Astronomers have discovered that the most massive galaxies in the early universe contain far more small stars than expected. As a result, these early galaxies were likely much more massive than initial measurements suggested. This discovery, which involved Penn State scientists, challenges current understanding about how the first galaxies formed. The results were recently published in Nature Astronomy.
An international team of researchers leveraged NASA's James Webb Space Telescope (JWST) to make this discovery. The team studied nine massive, mature galaxies that ceased forming stars billions of years ago by combining by combining JWST's unique ability to look deep into the distance universe with previous ground-based observations from the Very Large Telescope. The researchers used these observations to reliably determine the proportions of tiny, faint stars and giant, bright stars present in such distant galaxies for the first time.
"These galaxies are different," said Joel Leja, the Dr. Keiko Miwa Ross Mid-Career Associate Professor of Astronomy and Astrophysics at Penn State and coauthor of the paper. "They're different in the way that is really challenging to understand, because they are more massive than we expected - like a lot more massive, they have three or four times more mass than we expected."
To achieve these measurements, astronomers split the light from a galaxy into a spectrum, in which subtle differences in color reveal the types of stars within the galaxy. This spectrum is typically dominated by massive, bright stars, making the much fainter, low-mass stars extremely challenging to detect. Penn State researchers provided expert aid and advice on modeling of the light detected from the galaxy systems.
"If a galaxy were a city, the brightest stars would be the skyscrapers that immediately catch your eye from afar," said lead author Chloe Cheng, a recent doctoral graduate of Leiden University. "Our models demonstrate that a far more numerous population of low-mass stars is concealed by those rare, bright stars, like houses hidden between skyscrapers. As a result, this galaxy turns out to be much more massive than previous estimates suggested."
To estimate the unseen mass locked up in small, faint stars, astronomers historically assumed that stars formed in roughly the same proportions everywhere in the universe. These new results challenge that assumption and show that the most massive galaxies in the early universe contain a much larger fraction of low-mass stars than less massive galaxies, such as our own Milky Way. For one of the galaxies in the sample, the effect is particularly striking, said co-author Martje Slob, a doctoral candidate at Leiden University. That galaxy likely formed less than one and a half billion years after the Big Bang and may be as much as four times more massive than previous estimates indicated.
"Until recently, measurements like these were simply impossible," Slob said. "We needed not only a telescope capable of magnifying very distant galaxies, but also spectra of exceptional quality and new analysis techniques to reliably detect the subtle signatures of faint, low-mass stars hidden within these cosmic titans."
This discovery has crucial implications for our understanding of the early universe, Leja said. Since the launch of JWST, astronomers have found surprisingly massive and mature galaxies that already existed shortly after the Big Bang.
"This discovery has crucial implications for our understanding of the early universe," said Leja, who is also affiliated with The Penn State Institute for Computational and Data Sciences. "Since the launch of JWST, astronomers have found surprisingly massive and mature galaxies that already existed shortly after the Big Bang. These very early galaxies are thought to evolve into the type of galaxies studied in this work; adding up to four times more stars to these massive, early-forming galaxies sharpens these tensions further."
These new results suggest that these galaxies were probably even more massive than originally reported. Models of galaxy formation must now explain how such vast quantities of tiny stars could have formed so early in the history of the universe, the researchers said.
"This result shows that much more mass than previously thought is hidden in low-mass stars," said Mariska Kriek, who led the research and serves as professor of extragalactic astronomy at Leiden Observatory. "That has consequences for many areas of astronomy. For example, as many planets orbit low-mass stars, this could even indicate that more planets formed in the early universe than we had previously assumed."
In the next few years, the researchers plan to apply their method to even earlier galaxies. In doing so, they hope to probe even closer to the era when the first generations of stars and galaxies formed.
