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These clouds are hot enough to hang with black holes

A new study solves the mystery of clumpy clouds forming near black holes.

Amidst all the cosmic creatures that lurk in the shadows of the vast universe, perhaps none are as monstrous and unforgiving as the elusive black hole.

Nothing escapes the powerful grip of a black hole, not even light. This massive amount of matter is packed into a tight space, and has a gravitational tug that swallows up gas and dust from its surroundings.

However, massive cosmic clouds have been observed in the vicinity of black holes, unharmed. Until now scientists have been unsure of their origin, or how they got to be cool enough to hang with the black holes. But a new study may have just unlocked the secret behind this unlikely pairing.

The study, published in The Astrophysical Journal Letters, could help improve astronomers' observations of black holes, and their surrounding environment.

What are cosmic clouds? Clouds that exist in deep space are different from the ones we see in our daytime skies. Instead of being formed from water vapor, cosmic clouds are made up of dense, clumpy gas and can extend as far as the distance between Earth and the closest star to our Sun, Proxima Centauri, located more than 4 light years away.

An artist's depiction of a black hole located in the center of a galaxy, surrounded by clumpy clouds.Illustration by Nima Abkenar

Astronomers have been observing these massive clouds in the vicinity of even more massive black holes at the center of galaxies, but not really understanding how they came to be in the first place.

In order to help resolve that mystery, the team of researchers behind the new study created a computer model that simulates the outflow of gas from black holes.

As material such as gas and dust fall into the black hole, some gas will also flow outwards, away from the center of the galaxy. The area near the center of the galaxy tends to be highly energetic, and hot gas is created from all the particles moving around the black hole as it pulls them in with its gravity.

The clouds start off small, and then grow massively in size beyond 1 parsec, approximately 19 trillion miles, and move at a speed of around 20 million miles per hour.

However, this gas flow is not as smooth as scientists expect it to be. Instead, the gas forms clumpy clouds. If the gas flows fast enough, then it would not cool down enough to form clumps.

Daniel Proga, an astrophysicist at the University of Nevada, Las Vegas, and one of the authors behind the new study, compared it to cars waiting to get on a highway ramp. “Every now and then you have a bunch of cars,” he said in a statement.

In order to explain the clouds, the researchers behind the new study suggest that gas density around the shell of the gas is a little lower on the outer edges, which causes it to heat up faster than the gas further out. As a result, the effect is similar to the buoyancy that makes hot air balloons float with the heated air inside the balloon being lighter than the cooler air outside, and the difference in density makes the balloon rise, according to NASA.

"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 [active galactic nucleus],” Randall Dannen, a doctoral student at the University of Nevada, and lead author of the study, said in a statement. “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."

The researchers are going to further examine these cosmic clouds, hoping to find out why the clouds move so fast.

Abstract: One of the main mechanisms that could drive mass outflows on parsec scales in active galactic nuclei (AGN) is thermal driving. The same X-rays that ionize and heat the plasma are also expected to make it thermally unstable. Indeed, it has been proposed that the observed clumpiness in AGN winds is caused by thermal instability (TI). While many studies employing time-dependent numerical simulations of AGN outflows have included the necessary physics for TI, none have so far managed to produce clumpiness. Here we present the first such clumpy wind simulations in 1D and 2D, obtained by simulating parsec-scale outflows irradiated by an AGN. By combining an analysis of our extensive parameter survey with physical arguments, we show that the lack of clumps in previous numerical models can be attributed to the following three effects: (i) insufficient radiative heating or other physical processes that prevent the outflowing gas from entering the TI zone; (ii) the stabilizing effect of stretching (due to rapid radial acceleration) in cases where the gas enters the TI zone; and (iii) a flow speed effect: in circumstances where stretching is inefficient, the flow can still be so fast that it passes through the TI zone too quickly for perturbations to grow. In addition to these considerations, we also find that a necessary condition to trigger TI in an outflow is for the pressure ionization parameter to decrease along a streamline once gas enters a TI zone.
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