Computer simulations of a globular cluster called NGC 6101 reveal that it contains several hundreds of stellar-mass black holes, until now thought impossible.
This Hubble image shows the globular cluster NGC 6101. Globular clusters are spherical collections of old stars (10-13 billion years old) which orbit around a galactic center. Image credit: NASA / ESA.
NGC 6101 is a globular star cluster in the constellation Apus. It is located at a distance of about 47,600 light-years from the Sun and about 36,500 light-years from the Galactic center.
Also known as Dun 68, GCL 40 and ESO 69-SC4, this grouping of stars was discovered on June 1, 1826 by the Scottish astronomer James Dunlop.
Using advanced computer simulations, astronomers at the University of Surrey, UK, were able to see the un-seeable by mapping 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.
“Due to their nature, black holes are impossible to see with a telescope, because no photons can escape”, said Dr. Miklos Peuten, a researcher in the Department of Physics at the University of Surrey and the lead author on a paper published in the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint).
“In order to find them we look for their gravitational effect on their surroundings. Using observations and simulations we are able to spot the distinctive clues to their whereabouts and therefore effectively ‘see’ the un-seeable.”
In 2013, astronomers found individual black holes in globular clusters via rare phenomena in which a companion star donates material to the black hole.
The current study has shown 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,” said co-author Prof. Mark Gieles.
“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.”
The scientists chose to map NGC 6101 due to its recently found distinctive makeup, which suggested that it could be different to other clusters.
Compared to other globular clusters this one appears dynamically young in contrast to the ages of the individual stars.
Also NGC 6101 appears inflated, with the core being under-populated by observable stars.
Using computer simulation, the astronomers recreated every individual star and black hole in the cluster and their behavior.
Over the whole lifetime of 13 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 team showed that the unexplainable dynamical apparent youth is an effect of the large black hole population.
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M. Peuten et al. 2016. A stellar-mass black hole population in the globular cluster NGC 6101? MNRAS 462 (3): 2333-2342; doi: 10.1093/mnras/stw1726
