Astronomers may have found a new explanation for one of the James Webb Space Telescope’s (JWST) most puzzling discoveries—tiny, distant galaxies known as “little red dots.”
A new study from the Center for Astrophysics | Harvard & Smithsonian suggests these unusual galaxies may have formed inside rare dark matter halos that spin exceptionally slowly.
The work, led by astrophysicists Fabio Pacucci and Abraham (Avi) Loeb, appears in The Astrophysical Journal Letters.
Their findings offer a fresh perspective on why these mysterious galaxies look and behave so differently from anything we’ve seen before.
Little red dots are faint, compact galaxies spotted in JWST’s deep space images.
They are seen as they were over 12 billion years ago, when the universe was just one billion years old—a period known as the cosmic dawn.
Despite being only about one-tenth the size of typical galaxies, they appear surprisingly bright. Their reddish color likely comes from thick layers of dust or from older stars dominating their light.
For astronomers, these galaxies present a strange puzzle. If their light comes from stars, they seem far too dense—packing in more stars than should be physically possible for their size.
But if their light comes from giant black holes, those black holes would be unusually massive compared to the small galaxies hosting them. Either explanation stretches the limits of our current understanding.
Instead of debating whether the light comes from stars or black holes, Pacucci and Loeb decided to ask a different question: how could galaxies like this form in the first place? Their answer points to the role of dark matter halos—the massive, invisible structures made of dark matter that surround galaxies and guide their formation.
Dark matter halos typically spin, and this spin helps determine the shape and size of the galaxy that forms within them.
In most cases, faster spins spread out a galaxy’s material, creating larger structures. But the researchers suggest that little red dots formed inside halos that spin extremely slowly—slower than 99% of all others. This lack of spin would keep the galaxy’s matter tightly packed in the center, producing a small, ultra-compact galaxy.
Pacucci compares it to a carnival swing ride: the faster the ride spins, the farther out the swings fly. If the ride spins slowly, the swings stay close to the center. In the same way, a slow-spinning halo keeps a galaxy’s material close together.
This rare slow-spin scenario explains not only the compact size of little red dots but also their scarcity. Such halos represent only about 1% of all galaxies, though they are still more common than the extremely bright, black hole–powered quasars.
It also explains why little red dots are only seen in the early universe. Over time, halos grow larger and spin faster, making it harder for such tightly packed galaxies to form.
While the new theory doesn’t settle the debate over whether these dots are powered by stars or black holes, it suggests they are ideal environments for rapid growth of either. Their concentrated mass makes it easier for black holes to feed or for stars to form quickly. Some little red dots even show hints of black hole activity in their light spectra, though they lack the X-ray signals usually associated with active black holes.
Pacucci and his team are now working on new observations to learn more about these enigmatic objects. They hope to find similar galaxies closer to us in space, which would help reveal how little red dots evolve over time.
“These objects might be key to understanding how the first black holes formed and grew alongside galaxies,” Pacucci said. “Our work offers one possible piece of that puzzle.”
Source: Harvard-Smithsonian Center for Astrophysics.
