Study Reveals How Black Holes Grow

Australian astronomers have discovered how supermassive black holes grow – and it’s not what was expected.

Researchers have believed that supermassive black holes increased their mass in step with the growth of their host galaxy. However, new observations reveal a dramatically different behavior.

“Black holes have been growing much faster than we thought,” said Prof Alister Graham of Swinburne University of Technology, lead author of a paper accepted for publication in the Astrophysical Journal (arXiv.org version).

Within galaxies, there is a competition of sorts for the available gas; for either the formation of new stars or feeding the central black hole. For more than a decade the leading models and theories have assigned a fixed fraction of the gas to each process, effectively preserving the ratio of black hole mass to galaxy mass. The new reveals that this approach needs to be changed.

“We now know that each ten-fold increase of a galaxy’s stellar mass is associated with a much larger 100-fold increase in its black hole mass. This has widespread implications for our understanding of galaxy and black hole co-evolution,” Prof Graham said.

The astronomers have also found the opposite behavior to exist among the tightly packed clusters of stars that are observed at the centers of smaller galaxies and in disk galaxies like our Milky Way.

“The smaller the galaxy, the greater the fraction of stars in these dense, compact clusters,” said co-author Dr Nicholas Scott, also from Swinburne University of Technology. “In the lower mass galaxies the star clusters, which can contain up to millions of stars, really dominate over the black holes.”

Previously it was thought that the star clusters contained a constant 0.2 per cent of the galaxy mass.

The new study also appears to have solved a long-standing mystery in astronomy. ‘Intermediate mass’ black holes with masses between that of a single star and one million stars have been remarkably elusive. The research predicts that numerous galaxies already known to harbor a black hole – albeit of a currently unknown mass – should contain these missing `intermediate mass’ black holes.

“These may be big enough to be seen by the new generation of extremely large telescopes,” Dr Scott said.

“These black holes were still capable of readily devouring any stars and their potential planets if they ventured too close. Black holes are effectively gravitational prisons and compactors, and this may have been the fate of many past solar systems. Indeed, such a cosmic dance will contribute at some level to the transformation of nuclear star clusters into massive black holes,” Prof Graham said.

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Bibliographic information: Alister W. Graham, Nicholas Scott. 2013. The (black hole mass)-(host spheroid luminosity) relation at high and low masses, the quadratic growth of black holes, and intermediate-mass black hole candidates. ApJ, in press; arXiv: 1211.3199