It’s somewhat of a myth that black holes pull everything inward. Much of the matter that falls toward the black hole is actually spit back out, thanks to powerful magnetic fields that can levitate charged particles from the black hole’s accretion disk and accelerate them.
This material has long been assumed to flow radially from the black hole’s vicinity, either via a piercing jet that is collimated by magnetic fields, or material lifted up by streams of radiation emitted by the hot disk. However, there has always been a certain paradox at the heart of this theory: if the environment immediately around black holes is adept at removing material from danger, how can supermassive black holes feed on enough matter to grow to their massive masses of millions or even billions of times mass of our sun?
Now, observations of the active galaxy ESO320-G030, located 120 million light-years away, may have just provided the answer. In principle, a spiral magnetic vortex is found to revolve around a supermassive black hole in a distant galaxycreating conditions enabling the Black hole to eat hungry.
Using Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, astronomers led by Mark Gorski of Northwestern University, USA, discovered hydrogen cyanide gas blown by magnetohydrodynamic outflows – in other words, magnetic winds. Hydrogen cyanide itself is not particularly important, but as it represents the rest of the molecular gas in the system, it acts as a proxy that ALMA can detect.
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“We wanted to measure the light from molecules carried by winds from the galactic core, hoping to track how the winds are shot out by a growing – or soon to be growing – supermassive black hole,” said Suzanne Aalto of Chalmers University of Technology in Sweden, who worked with Gorski on the survey, in a statement. “With the help of ALMA, we were able to probe the light behind thick layers of dust and gas.”
ALMA was able to detect a Doppler effect in the submillimeter radio emission from hydrogen cyanide, which allowed Gorski’s team to track the movement of the gas. They found that it was carried by a rotating magnetic wind, unlike the typical radial flows expected from active black holes. This has a big effect on how the black hole feeds.
“In our observations, we see clear evidence of a rotating wind that helps regulate the growth of the galaxy’s central black hole,” Gorski said.
As the matter—gas and dust—approaches the black hole, it first accumulates in a rotating accretion disk, which is entwined with magnetic fields that become more intense as they evolve. Typically, they are able to lift charged particles up from the disk and repel them into a focused, magnetically collimated jet. The disk also gets really hot, radiating millions of degrees, and this stream of radiation can also push matter out of the black hole.
Rotating magnetic winds, however, are different. “We can see how the winds form a spiral structure spiraling out from the center of the galaxy,” Aalto said.
Gorski and Aalto’s research paper describes the magnetic wind as “spectacular.” This is because while the rotational wind can lift charged particles from the disc, the wind also steals some of the disc’s angular momentum as it also rotates. This causes the rotation of the accretion disk to slow down, and since the matter is no longer moving as fast as it was in the disk, the black hole’s gravity is able to pull more of that matter across the event horizon. This allows the black hole to grow faster than it would otherwise, as more matter falls into it.
By allowing more matter to fall into the supermassive black hole, this spinning magnetic wind could be the key to unlocking how AGN — active galactic nuclei, which is a supermassive black hole in a feeding frenzy — turns on, prompting a galaxy to turn into a quasar in the most extreme cases.
“Now that we know what to look for, the next step is to understand how common this is,” Gorski said. “And if this is a stage that all galaxies with supermassive black holes go through, what happens to them afterwards? Far from all the questions about this process have been answered.”
The study was published in April in the journal Astronomy and astrophysics.
