By Paul Rincon
Science reporter, BBC News, Austin
A wandering black hole is not something we need to worry about
Hundreds of rogue black holes, each several thousand times more massive than the Sun, may be roaming the Milky Way galaxy, scientists believe.
Models suggest that when black holes join, the merged object can be kicked out of its surrounding star cluster.
But astronomers say the wandering black holes produced by this process pose no threat to the Earth.
Details appeared at the meeting of the American Astronomical Society (AAS) held in Austin, Texas.
Astronomers already had ample evidence that small black holes, less than 100 times the mass of our Sun, were produced when giant stars exploded in supernovae.
There is similar evidence that so-called supermassive black holes should lie at the centres of most galaxies, if not all. These have masses millions or billions of times greater than our Sun.
Theory predicts that a class of intermediate mass black holes should exist with a mass equivalent to a few thousand Suns.
These should be found in globular clusters - ancient, gravitationally bound groups of between 100,000 and one million stars. There are roughly 200 globular clusters known in the Milky Way alone.
Scientists think that black holes grow as small ones merge to form heavier objects.
Efforts to simulate how these mergers occur show that when two black holes that are rotating at different speeds, or are of different sizes, combine, the resultant object receives a big "kick", which sends it hurtling off at speeds as high as 4,000 km/s.
The kick is generated by the ripples in space-time that result from the mergers.
A team led by Kelly Holley-Bockelmann, from Vanderbilt University in Nashville, US, ran computer simulations to investigate how intermediate black holes grew when they combined with a number of small black holes - of the type formed when giant stars explode.
These stellar-sized black holes are common within globular clusters.
Using their most conservative assumptions, Dr Bockelmann and colleagues found that even if every globular cluster started out with an intermediate mass black hole, only about 30% retained them through successive mergers.
Using their least conservative assumptions, the researchers found that less than 2% of the globular clusters should contain intermediate mass black holes today.
This effectively means that there should be hundreds of intermediate-sized black holes wandering invisibly through the Milky Way.
But Dr Bockelmann stressed that there was no need to worry.
"The danger zone for a black hole is... about 100km. The most dangerous thing I could think of would be a rogue black hole passing near to the Oort Cloud, causing comets to fall in (on the Solar System)," said Dr Bockelmann.
"But the probability of that is one in ten quadrillion per year, so it's not going to happen."