How Many?

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Artist’s concept of black hole (stock illustration) © nasa_gallery 
Since nobody is sure what ‘dark matter’ is, one other candidate for the glue that holds galaxies together is super massive black holes. 

A black hole is a region of spacetime exhibiting such strong gravitational effects that nothing can escape from inside it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. Quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. That means that they are essentially the ambient temperature of space.

A Columbia University-led team of astrophysicists has discovered a dozen black holes gathered around Sagittarius A* (Sgr A*), the supermassive black hole in the center of the Milky Way Galaxy. The finding is the first to support a decades-old prediction, opening up myriad opportunities to better understand the universe.

“Everything you’d ever want to learn about the way big black holes interact with little black holes, you can learn by studying this distribution,” said Columbia Astrophysicist Chuck Hailey, co-director of the Columbia Astrophysics Lab and lead author on the study. “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 study appears in the April 5 issue of Nature.

Charles J. Hailey, Kaya Mori, Franz E. Bauer, Michael E. Berkowitz, Jaesub Hong, Benjamin J. Hord. A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy. Nature, 2018; 556 (7699): 70 DOI: 10.1038/nature25029

For more than two decades, researchers have searched unsuccessfully for evidence to support a theory that thousands of black holes surround supermassive black holes (SMBHs) at the center of large galaxies.

“There are only about five dozen known black holes in the entire galaxy — 100,000 light years wide — and 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,” Hailey said, adding that extensive fruitless searches have been made for black holes around Sgr A*, the closest SMBH to Earth and therefore the easiest to study. “There hasn’t been much credible evidence.”

He explained that Sgr A* is surrounded by a halo of gas and dust that provides the perfect breeding ground for the birth of massive stars, which live, die and could turn into black holes there. Additionally, black holes from outside the halo are believed to fall under the influence of the SMBH as they lose their energy, causing them to be pulled into the vicinity of the SMBH, where they are held captive by its force.
While most of the trapped black holes remain isolated, some capture and bind to a passing star, forming a stellar binary. Researchers believe there is a heavy concentration of these isolated and mated black holes in the Galactic Center, forming a density cusp which gets more crowded as distance to the SMBH decreases.
In the past, failed attempts to find evidence of such a cusp have focused on looking for the bright burst of X-ray glow that sometimes occurs in black hole binaries
“It’s an obvious way to want to look for black holes,” Hailey said, “but the Galactic Center is so far away from Earth that those bursts are only strong and bright enough to see about once every 100 to 1,000 years.” To detect black hole binaries then, Hailey and his colleagues realized they would need to look for the fainter, but steadier X-rays emitted when the binaries are in an inactive state.

“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 said. “Isolated, unmated black holes are just black — they don’t do anything. 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.”

Hailey and colleagues turned to archival data from the Chandra X-ray Observatory to test their technique. They searched for X-ray signatures of black hole-low mass binaries in their inactive state and were able to find 12 within three light years, of Sgr A*. The researchers then analyzed the properties and spatial distribution of the identified binary systems and extrapolated from their observations that there must be anywhere from 300 to 500 black hole-low mass binaries and about 10,000 isolated black holes in the area surrounding Sgr A*.

“This finding confirms a major theory and the implications are many,” Hailey said. “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.”

10 thoughts on “How Many?

  1. "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."
    I highly recommend this video/book that points out how earth seems designed to be an observation platform.

  2. I watched the video you recommended. Good video. The volume of information that we have been able to amass about the Earth, the Solar System, the Galaxy and the really large structures in the universe is so vast — and the information keeps coming — that it's tough to keep up. The complex system that is Earth/Moon and the vital interaction that creates tidal action which keeps the core molten and keeps radiation belts in place to allow life to exist long-term alone is elegant in the extreme. It requires a pause and a silent moment of thanks (How great Thou art).

    And then we go back to look at the mess that we've made of things, the cruelty, the mindless indifference to suffering and on – and on. (sigh)

  3. I know, this is an astronomy post, so I'll resist the temptation to draw any parallels with defeated presidential candidates or life inside the Beltway. For example, references to SMBHs, Hillary, Huma and Yoko wouldn't be appropriate on a family science blog.

    To the stars.

  4. I simply found the revelation that there are tens of thousands of black holes orbiting Sagittarius A* (the super massive black hole at the center of the galaxy) an interesting one. Is Yoko Ono giving Hillary physical comfort the way she once did? The mere thought of those two coupling is a dietary aid.

  5. "When I consider thy heavens, the work of thy fingers, the moon and the stars which thou hast ordained; what is man that thou art mindful of him? and the son of man, that thou visitest him." –King David Psalm 8:3, 4.

  6. I think that the real thing is far more dramatic – a destroyer of worlds – but who knows "what's on the other side" of a black hole, if anything?

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