In the last couple of decades, it’s become increasingly clear that massive galaxies like our own Milky Way host supermassive black holes (SMBHs) in their centres.

How they became so massive and how they affect their surroundings are active questions in astronomy.

Astronomers working with the James Webb Space Telescope have discovered an SMBH in the early Universe that is accreting mass at a very low rate, even though the black hole is extremely massive compared to its host galaxy.

What’s going on with this SMBH, and what does it tell astronomers about the growth of these gargantuan black holes?

The black hole, named GN-1001830, was discovered as part of JADES (JWST Advanced Deep Extragalactic Survey).

It is one of the most massive SMBHs discovered by the JWST in the early Universe. While most present-day SMBHs account for about 0.1 % of the mass of their host galaxies, this one accounts for about 40% of its host galaxy’s mass.

The puzzling thing is that GN-1001830 is consuming the gas it needs to grow at a very low rate and is basically dormant. Is it taking a break? Did it experience accelerated bursts of growth in the past?

The findings are in new research published in Nature titled “A dormant overmassive black hole in the early Universe.” The lead author is Ignas Juodžbalis. Juodžbalis is a grad student at the Kavli Institute for Cosmology at the University of Cambridge.

The JWST has found many SMBHs already in place, only a few hundred million years after the Big Bang. Some of them are overmassive yet dormant, like GN-1001830. Researchers have developed multiple different models to explain them.

One model is the ‘heavy seed‘ model, where primordial gas clouds directly collapsed into black holes that grew to become SMBHs. Another model proposes light seeds that experience powerful bursts of accretion.

Both models hold promise, but there’s no certainty. “Yet, current datasets are unable to differentiate between these various scenarios,” Juodžbalis and his co-authors write in their research article.

These overmassive black holes that appear to be dormant are testing astrophysicists’ understanding of how SMBHs form and grow. It’s likely that they go through bursts of growth, and in between those bursts, they lie dormant.

One of the problems is that it’s very difficult to spot an SMBH that isn’t actively accreting gas. They’re visible when accreting because the accretion disk heats up and emits light.

This one was only spotted because it’s so massive.

“Even though this black hole is dormant, its enormous size made it possible for us to detect,” said lead author Juodžbalis. “Its dormant state allowed us to learn about the mass of the host galaxy as well. The early universe managed to produce some absolute monsters, even in relatively tiny galaxies.”

The Eddington Limit (also known as Eddington Luminosity) is an important factor in the growth of SMBHs.

It is a theoretical upper limit on the mass and luminosity of stellar objects, explaining the luminosity we observe in accreting black holes. The Eddington limit is reached when the outward pressure of radiation exceeds the object’s gravitational power, and it can’t accrete more matter.

Objects can also exceed this limit, and when that happens, it’s called Super Eddington accretion. Some researchers suggest that Super Eddington accretion was more common in the early Universe and that it explains not only this overmassive black hole but all of the other massive black holes the JWST has discovered in the Universe’s early times.

“It’s possible that black holes are ‘born big’, which could explain why Webb has spotted huge black holes in the early universe,” said co-author Professor Roberto Maiolino from the Kavli Institute and Cambridge’s Cavendish Laboratory. “But another possibility is they go through periods of hyperactivity, followed by long periods of dormancy.”

The research is based on the detection of broad H-alpha emissions from the SMBH. Those emissions showed that the overmassive black hole has an estimated mass of approximately 4 × 10? (40 million) solar masses. That’s extremely massive for an object only about 800 million years after the Big Bang. For comparison, Sagittarius A*, the SMBH in the Milky Way, has an estimated mass of about 4.3 million solar masses.

The SMBH in question is one of the most overmassive objects the JWST has found. Its mass is almost 50% of the stellar mass of its host galaxy. That’s about 1,000 times more massive than the relation in local galaxies.

The researchers conducted computer simulations to probe the issue. Their research suggests that the SMBH’s periods of hyperactivity likely exceed the Eddington Limit. The SMBH’s long periods of dormancy and inactivity can last for 100 million years, where the accretion rate is only 0.02 times the Eddington Limit, and are punctuated by episodes of Super Eddington accretion that last for about five or ten million years.

“It sounds counterintuitive to explain a dormant black hole with periods of hyperactivity, but these short bursts allow it to grow quickly while spending most of its time napping,” said Maiolino.

Since these SMBHs spend far more time dormant than they do active, they’re more likely to be spotted during dormancy. However, they’re far more difficult to spot when they’re not actively accreting and emitting radiation from their accretion rings. That’s part of what makes this detection so valuable.

These results are agnostic when it comes to heavy or light seeds. Instead, they’re all about Super Eddington episodes. “It is tempting to speculate that our result favours light seed models. However, the same result would also hold if the models had started with heavy seeds. The key feature that allows the properties of GN-1001830 to be matched is the fact that accretion goes through super-Eddington phases, regardless of the seeding mechanism,” the authors explain.

“This was the first result I had as part of my PhD, and it took me a little while to appreciate just how remarkable it was,” said Juodžbalis. “It wasn’t until I started speaking with my colleagues on the theoretical side of astronomy that I was able to see the true significance of this black hole.”

“It’s likely that the vast majority of black holes out there are in this dormant state—I’m surprised we found this one—but I’m excited to think that there are so many more we could find,” said Maiolino.

Written by Evan Gough/Universe Today.