Scientists find concentration of black holes 'only' 7,800 light years away

A supermassive black hole with millions to billions times the mass of our sun is seen in an undated NASA artist's concept illustration.
(photo credit: REUTERS/NASA/JPL-CALTECH/HANDOUT) 
Mergers of these black holes could be a source of gravitational waves.
Astronomers using the Hubble telescope made an unexpected discovery when they found a concentration of black holes in the heart of the NGC 6397 globular cluster, NASA reported.
The cluster, which is a dense stellar system with stars packed closely together, is one of the closest of its kind to Earth, at only 7,800 light years away. Like most globular clusters, NGC 6397 is very old, and is close to the age of the universe itself.
The core of NGC 6397 is very dense, and astronomers had thought an intermediate-mass black hole (between the stellar-mass black holes the size of a star formed when a star collapses and a supermassive black hole millions of times larger than that) would lie in the center. That alone would be significant, as the existence of these intermediate-mass black holes is itself a matter of debate.
But according to Eduardo Vitral, of the Paris Institute of Astrophysics (IAP), this did not add up to what they observed.
"We found very strong evidence for an invisible mass in the dense core of the globular cluster, but we were surprised to find that this extra mass is not 'point-like' (that would be expected for a solitary massive black hole) but extended to a few percent of the size of the cluster," he explained, according to NASA.
Using the velocity of nearby stars to find how the mass is distributed through the Hubble telescope and the European Space Agency's Gaia space observatory, Vitral and fellow IAP researcher Gary Mamon found that remnants of several collapsed stars were likely the source of the extra mass, meaning it was a concentration of multiple black holes.
Other recent studies had proposed similar ideas, but this study, the findings of which were published in the academic journal Astronomy & Astrophysics, was the first to provide "both the mass and the extent of... a collection of mostly black holes in the center of a core-collapsed globular cluster," Vitral explained.
The findings have significant implications, as the mergers of these black holes in globular clusters may be a source of gravitational waves.