Black holes in stellar guise: "Black Hole Stars" in the early universe

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Tiny, extremely bright, and strikingly red points of light from the early days of the cosmos have been occupying astronomers since the first deep-field images from the James Webb Space Telescope (JWST). Initially, these "Little Red Dots" appeared like surprisingly mature galaxies in a universe that is actually too young for them. Now, another interpretation is gaining traction: Many of these objects could be active black holes with dense gas envelopes – their spectrum more closely resembles that of a single hot celestial body than a galaxy of billions of stars.

From Galactic Nucleus to "Black Hole Star"

An analysis of 116 Little Red Dots published in November on arXiv, in the redshift range of 2.3 < z < 9.3, provides crucial clues: The UV to optical continuum of these objects can be described by modified blackbody radiation – typically with temperatures around 5000 Kelvin and a radiation maximum at about 0.65 micrometers. This is more akin to a stellar atmosphere than the light from billions of individual stars.

The energy source is at the center: an active, matter-devouring black hole. The surrounding gas envelope absorbs the hard radiation and re-emits it as reddish, thermalized light. Lead author Anna de Graaff explains the mechanism in New Scientist: "When material falls into the black hole, a lot of gravitational energy is released, and this could make the whole ball of gas around it glow like a star."

The designation "Black Hole Star" (BH*) originates from earlier work by the same team. Although no nuclear fusion occurs inside, the object appears from the outside like a giant star – powered not by fusion energy, but by the gravity of the central black hole.

De Graaff admits: "The name black hole star is, for sure, still controversial, but I do think that there is now a decent consensus in the community that we are looking at an accreting black hole that’s enshrouded in dense gas."

The authors draw an analogy to stellar evolution: The Little Red Dots lie on a sequence in the Hertzsprung-Russell diagram of stellar evolutionary phases, corresponding to the so-called Hayashi track – the evolutionary path that young stars follow.

Spectral Signatures as Key

The study shows close linear relationships between Hα line emission – a hydrogen emission line belonging to the Balmer series – and the optical continuum, as well as between Hα and the so-called OI 8446 line. Hotter Little Red Dots exhibit strong Balmer jumps (abrupt changes in brightness in the spectrum caused by hydrogen absorption), red UV spectral indices, and high optical luminosities. Cooler objects show weaker Balmer jumps and greater spectral diversity.

Another finding: The Balmer decrement – the intensity ratio of hydrogen emission lines – increases with luminosity and the strength of the Balmer jump. This suggests density-dependent processes in the gas envelopes, such as collision-driven excitation and resonance scattering. According to the analysis, the [O III] emission originates primarily from star formation in the host galaxies, not from the AGN itself.

A Piece of the Puzzle for the Early Universe

The BH* hypothesis fits into the picture that JWST has been painting since its commissioning: Supermassive black holes seem to have been present from the very beginning – almost like building blocks or seeds for galaxies. The discovery of "monster stars" of Population III, considered precursors to particularly massive black holes, also fits into this scenario.

Whether Black Hole Stars actually represent a distinct object class has not yet been definitively clarified. The authors of the study emphasize that their work provides a unified empirical description of the Little Red Dots and sets a reference framework for future models of early, gas-enshrouded active galactic nuclei.

For cosmology, this would be more than a footnote. It would mean that black holes are not a late byproduct of galaxy evolution, but possibly among their earliest architects.

(vza)