Australasian Science: Australia's authority on science since 1938

Black Holes’ Unexpected Ratio

By Stephen Luntz

The relationship between the size of a black hole and the size of the galaxy in which it is located has been recast by research published in The Astrophysical Journal.

Astronomers observed a linear relationship in the masses of galaxies, black holes and the central star clusters in which the black holes exist, concluding that black holes represent 0.2% of a galaxy’s mass.

However, Prof Alister Graham of Swinburne University’s Centre for Astrophysics and Supercomputing says this figure was based largely on observations of the largest and most luminous galaxies. For these there is indeed a linear relationship, although he estimates that black holes represent 0.5% of the mass. “Previous figures were dragged down because they included a small number of smaller galaxies,” Graham says.

At smaller sizes the relationship is quadratic rather than linear, so that a galaxy twice the mass will have a black hole four times heavier. Consequently, the black holes in smaller galaxies are many times smaller than previously realised.

Graham says the work may help solve the lack of intermediate mass black holes. While Swinburne researchers recently produced evidence for the first object fitting into this class (AS, October 2012, p.8), the shortage of such entities remains something of a mystery.

However, Graham suggests that many small galaxies should have black holes of less than 100,000 solar masses at their core. “These may be big enough to be seen by the new generation of extremely large telescopes,” says his Swinburne co-author, Dr Nicholas Scott.

Graham says the transition in growth rates seems to occur when the hole has 100 million solar masses. Up to this point larger galaxies collect more gas, of which the black hole can take a more than proportional share. Above this size, Graham says, “galaxies grow by merging, and if you have double the size of the galaxy you also double the size of the black hole”.

Not enough is known about the size, or even existence, of black holes in globular clusters to establish whether the relationship continues down to this scale. However, Graham says that “at the centre of very low mass galaxies are nuclear star clusters that can contain up to 10 million stars and are very similar to globulars. We know black holes exist there, and the same relationship seems to apply.”