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3D Simulations Show How Galactic Centers Cool Off

15 August 2017

The most extreme outbursts in the universe are the mysterious jets of energy and matter beaming from the center of galaxies at close to the speed of light. The narrow jets typically form in opposing pairs, and they are associated with supermassive black holes and other exotic objects. The mechanisms that drive and dissipate the jets are not understood.

A team of researchers has developed theories supported by 3D simulations to explain what’s at work.

"These jets are notoriously hard to explain," said Alexander "Sasha" Tchekhovskoy, a former NASA Einstein fellow who co-led the new study as a member of the Nuclear Science Division at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), and the Astronomy and Physics departments and Theoretical Astrophysics Center at UC Berkeley. "Why are they so stable in some galaxies and in others they just fall apart?"

This rendering illustrates magnetic kink instability in simulated jets beaming from a galaxy's center. The jets are believed to be associated with supermassive black holes. The magnetic field line (white) in each jet is twisted as the central object (black hole) rotates. As the jets contact higher-density matter the magnetic fields build up and become unstable. The irregular bends and asymmetries of the magnetic field lines are symptomatic of kink instability. The instability dissipates the magnetic fields into heat with the change in density, leading them to become less tightly wound. (Berkeley Lab, Purdue University, NASA)This rendering illustrates magnetic kink instability in simulated jets beaming from a galaxy's center. The jets are believed to be associated with supermassive black holes. The magnetic field line (white) in each jet is twisted as the central object (black hole) rotates. As the jets contact higher-density matter the magnetic fields build up and become unstable. The irregular bends and asymmetries of the magnetic field lines are symptomatic of kink instability. The instability dissipates the magnetic fields into heat with the change in density, leading them to become less tightly wound. (Berkeley Lab, Purdue University, NASA)

Almost half the jets’ energy escapes in the form of X-rays and stronger forms of radiation. The researchers showed how two different mechanisms that are both related to the jets’ interaction with surrounding matter, also known as “ambient medium” and serve to reduce half of the energy of the powerful jets.

"The exciting part of this research is that we are now coming to understand the full range of dissipation mechanisms that are working in the jet no matter the size or type of jet,” he said.

Tchekhovskoy co-led the study with Purdue University scientists Rodolfo Barniol Duran and Dimitrios Giannios. They concluded that the ambient medium itself has a lot to do with how the jets release energy.

"We were finally able to simulate jets that start from the black hole and propagate to very large distances—where they bump into the ambient medium," said Duran.

Tchekhovskoy has studied these jets for over a decade. He said that an effect known as magnetic kink instability causes a bend in the direction of some jets. This along with another effect that triggers a series of shocks within other jets appear to be the primary mechanisms for energy release. The density of ambient medium that the jets encounter serves as the key trigger for the types of the release mechanism.

"For a long time, we have speculated that shocks and instabilities trigger the spectacular light displays from jets. Now these ideas and models can be cast on a much firmer theoretical ground," said Giannios, assistant professor of physics and astronomy at Purdue.

The length and intensity of the jets can illuminate the properties of their associated black holes like their age and size and if they are actively “feeding” on surrounding matter. The longest jets extend for millions of light years into space.

"When we look at black holes, the first things we notice are the central streaks of these jets. You can make images of these streaks and measure their lengths, widths and speeds to get information from the very center of the black hole," Tchekhovskoy noted. "Black holes tend to eat in binges of tens and hundreds of millions of years. These jets are like the 'burps' of black holes—they are determined by the black holes' diet and frequency of feeding."

Nothing can escape a black hole’s interior, but jets manage to draw their energy from the black hole. In the black holes, the laws of physics allow them to spew energy and matter even when they suck in surrounding matter.

The friction and heating of gases spiraling in toward the black hole cause extreme temperatures and compression in magnetic fields. This results in energetic backlash and an outflow of radiation that escapes the black hole’s pull.

Earlier studies have shown how magnetic instabilities in the jets can occur when jets run into the ambient medium. A jet experiencing the instability can change direction when it rams into matter outside the black hole’s reach.

The same instability frustrated scientists working on early machines attempting to create and harness a superhot, charged state of matter known as plasma in efforts to develop fusion energy that powers the sun. The space jets, also known as active galactic nuclei (AGN) jets, are a form of plasma.

The latest study found that if an earlier jet had “pre-drilled” a hole in the ambient medium surrounding a black hole and the matter impacted by the newly formed jet was less dense, a different process is at work in the form of “recollimation” shocks.

These shocks, formed as matter and energy in the jet, bounce off the sides of the hole. The jet loses energy with every shock and immediately reforms a narrow column until its energy dissipates to the point that the beam loses its tight focus and spills out into a broad area.

"With these shocks, the jet is like a phoenix. It comes out of the shock every time," though with gradually lessening energy, Tchekhovskoy said. "This train of shocks cumulatively can dissipate quite a substantial amount of the total energy."

The researchers designed the models to smash against different densities of matter in the ambient medium to create instabilities in the jet that mimic astrophysical observations.

New, higher-resolution images of areas in space where supermassive black holes are thought to exist—from the Event Horizon Telescope (EHT), for example—should help inform and improve models and theories that explain jet behavior. Future studies could also include more complexity in the jet models, like a longer sequence of shocks.

"It would be really interesting to include gravity into these models, and to see the dynamics of buoyant cavities that the jet fills up with hot magnetized plasma as it drills a hole in the ambient medium," Tchekhovskoy said. "Seeing deeper into where the jets come from—we think the jets start at the black hole's event horizon (a point of no return for matter entering the black hole)—would be really helpful to see in nature these 'bounces' in repeating shocks, for example. The EHT could resolve this structure and provide a nice test of our work."

A paper on this study was published in Monthly Notices of the Royal Astronomical Society.

To contact the author of this article, email Siobhan.Treacy@ieeeglobalspec.com


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