How Big Can Black Holes Get?
December 19, 2012

More Ultramassive Black Holes Than Previously Thought

Lee Rannals for - Your Universe Online

According to a study using data from NASA's Chandra X-ray Observatory, there may be more ultramassive black holes in the Universe than previously thought.

A new analysis has looked at the brightest galaxies in a sample of 18 galaxy clusters in order to target the largest black holes.

The scientists' work suggests that at least ten of the galaxies contain an ultramassive black hole, weighing between 10 and 40 billion times the mass of the sun. Astronomers refer to black holes of this size as "ultramassive" black holes.

"Our results show that there may be many more ultramassive black holes in the universe than previously thought," said study leader Julie Hlavacek-Larrondo of Stanford University and formerly of Cambridge University in the UK.

The astronomers estimated the masses of the black holes in the sample by using an established relationship between masses of black holes, and the amount of X-rays and radio waves they generate.

This relationship, known as the fundamental plane of black hole activity, fits the data on black holes with masses ranging from 10 solar masses to a billion solar masses.

The black hole masses derived by the team 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 of the galaxy.

"These results may mean we don't really understand how the very biggest black holes coexist with their host galaxies," said 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 ultramassive black holes found in the study lie in galaxies at the centers of massive galaxy clusters. Outbursts powered by the central black holes are needed to help prevent this hot gas from cooling and forming enormous numbers of stars.

In order to power the outbursts, the black holes must swallow large amounts of mass. Because the largest black holes can swallow the most mass and power the biggest outbursts, ultramassive black holes had already been predicted to exist.

The extreme environment experienced by these galaxies may explain why the standard relationships for estimating black hole masses based on the properties of the host galaxy do not apply.

The results from the study can only be confirmed by making detailed mass estimates of the black holes in this sample. A study like has been carried out in the center of the galaxy M87.

"Our next step is to measure the mass of these monster black holes in a similar way to M87, and confirm they are ultramassive. I wouldn't be surprised if we end up finding the biggest black holes in the Universe," Hlavacek-Larrondo said. "If our results are confirmed, they will have important ramifications for understanding the formation and evolution of black holes across cosmic time."

The researchers published their findings in the Monthly Notices of the Royal Astronomical Society.