Ultramassive Black Holes May Be More Numerous Than Thought, Astronomers Say

According to a team of astronomers led by Dr Julie Hlavacek-Larrondo of Stanford University, so-called ‘ultramassive black holes’ may be more abundant in the Universe than previously thought.

This image shows a large elliptical galaxy located about 1.3 billion light years away in one of the 18 galaxy clusters studied by the team – the galaxy cluster PKS 0745 (NASA / CXC / Stanford / STScI / NSF / NRAO / VLA / Hlavacek-Larrondo J. et al)

Astronomers have long known about the class of the largest black holes, which they call supermassive black holes. Most galaxies are believed to contain a supermassive black hole at their centers, with masses ranging between a few million and a few billion times that of the Sun.

Results of an analysis of brightest galaxies in a sample of 18 galaxy clusters, reported in the Monthly Notices of the Royal Astronomical Society (arXiv.org version), suggest that at least ten of these galaxies contain a black hole weighing between 10 and 40 billion times the mass of the Sun – about ten times more massive than previously thought. Astronomers call these objects ‘ultramassive black holes.’

Dr Hlavacek-Larrondo said: “our results show that there may be many more ultramassive black holes in the Universe than previously thought.”

The team 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, called 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, 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,” explained study co-author Dr 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.”

Eleven of the ultramassive black holes found in this 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. Because the largest black holes can swallow the most mass and power the biggest outbursts, ultramassive black holes had already been predicted to exist, 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 based on the properties of the host galaxy do not apply.

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Bibliographic information: Hlavacek-Larrondo J et al. 2012. On the hunt for ultramassive black holes in brightest cluster galaxies. Monthly Notices of the Royal Astronomical Society, vol. 424, no. 1, pp. 224–231; doi: 10.1111/j.1365-2966.2012.21187.x