuclear reactions. So, the brightness of a galaxy is more directly related to how many stars it has formed in the last few million years than the mass of the galaxy as a whole.”
Therefore, these early galaxies initially appeared larger than they were, as a result of unexpectedly bright bursty star formation. This means that their existence during cosmic dawn—a period between 100 million years and 1 billion years after the Big Bang, during which stars and galaxies first formed—is not implausible under the laws of physics.
Scientists can see so far into the past because of these galaxies’ immense distance from Earth. Since light travels at a maximum speed, objects billions of light-years away appear to us the way they did billions of years ago.
“The JWST brought us a lot of knowledge about cosmic dawn,” Guochao Sun, a co-author of the paper and CIERA postdoctoral fellow at Northwestern, said in the statement.
“Prior to JWST, most of our knowledge about the early universe was speculation based on data from very few sources. With the huge increase in observing power, we can see physical details about the galaxies and use that solid observational evidence to study the physics to understand what’s happening,” Sun said.
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