Supermassive black holes put on weight

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 9 June, 2009

New computer modelling has found that the black hole at the heart of M87 is as much as three times more massive than previously thought, which could up the masses of other supermassive black holes, too.

Weighing in at 6.4 billion solar masses, M87 hosts the most
massive black hole measured with the new technique, which was presented by astronomers Karl Gebhardt of the University of Texas at Austin and Jens Thomas of the Max Planck Institute for Extraterrestrial Physics at the American Astronomical Society meeting yesterday.

The revised mass of this black hole has implications for the masses of other supermassive black holes, and has serious consequences for theories of how galaxies form and grow. “The crucial point is to determine whether the mass is in the black hole, the stars, or the dark halo,” says Thomas. “You have to run a sophisticated model to be able to discover which is which. The more components you have, the more complicated the model is.”

Streaming out from the centre of the galaxy M87 is a black-hole powered jet of electrons and other sub-atomic particles traveling at nearly the speed of light. New computer simulations will help astronomers learn about the characteristics and processes involved in supermassive black hole environments. Image: NASA and The Hubble Heritage Team (STScI/AURA).

For the case of M87, Gebhardt and Thomas used one of the world’s most powerful supercomputers, the Lonestar system at The University of Texas at Austin’s Texas Advanced Computing Center, to include greater detail than computing capabilities have previously allowed. Specifically, the models take into account the galaxy’s dark halo, a spherical region surrounding a galaxy that extends beyond its main visible structure, containing the galaxy’s mysterious dark matter, as well as the stars and central black hole.

The result was a mass for M87’s black hole several times more than what the team were expecting. “We did not expect it at all,” says Gebhardt. “We simply wanted to test our model on the most important galaxy out there.” M87 is considered by many as the anchor for supermassive black hole studies, since it is extremely massive, nearby, and has an active jet shooting light out of the galaxy’s core as matter swirls closer to the black hole, allowing astronomers to study the process by which black holes attract matter. The new results could mean that all black hole masses for the most massive galaxies are underestimated.

“If you change the mass of the black hole, you change how the black hole relates to the galaxy,” says Thomas. The relation between the galaxy and its black hole allows researchers to probe the physics of how galaxies grow over cosmic time, but increasing the black hole masses in the most massive galaxies will cause this relation to be re-evaluated. The result could, however, solve a paradox concerning the masses of quasars – active black holes at the centres of extremely distant galaxies, seen at a much earlier cosmic epoch. Quasars shine brightly as the material spirals in, giving off extreme amounts of radiation before crossing the event horizon, the region beyond which nothing, not even light, can escape.

“There is a long-standing problem in that quasar black hole masses were very large – 10 billion solar masses,” says Gebhardt. “But in local galaxies, we never saw black holes that massive, not nearly. The suspicion was before that the quasar masses were wrong. But if we increase the mass of M87 two or three times, the problem almost goes away.”

While the new results derive from computer models, the astronomers have also made new telescope observations of M87 and other galaxies using new powerful instruments on the Gemini North Telescope and the European Southern Observatory’s Very Large Telescope, the results of which support the model-based conclusions. The results will be published later this summer in The Astrophysical Journal.

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