Astronomers have found support for the existence of elusive medium-sized black holes, the journal Nature reports.
They provide the link between two other types of black hole: so-called stellar-mass black holes and the supermassive ones that reside in galaxy centres. Stellar-mass black holes are between two and 10 times the Sun's mass, while supermassive ones are between a million and a billion times the Sun's mass.
Simulations show medium-sized black holes can form in dense star clusters. This computer modelling fits with observations made by Nasa's Chandra X-ray space telescope of the galaxy M82.
One X-ray source is associated with a cluster of young stars called MGG 11. Its brightness corresponds to that expected of an intermediate-mass black hole of 300-900 solar masses.