A black hole deep in the cosmos, some 5 billion light-years away, could be the most massive ever found.
The galaxy SDSS J1148+1930 harbors a behemoth, around 36.3 billion times the mass of our Sun, according to new measurements. That places the black hole’s mass very close to the practical upper limit. For context, the Milky Way’s central black hole is a piddling 4.3 million solar masses.
This newly discovered black hole no longer fits within the category of supermassive – the giant beast of a thing is ultramassive.
Related: Black Holes Could Get So Humongous, Astronomers Came Up With a New Size Category
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“This is amongst the top 10 most massive black holes ever discovered, and quite possibly the most massive,” says astrophysicist Thomas Collett of the University of Portsmouth in the UK.
“Most of the other black hole mass measurements are indirect and have quite large uncertainties, so we really don’t know for sure which is biggest. However, we’ve got much more certainty about the mass of this black hole thanks to our new method.”
Supermassive black holes, larger than about a million solar masses, are thought to lurk in the heart of every full-sized galaxy, the gravitational hub around which everything else in the galaxy revolves.
Theoretically, there’s no limit to how massive a black hole could grow. In practical terms, other constraints, such as the growth rate, suggest that the maximum mass a black hole could achieve within the current 13.8-billion-year lifespan of the Universe is about 50 billion solar masses.
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The only way we can test these upper limits, however, is by finding the black holes involved. Which brings us to a fascinating feature in Earth’s sky known as the Cosmic Horseshoe. This is a horseshoe-shaped smear of light arcing around a central glowing blob – the result of a rare cosmic alignment called a gravitational lens.
Each of the two components – the smear and the blob – are along the same line of sight, at different distances. The blob is actually a galaxy so massive that its gravitational field warps and magnifies the light from a more distant light source. That’s what makes the smear.
We can learn a lot about distant stars and galaxies magnified this way, but in this case, it’s the foreground blob in which a team of astronomers led by Carlos Melo-Carneiro of the Federal University of Rio Grande do Sul in Brazil discovered their astonishing black hole.
“This discovery was made for a ‘dormant’ black hole – one that isn’t actively accreting material at the time of observation,” Melo-Carneiro says. “Its detection relied purely on its immense gravitational pull and the effect it has on its surroundings.”
In a lensed system, the extent of the lensing reveals the strength of the gravitational field, which is linked to the mass of the foreground object. Since the masses of supermassive black holes are proportional to the masses of their galaxies, this is one way to calculate the mass of a black hole at a galactic center.
Another tool for determining the mass of quiescent black holes involves stellar kinematics – the way stars and other material whirl around. Long-term observations of orbits around the Milky Way’s galactic center, for example, confirmed the presence of the black hole therein and provided a measure for its mass.
The Cosmic Horseshoe was discovered in 2007. Observations taken at intervals since then allowed the researchers to determine the motions at play in the galactic center. Combined with analysis of the radial arc of the more distant galaxy, the results gave what the researchers say is a very robust measurement.
Heavier black holes have been detected, but the measurements are perhaps a little less confident. TON-618 is a famous example. Its mass was initially thought to be around 66 billion solar masses; however, this was revised down in 2019 to about 40 billion solar masses based on galactic kinematics.
What makes SDSS J1148+1930 a little more exciting, however, is that it’s what is known as a fossil galaxy. This is a single, massive blob of a galaxy that was once a galaxy cluster. The researchers believe that, over time, the galaxies in the cluster – each with a supermassive black hole in its center – merged, their black holes also eventually glomming into one big 36 billion solar mass black hole.
It’s a huge clue about one of the Universe’s many open questions: how supermassive black holes get so enormous.
In the Cosmic Horseshoe, “we’re seeing the end state of galaxy formation and the end state of black hole formation,” Collett says.
The discovery has been published in the Monthly Notices of the Royal Astronomical Society.
