The illuminated compact region shining from the center of an active galaxy is known as an active galactic nucleus (AGN). Active galactic nuclei can produce jets of plasma thousands of light-years long. Consider popular images of galaxies speared by intense light. Many astronomers believe that this explosive energy output is powered by supermassive black holes in the center of the AGN’s host galaxy.
Active galaxies and active galactic nuclei are particularly intriguing to physicists because they emit more energy than would be expected. Researchers would like to study the fluid-like mechanics of these cosmic conundrums, but their efforts are stymied by the immense distance. Millions or billions of light-years separate scientists on Earth from the plasma jets, and viewing individual electromagnetic particles through a telescope is impossible.
“Understanding these plasma jets can help explain what is happening to the matter in these objects — how it is accelerated to such high energies and other fundamental physics out of our reach,” said Michael Bussmann, HZDR–Dresden Computational Radiation Physics group leader.
Since direct observation is not possible, a team from Germany’s HZDR–Dresden aimed to reproduce the process computationally. They hoped that a better understanding of plasma jet dynamics would reveal information about the source of the emissions, the active galactic nuclei themselves. The scientists used the number-one ranking US supercomputer, Titan, located at Oak Ridge National Laboratory, to simulate billions of particles in two passing jet streams.
The work earned the team a finalist nomination for the Association of Computing Machinery’s 2013 Gordon Bell Prize. The prize, which recognizes outstanding achievements in HPC, is presented by the Association for Computing Machinery each year in conjunction with the SC Conference series. This year’s winner will be announced at SC13, held November 17–22 in Denver, Colorado.
According to an article on the OLCF website, the research relied on a property of plasma turbulence known as Kelvin-Helmholtz instability (KHI), which occurs where passing plasma streams collide. The process of discovery unfolds by comparison. When two streams pass, KHI reveals information about their comparative density, velocity, direction and so forth. In this way, the scientists were able to discern patterns of particle behavior taking place inside these distant objects.
The plasma jet’s radiative signatures provided additional clues as to the plasma dynamics. While the jet’s particles cannot be viewed from Earth, the radiation can be observed through telescopes.
Ultimately, the scientists wished to know if it was possible to correlate the radiative signature with individual particles. As Bussmann asks: “Is there a chance to really see what’s happening inside the plasma just by looking at the radiation? We are very limited in our tools to connect plasma dynamics to what we observe, and this is where simulation comes in.”
The KHI simulations revealed structures in the turbulence, like mushrooms or whirlpools. Without the degree of resolution enabled by Titan, such patterns would never have come to light. The simulation on Titan was 46 times larger with a spatial resolution 4.2 times higher than any other kinetic KHI simulations previously performed.
The bulk of the processing power was attributed to Titan’s GPU accelerators. Both the plasma dynamics and emitted radiation computation was done on the GPUs.
With the data generated by Titan, researchers now have a map of radiative signatures that they can begin applying to actual plasma jets.
“We know every spectrum and every direction of the radiation from the Titan simulations, and we can use this information to map the radiative signatures to different objects,” Bussmann said. “By extension, we can use it as an input to predict the dynamics for different plasma jets we observe from Earth.”