Supermassive black holes are the most massive objects in the known universe. They live in the centers of most galaxies and can "weigh in" from millions to billions of times the mass of our sun. Our Milky Way has one, called Sagittarius A*, lurking in the galactic core some 20,000 light-years away with a mass of 4 million times the mass of our sun. Though we know these gravitational behemoths are out there, we don't fully understand how they grow so big and how their growth and activity is linked to the evolution of their host galaxies.
One thing we do know, however, is that if any object gets too close, it will be ripped to shreds and blended into a superheated gas called a plasma, like an extremely hot cosmic smoothie that's ready to be slurped down. This plasma forms an accretion disk that slowly feeds into the black hole's event horizon - the boundary surrounding a black hole where the gravitational warping of space-time becomes so great that even light cannot escape. As one would expect, these accretion disks are violent places, generating huge quantities of radiation. These powerful features are revealed in intense radio and X-rays and their presence is a signal that the central black hole is feeding.
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Though the physics of these accretion disks may seem pretty clear, there are many black holes that should be actively feeding , but do not produce intensely-radiating accretion disks. It's as if they are sneaking extra meals without letting the universe know about it. This includes Sagittarius A* (or Sgr A*), which poses a quandary. Though Sgr. A* has an accretion disk, astronomers call it "radiatively inefficient," meaning it's generating less radiation than one would expect.
"So the question is, why is this disk so quiescent?" said astrophysicist Matthew Kunz, of the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL), in a statement.
To work out what is going on with Sgr. A*'s accretion disk, Kunz's team focused on what they think is happening on the smallest of scales in the accretion disk. Though the disk is undoubtedly hot and filled with particles, their research suggests that this particular accretion disk is comparatively dilute, meaning the individual protons and electrons buzz around in the disk rarely hitting each other. This lack of interaction likely sets it apart from other more active accretion disks.
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The "classical" model of black hole accretion disks use established formulas from the 1990s that see the plasma as an electrically-conducting fluid with particles that, by a fluid's nature, strongly interact. But if you apply these formulas to our black hole's accretion disk, they don't produce the emissions as predicted by the classical model. This is a problem as if you apply this model to Sgr. A*, and assume the fluid is collisionless, the particles cannot spiral down into the event horizon and be consumed by the black hole. In short, if we apply our classical understanding of accretion disk science to our black hole, it would never be able to consume any of the matter held in its disk.
So, in new research published in the journal Physical Review Letters, the team modeled the motion of individual particles orbiting a black hole in a collisionless accretion disk in the hope of explaining the weak emissions that come from certain accretion disks. But to do this, complex code had to be written in an effort to "produce more predictive models of the emission from black-hole accretion at the galactic center for comparison with astrophysical observations," said Kunz.
Through the use of powerful computers, this new "kinetic" code appears to explain how a supermassive black hole, like ours, produces little radiation while still slurping down its dilute and weakly radiating accretion disk.
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