Monster black holes supersize their galactic greed

Monster black holes lurking in the centres of galaxies are hungrier than previously thought, Melbourne scientists have discovered.

Artist’s impression of a star being pulled into a black hole. Credit: Gabriel Perez Diaz

Astrophysicist Alister Graham and his team at Swinburne University have revealed that these so-called supermassive black holes consume a greater portion of their galaxy’s mass the bigger the galaxy gets. The discovery overturns the longstanding belief that these supermassive black holes are always a constant 0.2 per cent of the mass of all the other stars in their galaxy.

Alister and his colleagues found that the constant mass ratio rule was at odds with other properties that grow non-linearly with galactic size. Using new data gathered from the Hubble Space Telescope, the European Very Large Telescope in Chile and the Keck Telescope in Hawaii, they discovered that the ratio is only fixed for large galaxies formed when smaller, gas-poor galaxies merge.

“This non-linear growth is such that the black holes start out as much smaller seeds than previously realised,” says Alister. “They then grow rapidly by accumulating gas that would otherwise go into making stars.

“While small galaxies can contain both a black hole and a dense, centrally located star cluster, the bigger galaxies only harbour massive black holes containing about 0.5 per cent of the galaxy mass,” he says.

Interestingly, some galaxies are so small that they may contain the yet-to-be-observed ‘intermediate mass’ black holes, which lie between the supermassive monsters and those formed by collapsing stars.

Our own galaxy is relatively small, with a black hole only six million times the mass of the Sun. This was hard to explain under the old rules, which have now been updated to predict ratios of 10 or even 1,000 times less in the smallest galaxies.

This research was supported by Australian Research Council funding through grant FT110100263.

Photo: Artist’s impression of a star being pulled into a black hole.
Credit: Gabriel Perez Diaz

Centre for Astrophysics and Supercomputing, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Alister Graham, Tel: +61 3 9214 8784,