Webb Telescope Sharpens Understanding of “Little Red Dots”

Natural Sciences8 MIN READ

Astronomer Dale Kocevski leads efforts to explain new class of previously unseen objects in the early universe

Dale Kocevski, associate professor of physics and astronomy, in front of a large mosaic of data collected from the James Webb Space Telescope. Kocevski is the lead author of a new study that proposes the hotly debated "little red dots" are likely galaxies with growing black holes at their centers. (Photo by Gabe Souza)
Share
By Laura Meader
January 15, 2025

Look closely at images from the James Webb Space Telescope and you’ll see hundreds of small red objects scattered like rubies across the distant, young universe. These objects, never before seen, are astronomy’s most pressing mystery. 

Meet the “little red dots.”

Though abundant, researchers are perplexed by the dots’ nature, why they boast unique colors, and what they might reveal about the early universe. They have universally confounded astronomers.

“We don’t really know what they are, which is highly unusual,” said Dale Kocevski, associate professor of physics and astronomy at Colby and lead author of a new study drawing widespread attention that proposes a theory about the dots. “You rarely find things you’re at a complete loss to explain.”

Kocevski is a member of an international team of astronomers that receives data from the James Webb Space Telescope, or Webb for short.

These bright jewels of the universe are sparking new questions and theories about processes that occurred at the dawn of the universe, especially relating to early black hole growth.

A detailed view of one section of the Cosmic Evolution Early Release Science (CEERS) Survey. Images like this provided the first detection of little red dots, defined as a red, compact objects known to show signatures of a growing black holes, but with an unexplainable light distribution. (Photo courtesy of NASA, ESA, CSA, Steve Finkelstein (UT Austin), Micaela Bagley (UT Austin), Rebecca Larson (UT Austin))

Kocevski’s theory suggests that the little red dots are likely galaxies with growing black holes at their centers. Moreover, by measuring the objects’ cosmological redshift, astronomers found that these objects existed only in the early universe. They emerged in large numbers around 600 million years after the Big Bang and quickly declined in quantity around 1. 5 billion years after it.

He and members of CEERS, the Cosmic Evolution Early Release Science Survey, are fervently investigating their theory in a race to be the first to solve the mystery of the little red dots.

Kocevski presented the team’s results in a press conference at the 245th meeting of the American Astronomical Society Jan. 14, 2025. Their paper has been submitted for publication in the Astrophysical Journal.

A glimpse into early black hole growth

Kocevski, who studies galaxy evolution and why supermassive black holes grow at the center of some galaxies and not in others, was the first to draw attention to the possibility that growing black holes may power little red dots. In late 2023, he was investigating a small bright object he thought was a galaxy, only to find that it showed clear signs of a growing supermassive black hole at its center. In addition, the object had unusual characteristics.

Astronomers estimate a galaxy’s mass by looking at its light distribution, or the amount of light it gives off at shorter, blue wavelengths and at longer, red wavelengths. Then they employ a model that reveals how massive a galaxy is, how old it is, and how long ago it formed its stars.

“I fired off an email to my collaborator saying, ‘ I cannot figure out what is going on with the light coming from this galaxy. I can’t explain the light distribution I’m seeing,’” he recalled. He proposed a few scenarios: it could be a massive galaxy, a massive black hole, or some combination of the two.

That was the very first detection of a little red dot, defined as a red, compact object known to show signatures of a growing black hole, but with an unexplainable light distribution.

Soon after, Kocevski and his collaborators delved into datasets. That is when he realized they could identify galaxies with growing black holes based solely on their color distribution. “They all seemed really red at long wavelengths and really blue at short wavelengths,” said Kocevski. “That color combination alone seems to predict that there’s a growing black hole in the center of these sources.”

