NASA published this article on January 14, 2025. Edits by EarthSky.
What are Webb’s mysterious Little Red Dots?
In December 2022, less than six months after commencing science operations, NASA’s James Webb Space Telescope revealed something never seen before. It discovered an abundance of tiny red objects scattered across the sky. Scientists dubbed them ‘Little Red Dots.’ Since then, researchers have been perplexed by their nature, the reason for their color and what they convey about the early universe.
Now, a team of astronomers has compiled one of the largest samples of Little Red Dots to date. Nearly all of them existed during the first 1.5 billion years after the Big Bang. And they said on January 14, 2025, that a large fraction of the Little Red Dots in their sample are likely galaxies with supermassive black holes growing at their centers.
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A peek into early black hole growth?
By combing through data from several publicly available surveys, the researchers assembled a huge sample of these Little Red Dots. And they found the distribution of these objects across time to be intriguing. The Little Red Dots appear to emerge in large numbers around 600 million years after the Big Bang. They then underwent a rapid decline in quantity around 1.5 billion years after the Big Bang.
And they found that about 70 percent of the targets showed evidence for gas rapidly orbiting 2 million miles per hour (900 kilometers per second). That’s what you’d expect from an accretion disk around a supermassive black hole. So this suggests many Little Red Dots are accreting black holes, also known as active galactic nuclei (AGN).
Steven Finkelstein, a co-author of the study, said:
The most exciting thing for me is the redshift distributions. These really red, high-redshift sources basically stop existing at a certain point after the big bang. If they are growing black holes, and we think at least 70 percent of them are, this hints at an era of obscured [by gas and dust] black hole growth in the early universe.
Contrary to headlines, cosmology isn’t broken
When astronomers first discovered the Little Red Dots, some suggested that cosmology was ‘broken.’ If all of the light coming from these objects was from stars, it implied that some galaxies had grown so big, so fast, that theories could not account for them.
But the team’s research suggests that much of the light coming from these objects is from accreting black holes, not from stars. Fewer stars means smaller, more lightweight galaxies existing theories can explain.
Anthony Taylor, a co-author of the study, surmised:
This is how you solve the universe-breaking problem.
But questions remain around Little Red Dots
But the Little Red Dots evoke even more questions. For example, it’s still not clear why they don’t appear at lower redshifts. That is, more recently in the universe’s history. One possible answer is inside-out growth. As star formation within a galaxy expands outward from the nucleus, supernovae deposit less gas near the accreting black hole. It then becomes less obscured. So the black hole eventually sheds its gas cocoon, becoming bluer and less red. It therefore stops appearing as a Little Red Dot.
Additionally, Little Red Dots are not bright in X-ray light, unlike most black holes we see in the more recent universe. However, astronomers know that at certain gas densities, X-ray photons can become trapped. This reduces the amount of X-ray emission. Therefore, this quality of Little Red Dots could support the theory that these are heavily obscured black holes.
The team is taking multiple approaches to understand the nature of Little Red Dots. This includes examining the mid-infrared properties of their sample, and looking more broadly for accreting black holes to see how many fit the Little Red Dots criteria. Obtaining deeper follow-up observations will also be beneficial for solving this currently ‘open case’ about the mysterious Little Red Dots.
The researchers presented these results in a press conference at the 245th meeting of the American Astronomical Society in National Harbor, Maryland. They have also been submitted for publication in The Astrophysical Journal.
Bottom line: Researchers studying the mysterious Little Red Dots discovered by Webb have found evidence that they could be ancient galaxies with supermassive black holes growing at their centers.
Via NASA
Read more: 3 years of the Webb telescope: Here’s what it’s discovered