07 April, 2018
Researchers led by Columbia University published a study in Nature that shows that there are around a dozen black holes surrounding Sagittarius A*, which is the supermassive black hole at the center of the Milky Way. But scientists had no proof that these exotic objects had actually gathered together in the center of the Milky Way. Astronomers now believe that most galaxies have massive black holes at their center to help keep everything together.
There are only about five dozen known black holes in the entire galaxy stretching across 100,000 light years wide.
Black holes are greedy.
By analyzing the properties and spatial distribution of these X-ray binaries, the researchers extrapolated that 300 to 500 X-ray binaries may lurk in the core of the Milky Way, and about 10,000 isolated black holes without companion stars may also lurk there.
"I think this is a really intriguing result", says Fiona Harrison, an astrophysicist at Caltech. These types of black holes are referred to as black hole binaries. These are really exotic objects. "Theory is a lot more interesting when you have observations to compare against". Since the 1970s theorists studying this process have predicted a galactic center swarming with thousands of black holes bounded by an outer "cusp" beyond which the black holes' numbers should plummet.
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Simulations have long suggested that many smaller black holes - those with masses around the same as our sun - also exist in the Milky Way's centre and the middles of other galaxies, though only a single one has ever been spotted.
But the scientists have admitted the estimates are much graver, with as many as 20,000 black hole out there in the vast depths of deep space.
The discovery can also help refine our ideas of the freakish, rare binary systems that produce gravitational waves. "If we could find black holes that are coupled with low mass stars and we know what fraction of black holes will mate with low mass stars, we could scientifically infer the population of isolated black holes out there".
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HAILEY: Because they're so heavy, they naturally sink or gravitate towards the supermassive black hole in the center.
But these X-ray objects are extremely hard to find.
GREENFIELDBOYCE: Now, black holes aren't the only things at the center of the galaxy.
HAILEY: It's a place that's filled with a huge amount of gas and dust, and it's jammed with a huge number of stars.
The new study has used Chandra data to investigate X-ray sources close to Sagittarius A*.
Charles Hailey from Columbia University in NY and colleagues used archival data from Nasa's Chandra X-ray telescope to come to their conclusions. They knew that isolated black holes are nearly impossible to detect because they're black. But when black holes mate with a low mass star, the marriage emits X-ray bursts that are weaker, but consistent and detectable. "So they kind of just collect there, it's sort of like a junkyard ... they can't escape the pull from the supermassive black hole so they just kind of sit there".
"But this is the tip of the iceberg", he told AFP.
A group of astrophysicists analysed a set of X-rays blasted from the centre of the galaxy in a hunt for black holes.
So how do you "see" a black hole?
These early galaxies are 30 times smaller than the Milky Way - which is about 100,000 light-years in diameter - and are very compact - roughly only 3,000 light-years across - and full of hot stars. So this is a real high concentration.