How can young galaxies in the early universe have old stars? That’s the question a team of astronomers set out to answer using the JWST as a probe. They first discovered the massive objects in 2022 and are still working on an explanation for these things.
The infrared-sensitive JWST recently focused on three of these objects. Interestingly, it found that they were emitting light from stars that appeared to be mysteriously old. The galaxies themselves were seen as they appeared about 700 million years after the Big Bang. They look nothing like the spirals and ellipticals we see in the more “modern” universe.
“We’ve confirmed that these appear to be chock full of old stars — hundreds of millions of years old — in a universe that’s only 600-800 million years old. Remarkably, these objects hold the record for the earliest signatures of ancient starlight,” said Bingjie Wang, a postdoctoral researcher at Penn State University and lead author of a paper describing the discovery. “It was completely unexpected to find old stars in a very young universe. The standard models of cosmology and galaxy formation have been incredibly successful, but these luminous objects don’t fit comfortably into those theories.”
In addition to ancient stars, the JWST data also reveal the existence of unexpectedly large supermassive black holes in the galaxies. “It’s very puzzling,” said Joel Leja, an assistant professor of astronomy and astrophysics at Penn State. “You can make this fit uncomfortably into our current model of the universe, but only if we invoke an exotic, insanely fast formation at the beginning of time. This is without a doubt the most peculiar and interesting collection of objects I’ve seen in my career.”
What’s going on with such galaxies and old stars?
Because JWST can easily peer back into extremely early cosmic time, these results reveal what the earliest objects looked like. It still doesn’t explain how such old stars exist so soon after the birth of the universe. If they really are hundreds of millions of years old, then they began forming extremely early in cosmic time. It’s possible that they were born much earlier than astronomers think. Their formation is still hidden from our view, so it’s hard to know exactly when the process of star birth first began.
That’s a challenge that JWST’s infrared capabilities should help explain, especially as astronomers use it to delve deeper into the earliest cosmic epochs. It shows objects emitting non-infrared (UV) light, stretched out by the universe’s expansion into the infrared. This so-called “old” light streams from a myriad of objects, including stars, galaxies, and the active centers of galaxies. As Wang notes, it’s not clear which parts of the emitted light are coming from different objects. It could be coming from early galaxies that appear unexpectedly old and more massive than our own Milky Way. If so, they formed much earlier than astronomers suspect. It’s also possible that the light is coming from more normal-mass galaxies with “overmassive” black holes. Those supermassive monsters could be about a hundred to a thousand times more massive than such a galaxy would be in the “recent” universe.
“It’s challenging to distinguish between light from material falling into a black hole and light emitted by stars in these small, distant objects,” Wang said. “That inability to tell the difference in the current dataset leaves plenty of room for interpretation of these intriguing objects. Frankly, it’s exciting to have so much of this mystery to unravel.”
More about those unexpectedly large and early supermassive black holes
If the light the JWST is detecting is indeed coming from regions around supermassive black holes, then you have to wonder why they are so bright so early in history. Because they are luminous in the infrared, that means the original emissions were in ultraviolet light. And the brightness means there is a lot of activity going on to emit the light. That implies a lot more mass in the central SMBH than expected. And according to co-author Joel Leja, it is confusing because these objects do not seem to fit existing models of the early universe.
“Normally, supermassive black holes are paired with galaxies,” he said. “They grow up together and have all their important life experiences together. But here we have a fully formed adult black hole living in what should be a baby galaxy. That doesn’t really make sense, because these things should grow together, or so we thought.”
More data needed on old stars and baby galaxies
Leja explained that these systems are baby galaxies, only a few hundred light-years across. For comparison, our Milky Way is about 100,000 light-years across (or so). However, these small galaxies contain about the same number of stars as the Milky Way: between 10 billion and a trillion stars. “These early galaxies would be so full of stars: stars that would have formed in a way that we’ve never seen before, under conditions that we would never expect at a time when we would never expect them,” he said. “And for whatever reason, the Universe stopped making objects like these after just a few billion years. They’re unique to the early Universe.”
Clearly, JWST will need to do more research to explain these anomalous objects in the early universe. In particular, astronomers want to learn more about how ancient stars formed so early in history. Future observations should provide a deeper, longer spectral look at all of these objects. That will allow astronomers to separate the light from stars and the light from the regions around supermassive black holes. Each of these objects has its own specific “fingerprint” of light that tells astronomers what it is. The answers they get will help them understand not only the evolution of early stars and their galaxies, but also the ongoing questions surrounding the origin and growth of supermassive black holes so early in cosmic time.
For more information
Small bright objects discovered at the dawn of the universe amaze scientists
RUBIES: Evolved stellar populations with extended formation histories at z ~ 7–8 in candidate massive galaxies identified with JWST/NIRSpec