Tens of Thousands of Black Holes May Be at Milky Way's Center

Alan Olson
April 7, 2018

This illustration provided by Columbia University shows the supermassive black hole Sagittarius A, located at the center of the Milky Way Galaxy, surrounded by a cloud of dust and gas within which are 12 smaller black holes, and a closeup of one of the systems.(AP).

For decades, scientists theorized that circling in the center of galaxies, including ours, were lots of stellar black holes, collapsed giant stars where the gravity is so strong even light doesn't get out. But for every one such system astronomers have spotted, they've also detected many more black holes that don't have companions.

Instead, his team scoured through archived data from the Chandra X-ray space telescope to identify the X-ray signatures of black hole binaries.

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*.

"Isolated black holes just don't do much of anything", says lead author Hailey. As many as 20,000 black holes are predicted to settle in the central area of our galaxy (and all spiral galaxies) but so far, these black holes haven't been observed, and neither has their gravitational effect, despite astronomers' best efforts - that is, until now. "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".

HAILEY: Because they're so heavy, they naturally sink or gravitate towards the supermassive black hole in the center.

The work may also help shed light on how x-ray binaries form and develop, Harrison says.

The halo of gas and dust around Sgr A is thought to provide the flawless breeding ground for massive stars which collapse into black holes when they die. Most of the black holes remain isolated, but a small minority become attached to the orbits of low mass stars in the region, forming a binary system. Alternative explanations posit these anomalously massive black holes grew and merged in throngs of stars called globular clusters, but that process can easily require more time than the current age of the universe. The findings appear in a paper published Wednesday in Nature.

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GREENFIELDBOYCE: But Hailey says no one had actually seen any sign of this.

More information: Charles J. Hailey et al, A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy, Nature (2018). It's a busy place.

GREENFIELDBOYCE: And it's been studied by NASA's premier X-ray telescope. These black holes eventually are believed to congregate around the centre.

However, detecting them is a major challenge. And because scientists know what fraction of black holes will bind with low mass stars, after they discovered the 12 they were able to calculate the population of isolated black holes around Sgr A*.

Astronomer Cristina Martínez-Lombilla and co-authors set out to establish whether Milky Way-like galaxies are really getting bigger, and if so what this means for our own Galaxy.

GREENFIELDBOYCE: Fiona Harrison is an astrophysicist at Caltech.

Black holes are generally "pretty impossible" to see, according to the physicist. So this is a real high concentration.

The next step, Sobral said, is to discover more about these distant galaxies, including the distribution of various elements and the formation of different types of stars within them.

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