Black holes grow faster than first thought

By Joanna Egan 21 January 2013
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New research suggests black holes may not grow at the same rate as their galaxies.

GALAXIES AND THEIR CENTRAL black holes do not grow at the same rate, new research has revealed.

Until now, scientists believed the mass of a black hole was directly related to the mass of its parent galaxy, and that the two grew together at a constant rate. However, a pair of Australian astronomers from Swinburne University in Melbourne has shown that among galaxies, these mass ratios change.

This finding, which will be published in The Astrophysical Journal in February, not only challenges existing theories about the relationship between the speed of a galaxy’s growth and the that of its central black hole, but also indicates that widely-accepted methods for calculating the mass of a black hole are wrong.

“With this study, our understanding of black hole and galaxy evolution has been dramatically revised,” Professor Alister Graham, lead author of the study, told Australian Geographic. “As a result, current theories and computer models, which try and explain and reproduce galaxies, need to be significantly modified.”

Calculating mass of supermassive black holes

Prior to this study, astronomers thought the massive black holes that reside at the centre of most galaxies contained a near constant 0.2 per cent of their host galaxy’s total mass.

“We now know that these mass ratios change,” says Alister. He and co-author Dr Nicholas Scott combined data from the Hubble Space Telescope with observations from the European Very Large Telescope in Chile and the Keck Telescope in Hawaii, to determine the masses of 77 black holes.

“We compiled the largest sample to date of galaxies with directly measured supermassive black hole masses,” he says. “Coupled with our improved measurements of the galaxy masses, this enabled us to more accurately determine the relation between the mass of black holes and their host galaxies.”

The pair discovered that in small galaxies, black holes grow at much faster rates than previously thought.

“For some time now it’s been thought that the galaxy and black hole masses grew at the same rate, but one of the consequences of this work is that this may not be the case,” says Dr Sean Farrell, an astronomer at the University of Sydney. “Instead, this study indicates that black holes in low-mass galaxies grow faster than the ones in higher-mass galaxies.”

New class of black holes reside in smaller galaxies

Although black holes in these lower-mass galaxies contain a smaller proportion of their galaxy’s total mass, as their host galaxy expands, they grow at a much faster rate than supermassive black holes in larger galaxies.

“In these small galaxies, the black hole-to-galaxy growth rate is what we call ‘quadratic’, rather than ‘linear’,” says Alister. “If the galaxy doubles in mass then the black hole quadruples in mass, if the galaxy quadruples in mass then the black hole increases 16-fold, and so on.”

In the past, because of the difficulty involved in measuring the mass of black holes in small and distant galaxies, scientists have estimated their mass based on their brightness.

“Based on this new research, many of the black holes whose masses have been estimated using these techniques are potentially lower in mass than originally thought,” Sean explains.

This means they could be much smaller, and form part of a previously unidentified class of intermediate-mass black holes. Scientists have confirmed the existence of very small black holes (less than 100 times the mass of our Sun) and supermassive black holes (more than a million times the mass of our Sun), and although they have long suspected that intermediate-mass black holes exist, evidence for their existence has been very sparse.

“We now suspect that the previously missing class of intermediate-mass black holes reside in the dense, compact star clusters of small galaxies,” says Alister.

“This work is very interesting and if it is confirmed it will indeed represent a paradigm shift in the field,” Sean says. “I think for starters it will cause a lot of theorists to go back and look at their models of how early black holes in galaxies may have formed, as a lot of these models are based on assumptions regarding the masses of these black holes in the early stages of the Universe.”

However, Sean says controversial ideas such as this take time to be accepted by the field. “Further observations of low mass galaxies with the next generation of extremely large telescopes are needed to strengthen these results,” he says.