Astronomers solve mystery of "little red dots" spotted by James Webb Space Telescope

Ever since NASA’s James Webb Space Telescope was launched, it’s been revealing more about the universe than even most astronomers expected. Some of the data JWST has sent back to Earth have been awe-inspiring, but some of it has been downright confusing or challenging to dominant theories in cosmology.

Dr. Dale Kocevski, a professor of astronomy and physics at Colby College, announced on Tuesday that he had solved a mystery which could have broken what scientists thought they knew about the universe. In early 2023, JWST found a series of little red dots (LRDs) in photographs from the ancient universe, less than a billion years after the Big Bang. In theory, the 341 red dots should not exist, because according to cosmological theory, stars and other celestial material should not be able to accumulate in such a short period of time.

Yet according to Kocevski, the LRDs are not so inexplicable after all. They are just growing supermassive black holes, he said at a lecture at the 245th annual meeting of the American Astronomical Society, accompanied by a paper due to be published in The Astrophysical Journal.

“When LRD were first identified, people thought they were massive galaxies that existed very early in the history of the universe,” Kocevski said, adding that they were dubbed “universe breaking” because they would have been too massive to be formed so early in the universe’s history. “However, if some of the light from LRDs can be attributed to a growing supermassive black hole (which we think is the case), then this would reduce the implied stellar mass of these galaxies. This essentially solves the too massive, too early problem that LRDs seemed to originally present.”

A team of astronomers sifted through James Webb Space Telescope data from multiple surveys to compile one of the largest samples of “little red dots” (LRDs) to date. (NASA / ESA / CSA / STScI / Dale Kocevski (Colby College))

After further research into the LRDs, Kocevski and his team confirmed that 81 percent of the subset are active galactic nuclei, or AGN, meaning they have central black holes which cause them to glow at the brightness that led to such puzzlement in the first place. Yet while these observations about LRDs answer some questions, they also raise provocative new ones.

"The most exciting thing for me is the redshift distribution. These really red, high-redshift sources basically stop existing at a certain point after the big bang,” Steven Finkelstein, a co-author of the study at the University of Texas at Austin, said in a statement. “If they are growing black holes, and we think at least 70 percent of them are, this hints at an era of obscured black hole growth in the early universe.”

Indeed, the LRDs indicate that the formation of our universe needs to account for the abundance of black holes.

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“The other significance is that obscured black hole growth in the early universe is much more common than we previously observed,” Kocevski said. “There were predictions that this would be the case, but the prevalence of dust-reddened black hole growth in the LRDs, which are very numerous, is observational evidence of this.”

Writing about the LRDs for Big Think, theoretical physicist and science writer Ethan Siegel observed that Kocevski’s research validates the usefulness of JWST as a tool for learning more about the universe. When the mystery of the LRDs first became apparent, professional and citizen scientists alike bandied about alternative theories for the LRDs’ redness, all of which is healthy for the state of scientific discourse. One side argued that the stars in the given galaxies were very old, or “red-and-dead,” since the shortest lived stars burn bright and hot blue, and therefore redefined what we know about the universe. The other claimed that the galaxies were very dusty (which Kocevski says is the case with the LRDs), because the cosmic dust blocks the light that would otherwise emanate from stars and other objects behind the dust.

Now we know that the second theory was likely correct — and also why Kocevski seemed to “solve the puzzle.”

“We know that these little red dots have contributions from stars within the galaxy and also from the activity of the central black hole,” Siegel wrote. “Because you can quantify which components of the light you observe (the flat, rest-frame ultraviolet) is caused by the stars in these LRDs compared to the components (the rising, rest-frame optical and infrared) caused by the active black hole, you can make estimates for the stellar mass of each galaxy, the central black hole mass in each galaxy, and then determine both how massive the galaxy is and just how ‘overmassive’ each black hole is.”

Kocevski added that because of the new research, "we are witnessing the very early growth of the supermassive black holes that are found at the center of today’s massive galaxies.” Because the black holes appear to be overmassive, meaning they are larger relative to their host galaxies than scientists would anticipate, it indicates “massive holes may have formed first and then galaxies grow around them at later times."

For his part, Kocevski is thrilled that he was able to utilize the JWST to illuminate a great outer space enigma.

“The discovery of LRDs is a testament to the power of JWST in both identifying objects that we couldn’t see previously and providing the data (in this case infrared spectroscopy) needed to understand them,” Kocevski said.