A PAIR of Melbourne astronomers have overturned a decade-old theory on how supermassive black holes grow and, in the process, identified the location of a missing population of medium-sized black holes.
For years astronomers thought supermassive black holes, which are found at the centre of galaxies, grew by consuming gas clouds at a constant rate relative to the growth of nearby stars.
Black holes are celestial objects with a gravitational field so strong that virtually nothing can escape them.
Astronomer Alister Graham said the gigantic dense bodies, with masses many millions of times that of our sun, were believed to comprise a small fraction - 0.2 per cent - of the surrounding galaxy's total mass.
''We thought if we knew how big the galaxy was, then we knew how big the black hole would be,'' said Professor Graham, from the Centre for Astrophysics and Supercomputing at Swinburne University of Technology.
''But everybody had those relationships wrong.''
With fellow astronomer Nicholas Scott, the pair found that black holes in massive galaxies were more than twice as big as previously thought, containing up to 0.5 per cent of the galaxy's total mass.
''Black holes have been growing much faster than we thought,'' he said.
Small galaxies contain black holes with a much smaller fraction of their galaxy's total mass, often as low as 0.2 per cent.
It is within even smaller galaxies that Professor Graham and Dr Scott believe medium-sized or ''intermediate'' black holes exist. These black holes are smaller than supermassive black holes but larger than those formed when a massive star collapses, known as stellar mass black holes.
In these smaller galaxies, the researchers found a greater proportion of their mass was consumed by a dense cluster of stars at the galaxy's core.
Most galaxies have either a massive black hole or a dense cluster of stars at their centre.
''The smaller the galaxy, the greater the fraction of stars in these dense, compact clusters,'' Dr Scott said. ''In [these] galaxies the star clusters really dominate over the black holes.''
The pair made their finding by observing 72 galaxies with supermassive black holes.
Professor Graham said the findings would likely cause a stir. ''There have been about 1000 astronomy papers assuming this constant ratio of 0.2 per cent for black holes,'' he said. ''Theories will be modified, models will be refined.''