clouds around supermassive black hole
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Credit: NASA

Why are black holes surrounded by a haze of eerie clouds, and do they eat them?

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May 23, 2020, 2:49 PM EDT (Updated)

Whatever you thought you knew about clouds as those white fluffy things in the sky is about to be destroyed.

Supermassive black holes are surrounded by strange clouds. These clouds are actually outflows of gas that somehow escape the gaping maw of a black hole and end up clumping around it, which is more proof that even these cosmic monsters don’t devour everything in sight. Some material that ventures closer and closer to the dreaded event horizon ends up getting flung far out instead of vanishing forever (though there are scientists who may question “forever”). UNLV doctoral student Richard Dannen and his research team have now shed light on why these clumps occur deep in the vacuum of space.

"This work is important because astronomers have always needed to place clouds at a given location and velocity to fit the observations we see from [supermassive black holes],” Dannen said (via UNLV's news center). “They were not often concerned with the specifics of how the clouds formed in the first place, and our work offers a potential explanation for the formation of these clouds."

Dannen, whose team recently published a study in Astrophysical Journal Letters, created a computer model that demystifies how these clouds supposedly form. Heat gets really intense near a supermassive black hole, which allows for superfast plasma outflows that still aren’t too fast to form clumps. Gas that escapes the event horizon too fast will not have the time to cool into space clouds. The model was designed to look into the outer shell of the clumps, which takes thousands of years to accumulate. Turns out that there is a disturbance near that shell that lowers gas density and lets the gas heat up and lifts out colder gas further away.

Credit: NASA

“One of the main mechanisms that could drive mass outflows on parsec scales in active galactic nuclei (AGN) is thermal driving,” Dannen explained in the study. “The same X-rays that ionize and heat the plasma are also expected to make it thermally unstable.”

These outflows were previously expected to be smooth, but have now been found to form in clumps, some of which are more than 1 parsec (3.3 light years) wide. The team’s research, supported by NASA, involved creating digital versions of clumpy cosmic wind by simulating outgoing gas that has been exposed to the black hole’s extreme radiation. They were able to prove that previous studies couldn’t confirm the existence of these clumps for several reasons. Sometimes there wasn’t enough radiation for plasma to enter the zone where it would become thermally unstable. Other models showed that gas would accelerate at a speed that allowed it to stretch and stabilize once it was in that zone, or that it would zoom through space too fast for clumps to form.

Lurking in the center of every galaxy is a supermassive black hole. Those that feed ravenously on the gas and dust around them are also known as active galactic nuclei, extremely dense objects at least 100,000 times more massive than the Sun. Quasars are the brightest type of active galactic nuclei, flashy enough to overpower all the stars in their galaxies while releasing more energy than that of a hundred average galaxies. Sometimes these beasts end up surrounded by more material than they could possibly eat. While the event horizon is the point of no return, it is so energetic that when fast-moving particles around it produce high-energy radiation, it can fling out plasma up to several light years from the black hole they evaded.

There are still more questions floating in the cosmos. What happens beyond the gaseous shell? Does that gas escape from the black hole’s accretion disc? Why do some of these clouds take off at 20 million miles an hour? That should keep you awake at night.


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