Astronomers discover a clump of black holes around our galaxy's center

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And since most black holes can't even be spotted that way, they calculate that there are likely thousands of them there. Or so it was supposed until now because the accumulation of objects in the area confused astronomers trying to find out what happens in that key region of the galaxy.

For the first time, an worldwide team of astronomers led by astrophysicist Charles Hailey from Columbia University in New York, US, has found have found a dozen of these stellar mass black holes within 3.3 light-years (about 30 trillion kilometres) of Sagittarius A*.

Sifting through data accumulated by the Chandra X-ray Observatory over the past 12 years, they looked for the fainter, but steadier, X-ray signature produced by inactive X-ray binaries, where the black hole is paired with a low-mass star.

While black holes can form quite far from the centre of their galaxies, they gradually lose energy and start to migrate towards the galactic centre, Professor Hailey said, "much as sediment dumped in water will sink down under the influence of gravity". In addition, "the galactic center is very crowded so it is hard to differentiate systems in which a black hole is present from other more prosaic X-ray sources such as white dwarfs". This isn't a hugely effective way to spot black holes, because the distance between the Earth and the center of the galaxy mean that for smaller black holes, these X-rays are too small and weak to be discernible. These types of black holes are referred to as black hole binaries.

For some time, the theory has been that the supermassive black holes at the centre of most galaxies, including our own Milky Way, are accompanied by smaller black holes. There are stellar-mass black holes - the super-dense remnant left when a star more than eight times the mass of the Sun dies.

It is surrounded by a halo of gas and dust in which massive stars are born. "They would already have submerged their planets", Charles Hailey explained. But you can see evidence of a black hole's meal.

So how do you "see" a black hole?

Despite telescopes being trained on the galactic centre for more than a decade, astrophysicists have had no joy.

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It's also crowded with other X-ray-emitting objects, such as binary pairs that contain a white dwarf star. These black holes eventually are believed to congregate around the centre.

We really think we basically understand how things are happening at the centre of the galaxy. "Some of them were formed comparatively recently".

Hailey said the new findings could significantly advance research into one of the most exciting fields of astronomy right now: gravitational waves. Gravitational waves are ripples caused by the black holes colliding with each other.

"However, as our instruments get better it will be possible to untangle these uncertainties and gain a better understanding of the centre of our galaxy", says Harvey-Smith.

However at the moment a "density cusp", or localised increase in number, indicating this presence has not yet been detected.

"It would be so easy if black hole binaries routinely gave off big bursts like neutron star binaries do, but they don't, so we had to come up with another way to look for them", Hailey explained. And he hopes that other astronomers will use publicly available data to further seek out these somewhat elusive mysteries of the universe.

"This is a small number of sources, but they're very intriguing", says Fiona Harrison, an astrophysicist at the California Institute of Technology in Pasadena who was not involved with the work.

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