A New Theory for Little Red Dots: Shredded Stars Feeding Growing Black Holes

When JWST peered back into the early universe, its keen eyes revealed an unexpected population of galaxies nicknamed “little red dots” for their compact size and bright red hue. A new research article proposes that tidal disruption events could be responsible for the surprising appearance of these galaxies.

Galactic Surprises

Using JWST, researchers have discovered more than 300 little red dots. Most of these galaxies are concentrated around when the universe was 600 million years old, though they’ve been spotted out to about 1.6 billion years after the Big Bang. But why do these galaxies look the way they do? Do they host enormous, active black holes, as many researchers suspect? Do they get their characteristic red color from a population of old, evolved stars? Or could rapid, dust-shrouded star formation be the cause?

A successful theory of little red dot identity must account for several observed properties: “V”-shaped spectra, broad H-alpha lines, and little or no emission at X-ray wavelengths. In a recent research article, Jillian Bellovary (Queensborough Community College; CUNY Graduate Center; American Museum of Natural History) has theorized that little red dots get their curious characteristics from stars being torn apart by a young and growing black hole.

A New Hypothesis

In this scenario, little red dots arise in the extremely dense star clusters created in the early universe. Because of their extraordinary density, these star clusters are vulnerable to gravitational collapse, causing the packed-together stars to crash into one another. The colliding stars combine to form a supermassive star that collapses, leaving behind an intermediate-mass black hole — a seed that could someday grow into a supermassive black hole.

Situated at the center of a dense star cluster, an intermediate-mass black hole would be poised to ensnare stars in its gravitational web, pulling the stars apart with its powerful tidal forces and stealing their mass. These star-shredding events are known as tidal disruption events.

Simulations predict that in the era during which little red dots have been observed, a cubic megaparsec should contain about 0.3–1 intermediate-mass black holes. (These intermediate-mass black holes could arise through the cluster collapse scenario outlined above, or through other pathways like the collapse of the first stars in the universe.) Observations show roughly 10,000 fewer little red dots in this same volume.

Because tidal disruption events tend to remain bright for about a year, this implies that a tidal disruption event rate of one per 10,000 years is necessary for tidal disruption events to explain little red dots.

Matching Predictions

Using analytic and numerical models, Bellovary showed that this tidal disruption event rate is reasonable for intermediate-mass black holes with masses between 1,000 and 100,000 solar masses. But would tidal disruption events provide a match for other little red dot characteristics? Tidal disruption events tend to be faint at X-ray wavelengths and sport broad H-alpha lines, much like little red dots. Another plus is that unlike in the active black hole scenario, the H-alpha emission from a tidal disruption event doesn’t scale with the mass of the black hole; this means that black holes categorized as over-massive based on the strength of their H-alpha emission may not be over-massive after all. One drawback of the tidal disruption event scenario is that it doesn’t necessarily predict the characteristic red color of a little red dot; this color might come from the galaxy’s stars instead.

As Bellovary notes, little red dots could have multiple causes. Some might be due to tidal disruption events, while others could contain active black holes, ancient populations of red stars, or dusty starburst galaxies, as others have suggested.

To verify the tidal disruption event hypothesis, astronomers would need to find evidence for time variability at rest-frame ultraviolet wavelengths, as well as a characteristic decrease in brightness over time. These changes are expected to unfold over the course of years or decades, meaning that while observations may settle this question in due time, there will be plenty of time to ponder the mysteries of little red dots while we wait.

Citation

“Little Red Dots Are Tidal Disruption Events in Runaway-Collapsing Clusters,” Jillian Bellovary 2025 ApJL 984 L55. doi:10.3847/2041-8213/adce6c