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Astronomers at the Center for Astrophysics | Harvard & Smithsonian have proposed a new explanation for some of the universe's most puzzling early galaxies, nicknamed "little red dots."

In the study, , authors Fabio Pacucci and Abraham (Avi) Loeb suggest that these galaxies are the result of very slowly spinning dark matter halos, an extremely rare cosmic structure.

These faint, compact objects, discovered in deep space images from the James Webb Space Telescope (JWST), have challenged scientists' understanding of how galaxies and black holes formed in the early universe.

Their paper, "Cosmic Outliers: Low-Spin Halos Explain the Abundance, Compactness, and Redshift Evolution of the Little Red Dots," offers a physical explanation for the dots' distinctive properties.

"Little red dots are very compact and red distant galaxies that were completely undetected before the James Webb Space Telescope," said Pacucci. "They are arguably the most surprising discovery by JWST to date. Our work shows that these could naturally form in dark matter halos with very low spin."

A puzzle in the early universe

These galaxies are primarily visible when the universe was just one billion years old, but likely formed much earlier, Pacucci said, during a time known as the cosmic dawn. Despite being about one-tenth the size of typical galaxies, astronomical observations show them to appear unusually bright. Astronomers believe their striking red color suggests they are shrouded in dust or filled with older stars.

For years, astronomers have debated whether the light we observe from these objects is generated by stars or central supermassive black holes.

"It's a fundamental mystery," said Pacucci. "If they contain black holes, those black holes are enormous for such small galaxies. But if they only contain stars, the galaxies are too compact to contain all of them, reaching central stellar densities that are unthinkable."

Rather than focusing on what powers the luminous dots, Pacucci and Loeb took a different approach: they examined how such objects might form in the first place.

The low-spin cycle

Dark matter halos are the invisible, spinning scaffolding around which galaxies form. In their paper, the authors show that the luminous dots are formed in halos that are in the lowest 1% of the spin distribution. In other words, 99% of all halos spin faster than those. These low-spin halos would naturally create extremely compact galaxies. Much like the swings ride at a carnival, the faster the halo spins, the farther out the swings stretch, causing the galaxy forming at its center to expand; likewise, a slow spin keeps the swings' radius smaller.

This hypothesis also explains why luminous dots are relatively rare: they represent just 1% of the abundance of typical galaxies, but are more common than quasars, the extremely bright centers supermassive black holes that shine at the center of some galaxies.

In addition, it helps clarify why luminous dots are only observed during a brief 1-billion-year period in the early universe. As the universe evolves, dark matter halos grow larger and gain more angular momentum, making it more difficult to form compact, low-spin galaxies.

"Dark matter halos are characterized by a rotational velocity: some of them spin very slowly, and others spin more rapidly," Loeb said. "We showed that if you assume the little red dots are typically in the first percentile of the spin distribution of dark matter halos, then you explain all their observational properties."

Prime environments for black holes

While the paper does not resolve whether little red dots are powered by stars or black holes, it suggests they are prime environments for rapid stellar or black hole growth.

"Low-spin halos tend to concentrate mass in the center, which makes it easier for a black hole to accrete matter or for stars to form rapidly," said Pacucci.

Some of the dots show broad emission lines in their spectra, which are possible signs of active black holes, but they lack the X-ray emission typically associated with them. Pacucci is leading new programs to understand better the nature of these peculiar astrophysical sources. For example, finding similar nearby galaxies will clarify what they evolve into further out in space.

"Our work is a step toward understanding these mysterious objects," he said. "They might help us understand how the first black holes formed and co-evolved with galaxies in the early universe."