Astronomers have identified and measured the most massive black hole to date, which tops out at a mass of 36 billion solar masses. This ultramassive black hole is close to the theoretical upper limit of what is possible in the Universe and is roughly 10,000 times heavier than Sagittarius A*, a supermassive black hole at Milky Way’s center.

The newly-discovered ultramassive black hole resides within the Cosmic Horseshoe gravitational lens system, where the lens galaxy is one of the most massive strong gravitational lenses known to date.

Also known as SDSS J1148+1930 and CSWA 1, this lens system is located 5 billion light-years away in the constellation of Leo.

“Typically, for such remote systems, black hole mass measurements are only possible when the black hole is active,” said Carlos Melo, a Ph.D. candidate at the Universidade Federal do Rio Grande do Sul.

“But those accretion-based estimates often come with significant uncertainties.”

“Our approach, combining strong lensing with stellar dynamics, offers a more direct and robust measurement, even for these distant systems.”

“The newly-discovered black hole is amongst the top 10 most massive black holes ever discovered, and quite possibly the most massive,” said Professor Thomas Collett, of the University of Portsmouth.

“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.”

The authors detected the Cosmic Horseshoe black hole using a combination of gravitational lensing and stellar kinematics.

The latter is seen as the gold standard for measuring black hole masses, but doesn’t really work outside of the very nearby Universe because galaxies appear too small on the sky to resolve the region where a supermassive or ultramassive black hole lies.

“Adding in gravitational lensing helped us push much further out into the Universe,” Professor Collett said.

“We detected the effect of the black hole in two ways — it is altering the path that light takes as it travels past the black hole and it is causing the stars in the inner regions of its host galaxy to move extremely quickly (almost 400 km/s).”

“By combining these two measurements we can be completely confident that the black hole is real.”

“This discovery was made for a ‘dormant’ black hole — one that isn’t actively accreting material at the time of observation,” Melo said.

“Its detection relied purely on its immense gravitational pull and the effect it has on its surroundings.

“What is particularly exciting is that this method allows us to detect and measure the mass of these hidden ultramassive black holes across the Universe, even when they are completely silent.”

An interesting feature of the Cosmic Horseshoe system is that the host galaxy is a so-called fossil group.

Fossil groups are the end state of the most massive gravitationally bound structures in the Universe, arising when they have collapsed down to a single extremely massive galaxy, with no bright companions.

“It is likely that all of the supermassive black holes that were originally in the companion galaxies have also now merged to form the ultramassive black hole that we have detected,” Professor Collett said.

“So we’re seeing the end state of galaxy formation and the end state of black hole formation.”

The team’s paper was published today in the Monthly Notices of the Royal Astronomical Society.

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Carlos R Melo-Carneiro et al. 2025. Unveiling a 36 billion solar mass black hole at the centre of the Cosmic Horseshoe gravitational lens. MNRAS 541 (4): 2853-2871; doi: 10.1093/mnras/staf1036