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Black holes growing faster than expected

Black hole find Existing theories on the relationship between the size of a galaxy and its central black hole are wrong according to a new Australian study.

The discovery by Dr Nicholas Scott and Professor Alister Graham, from Melbourne's Swinburne University of Technology, found smaller galaxies have far smaller black holes than previously estimated.

Central black holes, millions to billions of times more massive than the Sun, reside in the core of most galaxies, and are thought to be integral to galactic formation and evolution.

However astronomers are still trying to understand this relationship.

Scott and Graham combined data from observatories in Chile, Hawaii and the Hubble Space Telescope, to develop a data base listing the masses of 77 galaxies and their central supermassive black holes.

The astronomers determined the mass of each central black hole by measuring how fast stars are orbiting it.

Existing theories suggest a direct ratio between the mass of a galaxy and that of its central black hole.

Inconsistency

"This ratio worked for larger galaxies, but with improved technology we're now able to examine far smaller galaxies and the current theories don't hold up," says Scott.

In a paper to be published in the Astrophysical Journal, they found that for each ten-fold decrease in a galaxy's mass, there was a one hundred-fold decrease in its central black hole mass.

"That was a surprising result which we hadn't been anticipating," says Scott.

The study also found that smaller galaxies have far denser stellar populations near their centres than larger galaxies.

According to Scott, this also means the central black holes in smaller galaxies grow much faster than their larger counterparts.

Black holes grow by merging with other black holes when their galaxies collide.

"When large galaxies merge they double in size and so do their central black holes," says Scott.

"But when small galaxies merge their central black holes quadruple in size because of the greater densities of nearby stars to feed on."

Somewhere in between

The findings also solve the long standing problem of missing intermediate mass black holes.

For decades, scientists have been searching for something in between stellar mass black holes formed when the largest stars die, and supermassive black holes at the centre of galaxies.

"If the central black holes in smaller galaxies have lower mass than originally thought, they may represent the intermediate mass black hole population astronomers have been hunting for," says Graham.

"Intermediate sized black holes are between ten thousand and a few hundred thousand times the mass of the Sun, and we think we've found several good candidates."

"These may be big enough to be seen directly by the new generation of extremely large telescopes now being built," says Graham.