Astronomers have found that some of the largest galaxies in the early universe contained many more small stars than scientists previously realized. That hidden population means these ancient galaxies may have been significantly more massive than earlier estimates indicated.
The discovery, made by an international team that included Penn State researchers, adds to a growing challenge for theories about how the first galaxies formed and evolved. The findings were published in Nature Astronomy.
JWST Uncovers Faint Stars in Ancient Galaxies
Using NASA’s James Webb Space Telescope (JWST), researchers examined nine massive, mature galaxies that stopped producing new stars billions of years ago. They combined JWST observations of the distant universe with earlier ground-based data from the Very Large Telescope.
For the first time, the team was able to reliably estimate the relative numbers of small, faint stars and much larger, brighter stars in galaxies at such great distances.
“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.”
Astronomers made the measurements by separating each galaxy’s light into a spectrum. Small variations in color within that spectrum can reveal which kinds of stars are present.
The difficulty is that large, luminous stars dominate the light, while smaller stars are far dimmer and much harder to identify. Penn State researchers contributed expertise and guidance for modeling the light detected from the galaxy systems.
Bright Stars Can Hide Vast Stellar Populations
“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.”
Traditionally, astronomers estimating the mass hidden in small, faint stars have assumed that stars formed in roughly similar proportions throughout the universe. The new findings challenge that long-standing assumption.
The researchers found that the most massive galaxies in the early universe appear to contain a much greater share of low-mass stars than smaller galaxies such as the Milky Way.
One galaxy stood out in particular, according to coauthor Martje Slob, a doctoral candidate at Leiden University. It likely formed less than one and a half billion years after the Big Bang and could be as much as four times more massive than previous calculations suggested.
“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.”
An Even Bigger Early Universe Puzzle
The findings could have major consequences for scientists trying to understand how galaxies developed in the young universe.
Since JWST began observing the cosmos, astronomers have repeatedly identified unexpectedly massive and mature galaxies that existed relatively soon after the Big Bang. Those discoveries have already put pressure on existing models of galaxy formation.
“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.”
If galaxies like these contained substantially more stars than researchers previously estimated, theories of galaxy formation must account for how such enormous numbers of small stars could have appeared so early in cosmic history.
Hidden Stars Could Also Mean More Early Planets
The implications may extend beyond galaxy formation.
“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.”
Over the next several years, the researchers plan to use the same technique to study galaxies from even earlier periods. Their goal is to push observations closer to the time when the universe’s first generations of stars and galaxies began to emerge.
