Artist’s impression of a black hole star (not to scale). T. Müller/A. de Graaff/Max Planck Institute for Astronomy (CC BY-NC-ND)
Astronomers may be one step closer to unlocking the missing link in supermassive black hole evolution after finding mysterious tiny red objects may in fact be an entirely new class of celestial objects – ‘black hole stars’.
The new study analysed data collected by the James Webb Space Telescope in 2022. At the time of the data’s release, researchers from Pennsylvania State University in the US noticed mysterious little red dots which they called ‘universe breakers’.
These dots were thought to be galaxies as ancient as the Milky Way – roughly 13.6 billion years old – but from just 500 to 700 million years after the Big Bang. The objects called into question how scientists thought galaxies formed.
Now, after revisiting the data, they believe the red dots may actually be giant spheres of hot gas that are so dense they look like the atmospheres of stars. However, they aren’t typical stars.
“Basically, we looked at enough red dots until we saw one that had so much atmosphere that it couldn’t be explained as typical stars we’d expect from a galaxy,” says co-author of the study, Joel Leja, an associate professor of astrophysics at Penn State.
Rather than being powered by nuclear fusion like stars, the researchers believe objects are driven by supermassive black holes in their centres that pull in matter and convert it into energy which gives off red light.
“It’s an elegant answer really,” says Leja.
“We thought it was a tiny galaxy full of many separate cold stars, but it’s actually, effectively, one gigantic, very cold star.”
The findings have been published in Astronomy & Astrophysics.
The colour of a star helps indicate to astronomers its temperature. Most stars in the universe are colder, low-mass stars that emit a red or near-infrared glow that often gets washed out by rarer hot luminous stars.
When the astronomers first started investigating the small red dots, they noticed that they were much larger than what galaxy models predict. Initially, Leja and his colleagues assumed they must be mature galaxies, which tend to get redder as the stars within them age. But the objects were still too bright to fit this hypothesis.
So, the researchers went back to square one and set out about getting a spectrum to provide information on how much light the objects emit at different wavelengths.
They collected one of the largest spectroscopic datasets to date, using almost 60 hours of footage to obtain the spectra of 4,500 distant galaxies.
Within the dataset, the researchers noticed an object whose spectrum indicated a huge amount of mass, which they nicknamed ‘The Cliff’.
“The extreme properties of The Cliff forced us to go back to the drawing board, and come up with entirely new models,” says Anna de Graaf, an author of the paper from the Max Planck Institute for Astronomy, Germany.
The light from this object took roughly 11.9 billion years to reach Earth.
To the researchers’ surprise, this light wasn’t coming from thick clusters of stars but one giant object. Taken with the object’s spectral qualities, the authors propose The Cliff would be best explained by a supermassive black hole surrounded by a ball of hydrogen gas.
“These black hole stars might be the first phase of formation for the black holes that we see in galaxies today – supermassive black holes in their little infancy stage,” says Leja.
The JWST has found signs of high-mass black holes in the early universe before, though not much is known about their origin.
The researchers suggest these new black hole star objects, which are essentially turbocharged mass-builders, could help explain the early evolution of the universe.
The team plan to test the density of gas of these novel black hole stars in hopes of finding a clearer answer.
“This is the best idea we have and really the first one that fits nearly all of the data, so now we need to flesh it out more,” Leja said.
“It’s okay to be wrong. The universe is much weirder than we can imagine and all we can do is follow its clues. There are still big surprises out there for us.”