Hundreds of undiscovered black holes spotted in star cluster

Scientists have identified a globular cluster of stars that may host several hundred black holes, a phenomenon that until recently was thought impossible.

Globular clusters are spherical collections of stars which orbit around a galactic centre such as our Milky-way galaxy.

Using advanced computer simulations, researchers at the University of Surrey mapped a globular cluster known as NGC 6101, from which the existence of black holes within the system was deduced.

These black holes are a few times larger than the Sun, and form in the gravitational collapse of massive stars at the end of their lives.

It was previously thought that these black holes would almost all be expelled from their parent cluster due to the effects of supernova explosion, during the death of a star.

"Due to their nature, black holes are impossible to see with a telescope, because no photons can escape. In order to find them we look for their gravitational effect on their surroundings," said Miklos Peuten of the University of Surrey in the UK.

"Using observations and simulations we are able to spot the distinctive clues to their whereabouts and therefore effectively 'see' the un-seeable," said Peuten.

It is only as recently as 2013 that astrophysicists found individual black holes in globular clusters via rare phenomena in which a companion star donates material to the black hole.

The new study showed that in NGC 6101 there could be several hundred black holes, overturning old theories as to how black holes form.

"Our work is intended to help answer fundamental questions related to dynamics of stars and black holes, and the recently observed gravitational waves," Professor Mark Gieles, University of Surrey.

"These are emitted when two black holes merge, and if our interpretation is right, the cores of some globular clusters may be where black hole mergers take place," said Gieles.

The researchers chose to map this particular ancient globular cluster due to its recently found distinctive makeup, which suggested that it could be different to other clusters.

Compared to other globular clusters NGC 6101 appears dynamically young in contrast to the ages of the individual stars. Also the cluster appears inflated, with the core being under-populated by observable stars.

Using computer simulation, researchers recreated every individual star and black hole in the cluster and their behaviour.

Over the whole lifetime of thirteen billion years the simulation demonstrated how NGC 6101 has evolved.

It was possible to see the effects of large numbers of black holes on the visible stars, and to reproduce what was observed for NGC6101.

From this, the researchers showed that the unexplainable dynamical apparent youth is an effect of the large black hole population.

The study was published in the journal Monthly Notices of the Royal Astronomical Society.