By Paul Rincon
BBC News Online science staff
Astronomers have found support for the existence of elusive medium-sized black holes, the journal Nature reports.
The X-ray sources are from galaxy M82, 11 million light-years away
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.
Collisions between stars in the flattened disc of spiral galaxies are very unlikely, because the spaces in between the stars are so great.
Stellar collisions are more likely to occur in clusters of new stars, where the more massive stars sink in towards the cluster's core, collide and merge.
Simon Portegies Zwart, of the University of Amsterdam, Netherlands, and colleagues ran computer simulations using MGG 11 as their model.
They demonstrated that repeated collisions between stars could produce a so-called "runaway star" with increasing mass.
Once those stars exceed 260 solar masses, they collapsed to form black holes without a significant loss of their mass.
The researchers believe that this process may produce medium-sized black holes of between about 100 and 1,000 solar masses.
Some also believe it could provide a foundation for the formation of supermassive black holes.
"The result is tantalising - this could well be how the building blocks of supermassive black holes formed," says Nate McCrady, from the University of California at Berkeley, US.