SDSS J1148+1930: Astronomers detect monster black hole pushing limits of physics

Far beyond our galaxy, about 5 billion light-years away, lies a black hole so vast it could set a new record. New measurements suggest the monster in galaxy SDSS J1148+1930 weighs in at around 36.3 billion times the mass of our Sun — perilously close to the practical upper limit for black holes. For comparison, the Milky Way’s central black hole is just 4.3 million solar masses.

This object isn’t just supermassive — it’s what astronomers are calling ultramassive.

The SDSS J1148+1930 black hole

"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," the space scientist adds.

Black holes larger than a million solar masses are thought to anchor the centre of almost every large galaxy. In theory, there’s no upper mass limit, but in practice, the age of the Universe — 13.8 billion years — caps growth at around 50 billion solar masses. Finding objects close to that ceiling is one way to test the limits of cosmic evolution.

The key to this discovery lies in an unusual feature in the night sky called the Cosmic Horseshoe — a luminous arc wrapped around a glowing blob, caused by a rare cosmic alignment known as gravitational lensing. The blob is a massive foreground galaxy whose gravity bends and magnifies light from a more distant source, creating the arc.

In this case, the “blob” turned out to be home to the colossal black hole.

"This discovery was made for a 'dormant' black hole – one that isn't actively accreting material at the time of observation," says Carlos Melo-Carneiro of the Federal University of Rio Grande do Sul in Brazil. "Its detection relied purely on its immense gravitational pull and the effect it has on its surroundings."

By studying the extent of the lensing and tracking the movement of stars near the galactic centre, the team produced what they describe as a highly reliable mass measurement. Observations of the Cosmic Horseshoe since its discovery in 2007 provided the long-term data needed.

Some heavier black holes have been reported, like TON-618, once estimated at 66 billion solar masses but revised down to around 40 billion in 2019. The key difference here is the confidence in the measurement.

What makes SDSS J1148+1930 even more intriguing is that it’s a fossil galaxy — once a cluster of galaxies that merged into a single massive one. Over time, their central black holes likely merged too, forming this single, ultramassive object.

In Collett’s words: "In the Cosmic Horseshoe, we're seeing the end state of galaxy formation and the end state of black hole formation."

The findings appear in the Monthly Notices of the Royal Astronomical Society.