Astronomers knew the James Webb Space Telescope would reveal new things about the cosmos. The most powerful space telescope ever built, JWST can look deeper into the universe—and thus farther back in time—than humans had ever seen.
JWST launched in December 2021 and started science operations about six months later. And it wasn’t long before the telescope delivered on its promise.
When data started coming in as part of the Cosmic Evolution Early Release Science Survey, CEERS, astronomers noticed something unusual: The early universe is freckled with small, red galaxies that they’d never seen before.
These came to be known as “little red dots.” Their properties were so strange that they were deemed an entirely new class of cosmic object.
And the more we learn about them, the more befuddling they seem. “Every time we think we understand something [about LRDs], they surprise us again by not having the properties we expect,” says Jenny Greene, a professor of astrophysical sciences at Princeton University.
Evidence so far points to two possible explanations: Little red dots are likely either galaxies that host surprisingly large or luminous supermassive black holes, or they are compact galaxies with inexplicably huge numbers of old stars.
“The interpretations are kind of uncomfortable either way,” says Hollis Akins, a PhD student at the University of Texas at Austin who is studying LRDs.
Whatever they are, the nature of little red dots will have profound implications for our understanding of black holes, galaxy formation and cosmic evolution.
“It feels like a once-in-a-career moment of discovery,” says Greene.
What we know
The first LRDs emerged from the Cosmic Evolution Early Release Science Survey, but even more have emerged from subsequent data releases, the JWST Advanced Deep Extragalactic Survey and the Next Generation Deep Extragalactic Exploratory Public survey.
They are distinctive. Their spectra are characterized by a V shape caused by a slope decreasing in ultraviolet wavelengths and rising in optical wavelengths. By the latest tally, we’ve recorded about 340 of them.
As the name implies, they are little. A typical LRD has a radius of no more than 500 light-years—about 1% that of the Milky Way galaxy—and some are even smaller. And they are red, suggesting that either they emit primarily red light, or they emit red and blue light, but their blue light is filtered out by dust.
LRDs seem to only have existed in the early universe, emerging around 600 million years after the Big Bang and disappearing by about 1.5 billion years ago. We don’t see them at all in the local universe.
Despite their size, they are incredibly luminous, a fact that has caused quite a hubbub. “If you look at a very bright object in the early universe and assume it’s all stars, it comes out looking extraordinarily massive—almost too massive to have assembled in the age of the universe up to that time,” says Anthony Taylor, a postdoctoral researcher at UT Austin.
In fact, some estimates say little red dots require up to 100 billion stars to achieve their luminosity. That’s approximately the number of stars in the Milky Way, but in a hundredth of the space.
It’s a phenomenon that’s been compared to seeing a toddler who is 6 feet tall—there just wasn’t enough time or material for it to grow to that size. The discovery caused some to suggest at first that cosmology could be “broken.”
The presence of a supermassive black hole could explain the extreme luminosities of LRDs. Most galaxies have a supermassive black hole at their centers, so LRDs might not be any different. If astronomers attribute that brightness to an active galactic nucleus instead of just stars, the cosmological theories aren’t as strained.
New research, presented in January by a group including Taylor, used data from Red Unknowns: Bright Infrared Extragalactic Survey, a.k.a. RUBIES, to examine spectra of some LRDs. They found over 70% of the sample showed evidence of rapidly rotating gas—a hallmark sign of an accreting black hole.
But if their luminosities are powered by black holes, there’s something strange going on. Supermassive black holes that we see in our local universe have masses that are typically about 0.1% the mass of their host galaxies. In some little red dots, however, the masses are estimated to take up a ridiculous ratio: 10% to 50% of the mass of the galaxy.
“Either they’re very little black holes that are making copious amounts of light, or we’re growing the black hole mass way faster than we ever thought before,” says Greene. “Both of those are really exciting and interesting in different ways.”
What we don’t know
Astrophysicists still have a lot of questions about little red dots.
Of course, they still don’t know exactly what they are. What are their structures? To what degree are these powered by active galactic nuclei? Or stars? In either case, how did they form? If it’s a mix of both, then the question becomes, what’s the interplay between the two?
If researchers find more evidence that LRDs are accreting black holes, it could indicate that these are the early stages of the supermassive black holes that pockmark the universe today.
Researchers have two theories for how supermassive black holes are formed: They either start as many small, solar-mass seed black holes that merge and grow quickly, or as one very massive seed black hole. “For either of those scenarios, the little red dots seem like they could be telling us something about the conditions in which you can grow black holes in the very early universe,” says Akins.
Whatever makes them appear red and compact is a short-lived phenomenon, Akins says. “Then they evolve and become something else. That’s exciting, because it tells us they are the progenitors of some other population.”
But it’s still unclear whether LRDs do contain black holes. Black-hole-powered active galactic nuclei emit light in ultraviolet and X-ray wavelengths, but little red dots do not.
Astronomers thought it was possible that LRDs had a lot of dust in the way that absorbed the ultraviolet photons, giving them their red (and not blue) appearance. But a recent study found no evidence for UV-blocking dust. Their lack of X-ray emission is still a mystery being explored.
There is plenty of room for theoretical astrophysicists to work on new models to explain the puzzling phenomenon of little red dots—whether they house black holes or ancient stars. “We don’t really have good models for what these should look like,” says Akins. “Everything kind of goes out the window when you get to these really extreme objects.”
To get answers, astronomers need more data on LRDs. Astronomers want to look at little red dots in X-ray and radio wavelengths. And the community is anxiously awaiting the next round of data from JWST.
Many programs in JWST’s fourth observation cycle, which starts July 1, are examining LRDs, taking their spectra and trying to measure variations in luminosity. If astronomers see the brightness changing over short time periods, that’s a tally in the black hole column. On the other hand, if they see certain features in spectra, like specific absorption lines or a Balmer break—a drop in the flux at a certain wavelength—it could indicate LRDs are star-dense galaxies.
A recent paper suggests an alternative: that little red dot could be “black hole stars”— supermassive black holes surrounded by extremely dense, turbulent gas. Their cocoons of gas could account for the Balmer breaks and absorption features typically indicative of aged stellar populations.
Clearly, astronomers studying LRDs must be on their toes, and it can be tiring to keep up with the ever-changing landscape.
“That’s just part of it being a very exciting and new field,” says Taylor. “We found something we haven’t seen before. We have competing explanations for it, and it makes it really, really fun to work on.”
