How big are the biggest black holes?

Astronomers have had to come up with a whole new word for black holes that are too big to be described as merely ‘supermassive’ – and now say there may be a lot more of them around then previously thought.

While supermassive black holes typically have masses ranging between a few million and a few billion times that of our sun, a new analysis shows that many get a great deal larger. Some weigh in at between 10 and 40 billion times the mass of the sun, qualifying for the term ‘ultramassive’.

The Stanford University-led team examined the brightest galaxies in a sample of 18 galaxy clusters, estimating the masses of the black holes within by using an established relationship between masses of black holes and the amount of X-rays and radio waves they generate.

The black hole masses they came up with were about ten times larger than those derived from standard relationships between black hole mass and the properties of their host galaxy. One of these relationships involves a correlation between the black hole mass and the infrared luminosity of the central region, or bulge, of the galaxy.

“These results may mean we don’t really understand how the very biggest black holes coexist with their host galaxies,” says co-author Andrew Fabian of Cambridge University. “It looks like the behavior of these huge black holes has to differ from that of their less massive cousins in an important way.”

All of the potential ultramassive black holes found in the study lie in galaxies at the centers of massive galaxy clusters containing huge amounts of hot gas. Outbursts powered by the central black holes are needed to prevent this hot gas from cooling and forming enormous numbers of stars.

To power the outbursts, the black holes must swallow large amounts of mass in the form of hot gas – and because the largest black holes can power the biggest outbursts, ultramassive black holes had already been predicted to explain some of the most powerful outbursts seen.

The extreme environment experienced by these galaxies may explain why the standard relations for estimating black hole masses don’t apply.

The only way to check is to make detailed mass estimates of the black holes in the sample, by modeling the motion of stars or gas in the vicinity of the black holes. This has already been done for the black hole in the center of the galaxy M87, the nearest galaxy cluster to Earth.

And the mass of M87’s black hole, as estimated from the motion of the stars, is significantly higher than the estimate using infrared data, approximately matching the new predictions.

“Our next step is to measure the mass of these monster black holes in a similar way to M87, and confirm their existence. I wouldn’t be surprised if we end up finding the biggest black holes in the universe,” says Stanford’s Julie Hlavacek-Larrondo.

“If our results are confirmed, they will have important ramifications for understanding the formation and evolution of black holes across cosmic time.”