Mysterious “Little Red Dots” Offer Clues to Black Hole Formation

When the James Webb Space Telescope (JWST) first allowed astronomers to peer at our universe’s infancy, an abundance of red pinpricks stared back. Dubbed “little red dots (LRDs),” these pinpricks appeared at first to have “broken the universe,” as they seemed to be galaxies too massive to have formed so early on in its history.

Now, new research presented at the 245th meeting of the American Astronomical Society in National Harbor, Maryland, and to be published in the Astrophysical Journal, shows that while these dots may not break the universe, they formed earlier on than expected and are even more abundant than previously thought. Moreover, they may help astronomers identify how our earliest black holes came to be.

“They’re one of the more unusual things that Webb has identified,” said team lead Dale Kocevski (Colby College). “We’ve only known about them for about a year, now.”

In previous work, Kocevski had already found that some LRDs are likely not giant, universe-breaking galaxies, but rather host hungry black holes that guzzle gas and emit more light than all of the galaxy’s stars. Such galaxies with bright centers are known as active galactic nuclei (AGN). But it remained unclear if all LRDs, or simply the brightest among them, fit this description.

To find out, Kocevski and his team sifted through about 1,000 JWST images looking for small, compact objects that appear red in JWST images. That selection yielded 341 little red dots — one of the largest samples ever compiled.

Adding JWST data from the Red Unknowns: Bright Infrared Extragalactic Survey (RUBIES), Kocevski found that more than 80% of LRDs showed signs of gas and dust spiraling at more than 1,000 km/s. Gas doesn’t revolve that fast unless it’s around a central black hole — solid evidence that the little red dots are indeed AGNs.

But the LRDs presented another conundrum. They seemed to violate a key theory governing our understanding of galaxies: that big black holes live in big galaxies while small ones abide in small galaxies. At such small sizes, the little red dot galaxies would be unlikely to hold such massive black holes.

Overly massive black holes don’t break the universe, either, though — they actually support a theory about the formation of the earliest black holes. Called direct collapse, this scenario suggests that early black holes were not born when a giant star collapsed in on itself, growing over time, but instead started off massive ­— around 100,000 times the mass of the Sun — when a huge pile of dust collapsed under its own weight. Over time, the galaxy would catch up to size by forming stars, with the black hole acting as its “seed.”

“If there are so many overmassive black holes so early on, then we really have to rethink a lot…about our existing hypotheses,” said Tonima Ananna (Wayne State University) who has studied LRDs. She thinks the new research “tips the scale towards the direct collapse” theory.

Kocevski’s work doesn’t just help explain early black holes; it also better describes the mysterious little red dots themselves. Based on how often they show up in any given patch of sky, Kocevski estimates that LRDs represent 1–5% of all galaxies in the early universe, more than 10 times the number expected for galaxies hosting gas-guzzling black holes. He thinks that number could jump even higher at earlier eras.

In general, little red dots represent a galaxy type most common when the universe was about 800 million years old, dropping off after it reached about 1.6 billion years.

The dots still provide a handful of curiosities — they don’t emit any X-rays, for example, which typical AGN do. Nevertheless, the new research suggests these galaxies aren’t just oddballs — they’re a common feature of our early universe.

 “If you take an image with JWST of the early universe, you’re likely going to have a handful of little red dots in every image that you see,” says Kocevski.  That’s “one of the reasons why folks actually want to figure out what they are.”