Are mysterious 'Little Red Dots' discovered by the James Webb Space Telescope actually nurseries for direct-collapse black holes?

Little Red Dots, mysterious objects discovered by the James Webb Space Telescope (JWST), could be nurseries for massive black holes that didn't form from collapsing stars, but instead emerged directly from vast gas clouds.

If this is the case, then it could solve not only the puzzle of the nature of Little Red Dots, but also another mystery uncovered by the JWST since it began operations in 2022. That is the discovery of a large population of supermassive black holes as early as 500 million years after the Big Bang.

This is problematic because the merger-driven process, thought to give rise to supermassive black holes with masses millions of even billions of times that of the sun, had previously been theorized to take at least a billion years to forge these cosmic titans that sit at the heart of large galaxies

That problem could be solved if supermassive black hole mergers begin with a "heavy seed", a direct collapse black hole, created when vastly overdense regions within primordial gas clouds collapse. This contrasts with a "light seed", formed when stars reach the end of their lives and explode as supernovae, leaving behind stellar-mass black holes

Not only would heavy seeds remove mass restrictions on the black holes that begin this merger process, but they would also allow it to get underway before the first generation of massive stars had even lived and died.

"All galaxies likely harbor a supermassive black hole at their centre, whose origin represents one of the frontier mysteries of modern astrophysics. One theoretical pathway to the formation of the heaviest black holes is that of direct collapse," research team leader Elia Cenci of the University of Geneva told Space.com. "In this scenario, black holes form following the collapse of a short-lived supermassive star that in turn forms from pristine gas that collapses at the centre of dark matter haloes that satisfy a number of stringent criteria. These criteria are mostly concerned with avoiding the formation of molecular hydrogen, which can efficiently cool the gas at high redshift, favoring the formation of smaller stars."

Cenci explained that Little Red Dots are weird sources of light that mostly emerged when the universe was less than a billion years old. Discovered through deep extragalactic surveys carried out with the JWST, they appear red and exceptionally compact, hence their name.

Little Red Dots are unusual for a number of other reasons, from the pattern of the light they emit, their spectra, to their physical properties, and the fact that they disappear early in the history of the 13.8 billion-year-old universe.

"A popular explanation for these objects is that we are looking at an abundant population of faint massive black holes of the early universe surrounded by very dense gas and stars that we would not been able to discover with previous instrumentation," Cenci said.

Cenci and colleagues connected Little Red Dots and direct collapse black holes while running high-resolution simulations of cosmic evolution in the early universe.

"Our results show that direct-collapse black holes that are newly formed naturally match the overall abundance and key physical characteristics inferred for the enigmatic Little Red Dots discovered with the JWST," Cenci said. "It is exciting to think that, if future studies confirm our proposed connection with direct-collapse black holes, Little Red Dots may represent the first direct observational evidence of the birth of the most massive black holes in the universe.

"For the first time, we would have real laboratories to understand the conditions under which giant black holes have formed."

Supermassive black holes could get a head start in Little Red Dots

Cenci explained that the advantage of direct collapse black holes is that they can act as so-called heavy seeds for black hole formation. This means that they can already be tens of thousands to a million times the mass of the sun when they form, unlike black holes formed via the death of stars, the mass of which is limited by the mass of the progenitor stars.

That provides a significant head start in growing supermassive black holes.

"Compared to lighter black hole seeds, they can grow more easily to the giant black holes that we observe in the short time available since the Big Bang, in astronomical terms at least," Cenci said.

The University of Geneva researcher also explained why direct collapse black holes and their nurseries aren't found in the local, modern-day universe, saying that the conditions needed include a lack of elements heavier than hydrogen and helium. Elements that are forged by stars and seeded in galaxies are released when these stars reach the end of their lives and explode as supernovae.

"In order to form direct collapse black holes, the gas should not form stars on its way to collapse in a monolithic fashion. Therefore, their 'nursery' environment must be pristine, not forming heavier molecules nor being polluted by the heavy elements produced by stellar evolution," Cenci said. "Practically speaking, these conditions are only possible in the early universe."

One of the most curious aspects of Little Red Dots is that they appear to vanish from the universe around 1.5 billion years after the Big Bang — or, as astronomers like Cenci describe it, at around redshift z~6. She believes this disappearance can be explained if Little Red Dots are hubs for direct collapse black hole formation.

"After z~6, the non-linear interplay of processes such as stellar evolution and feedback will make haloes hostile environments for the formation of direct collapse black holes, being more polluted with heavy elements and experiencing less intense inflows of gas that would favour the monolithic collapse scenario," Cenci said. "The decline in the population of newborn direct collapse black holes after z~6 is a natural consequence of the criteria determining where these objects can form."

Observational evidence confirming Little Red Dots as direct collapse black hole nurseries will require higher-resolution astronomical data and a more complete spectral coverage, Cenci explained. This would put additional constraints on the importance of the role black holes and stars play in Little Red Dots, as well as confirming the dynamics and physical state of their dense gas reservoirs. Until then, she and her team will continue to simulate conditions in the early universe to better understand this potential relationship.

"We are running a large suite of high-resolution simulations to test the implications of a number of different formation conditions for direct collapse black holes," "Our work will focus on understanding and characterising the population of direct collapse black holes in a cosmological context, and we will definitely be able to provide further insights on to what extent we can relate direct collapse black holes and Little Red Dots.

The team's research was published in the journal Monthly Notices of the Royal Astronomical Society.