Some of the mysterious pinpricks of light at the dawn of the Universe could be a type of object we’ve never seen before.
According to a new analysis of a “little red dot” (LRD) nicknamed The Cliff, these unexplained objects could be supermassive black holes wrapped in huge, dense clouds of gas, like an atmosphere surrounding a stellar core.
It’s a very tidy explanation that solves a problem astronomers are struggling to reconcile: a ‘break’ in the LRDs’ light that makes galaxies in the early Universe seem older than possible.
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“We … conclude that the rest-optical and near-infrared continuum of The Cliff cannot originate from a massive, evolved stellar population with an extremely high stellar density,” writes a team led by astrophysicist Anna de Graaff of the Max Planck Institute for Astronomy in Germany.
“Instead, we argue that the most plausible model is that of a luminous ionising source reddened by dense, absorbing gas in its close vicinity. Currently, the only model capable of producing both the strength and shape of the observed Balmer break is that of a black hole star.”
A Balmer break is a sharp change in the spectrum of an object in space occurring in the ultraviolet part of the spectrum, where shorter-wavelength light on one side of the line is much lower in intensity than the higher-wavelength light on the other side of the line. This feature is created by the absorption of shorter-wavelength light by hydrogen atoms.
A strong Balmer break is associated with galaxies that have a dominant population of A-type stars, which are just the right temperature to absorb light at the requisite wavelength.
Here’s the kicker: to display that strong Balmer break, those galaxies have to be old enough for the earliest dominant population of O and B stars to have largely died off, leaving the A-type stars responsible for most of the galaxy’s light, with little to no new star formation.
Many LRDs exhibit a strong Balmer break, at an epoch starting just 600 million years after the Big Bang, 13.8 billion years ago. Scientists believe this is too early in the lifespan of the Universe for a galaxy to have reached the stage of a dominant type-A population.
In turn, this has led to some investigation into what these small red lights at the dawn of spacetime might be – from primordial black holes to the seeds of supermassive stars.
The Cliff represents a whole new level of challenge, with light that has travelled for 11.9 billion years, with a Balmer break that is the most pronounced yet for an LRD.
“The extreme properties of The Cliff forced us to go back to the drawing board, and come up with entirely new models,” de Graaff explained.
Now, galaxies are not the only objects that exhibit Balmer breaks. If a whole bunch of stars collectively can produce a Balmer break in a galaxy’s spectrum, it stands to reason that each of those individual stars also exhibits the feature. The researchers noted that the spectrum of The Cliff looked closer to what they would expect to see in a single star rather than an entire galaxy.
With this peculiarity in mind, the researchers developed a model they called the black hole star: a supermassive black hole actively feeding from an accretion disk, surrounded and reddened, not by dust, but by a thick envelope of hydrogen gas.
The structure is somewhat similar to a star wrapped in scorching hot plasma, but instead of a core fusing atoms as we see in stars, the center of the black hole star is… well… a black hole, similar to an active galactic nucleus at the heart of a galaxy, that heats the turbulent hydrogen roiling around it.
It’s just a model at this point, but the team’s simulation of the black hole star replicated the spectrum observed in The Cliff extremely well. This suggests that at least some of the LRDs hanging out in the early Universe could be these strange black holes masquerading as galaxies.
Currently, the theory is very much just that: a theory. Further research is needed to determine not only if black hole stars are real, but also how they form and evolve, and what other features of their spectra might signify. However, it certainly seems plausible, and would at least partially help solve the problem of LRDs without resorting to breaking our understanding of how the Universe evolved.
“The Cliff presents the strongest direct evidence to date that the Balmer break and rest optical to near-infrared spectral energy distribution in LRDs can be dominated by emission from an active galactic nucleus, rather than evolved stellar populations, although many open questions regarding the black hole and host galaxy properties remain,” the researchers write.
“Because of its comparatively modest redshift, the high-quality spectrophotometric coverage of JWST extends over a wide rest-frame wavelength range. These stringent constraints make The Cliff the ideal benchmark for future active galactic nucleus and black hole star models.”
The research has been published in Astronomy & Astrophysics.