Six of the 340 little red dots Dale Kocevski and a team of astronomers identified using James Webb Space Telescope data from multiple surveys to compile one of the largest samples of little red dots to date. The team started with the Cosmic Evolution Early Release Science (CEERS) survey before widening their scope to other extragalactic legacy fields, including the JWST Advanced Deep Extragalactic Survey (JADES) and the Next Generation Deep Extragalactic Exploratory Public (NGDEEP) survey. (Composite courtesy NASA, ESA, CSA, STScI, Dale Kocevski)

Kocevski’s team identified 340 little red dots, the largest sample to date, using their dataset and others publicly available. They found that 70 percent to 80 percent of the targets showed evidence of gas rapidly orbiting two million miles per hour (1,000 kilometers per second)—a sign of an accretion disk around a supermassive black hole. This suggests that many little red dots are accreting black holes, also known as active galactic nuclei, which are bright, compact regions at the center of certain galaxies.

Their findings hint at an era of black hole growth obscured by cosmic dust in the early universe, said Kocevski’s colleague Steven Finkelstein, a coauthor of the study, at the University of Texas at Austin.

Saving cosmology from breaking

Of course, with so many other researchers clamoring for ways to explain the little red dots, other theories have emerged. Aside from Kocevski’s, the other predominant theory suggests the dots are massive galaxies with no growing black holes.

 “The problem with that interpretation,” said Kocevski, “is that the galaxies’ brightness implies that they’re very, very massive. Basically, more massive than the Milky Way. But since they existed so early in the history of the universe, you’ve got a problem. You can’t make galaxies that big that early.” There simply wasn’t enough material available, he said.

This contradiction led to headlines that the little red dots were “universe-breaking galaxies” and that cosmology was “broken.” Some suggested that perhaps the universe is older than we think it is because that would allow more time for massive galaxies to form.

Kocevski’s theory, however, alleviates the problem. His team’s research supports the argument that much of the light coming from these objects is from growing black holes and not from stars within a galaxy. Fewer stars mean smaller, more lightweight galaxies that can be understood by existing theories.

Breaking degeneracy

One issue still up for debate in Kocevski’s theory is how much of the dots’ light is coming from the black hole versus the galaxy.

All galaxies contain supermassive black holes at their center, but the galaxies associated with the little red dots are extremely tiny. In fact, when Kocevski uses a model to subtract the light of the black hole, the red dot completely disappears. There is no galaxy visible.

“Some people have gone as far as to say there’s no galaxy there. These are just free-floating black holes roaming around the early universe without a galaxy,” he explained. Such objects are highly unusual, he said, because we don’t see them in our local universe.  

One of Dale Kocevski’s diagrams that shows brightness measurements (purple points) for little red dot CEERS 746 at various observed wavelengths.  The steep rise in brightness at long wavelengths and the flat emission at short wavelengths can be explained by a combination of dust-reddened starlight from a galaxy (green curve) and emission from an obscured supermassive black hole (red curve).  Astronomers are still uncertain which emission mechanism dominates the long wavelengths versus short wavelength light seen from little red dots.  Future observations with Webb are being planned to resolve this issue and break this degeneracy.

To help them understand this conundrum, Kocevski and his collaborators plotted data about little red dots obtained from CEERS observations on a graph relating brightness as a function of wavelength. On part of the curve, at shorter wavelengths, the data points match light-distribution models for galaxies. But at longer wavelengths, the points match models for black holes. However, the long-wavelength data points can also be explained by dust-obscured galaxy light, potentially without the need for emission from the black hole.

“That means these models are degenerate in the sense that we can’t figure out which one is the right one,” said Kocevski.

New data from additional observations from Webb at longer wavelengths—including the mid-infrared—is key to breaking the degeneracy.  Understanding the temperature of the dust possibly obscuring the black holes is another factor Kocevski is investigating.

Precursors to today’s black holes?

In the months ahead, Kocevski and his collaborators will continue looking broadly for accreting black holes to see how many fit the little red dot criteria. Obtaining deeper spectroscopy and select follow-up observations must be balanced with modeling in a continuous exchange to solve this “open case.”

If Kocevski can confirm the presence of black holes in the distant, little red dots, it would suggest they are progenitors of the supermassive black holes we see in the present-day universe.

“That’s the whole point of studying really early things,” said Kocevski. “It’s to look for the origin of the objects we see around us today.”

related

Highlights