Tens of Thousands of Black Holes Exist At Our Galaxy's Center, Study Suggests

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The thought that an omnivorous supermassive black hole exists at the centre of our galaxy, is enough to make the bravest soul feel a little vulnerable. But now scientists have discovered that there are actually tens of thousands of black holes spinning at the heart of the Milky Way.

This image of the area around galactic centre is a composite of images taken in X-ray, optical, and infrared wavelengths

Scientists now think that all large galaxies have a supermassive black hole in the middle, and astronomers first picked up a signal from Sagittarius A*, the behemoth at the centre of our own galaxy, in 1931. For around 20 years, astrophysicists have predicted that thousands of smaller black holes should exist around Sagittarius A* because it is surrounded by enormous halos of gas and dust, which provide a perfect breeding ground for the birth and, ultimately the death, of stars.

As stars die, some collapse in on themselves forming new black holes. Supermassive black holes can also pull in black holes from elsewhere, but until now the search for them has proved fruitless. But now Columbia scientists have come up with a new method for detecting the smaller space voids.

Sagittarius A* sits at the centre of our Milky Way galaxy
Sagittarius A* sits at the centre of our Milky Way galaxy 
“There are supposed to be 10,000 to 20,000 of these things in a region just six light years wide that no one has been able to find,” said Dr Chuck Hailey, co-director of the Columbia University’s Astrophysics Lab and lead author on the study. “There hasn't been much credible evidence."

While most black holes remain solitary, some capture passing stars in their huge gravity fields, creating a binary system in a process dubbed ’mating’. Crucially, that binding action sets off a constant stream of x-rays busts, which can be detected on Earth.

“Isolated, unmated black holes are just black -- they don't do anything,” said Dr Hailey. “So looking for isolated black holes is not a smart way to find them either. But when black holes mate with a low mass star, the marriage emits X-ray bursts that are weaker, but consistent and detectable. 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.”
 

The team looked at archive data from the Chandra X-ray Observatory, Nasa’s flagship telescope, and found 12 within three light years of Sagittarius A* and after analysing the properties and spatial distribution of the binary systems and extrapolating their observations they concluded there must be anywhere from 300 to 500 binaries and about 10,000 isolated black holes in the area surrounding area.

The findings could also help scientists hunt for gravitational waves - the ripples in space-time predicted by Albert Einstein which occur when catastrophic events happen in space, such as two black holes colliding.

Thousands of stars swirl around Sagittarius A*, the black hole at the centre of our galaxy

“This confirms a major theory and the implications are many," added Dr Hailey. “It is going to significantly advance gravitational wave research because knowing the number of black holes in the center of a typical galaxy can help in better predicting how many gravitational wave events may be associated with them. All the information astrophysicists need is at the center of the galaxy. The Milky Way is really the only galaxy we have where we can study how supermassive black holes interact with little ones because we simply can't see their interactions in other galaxies. In a sense, this is the only laboratory we have to study this phenomenon.”

The research was published in the journal Nature.

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