A human lifespan is only a tiny moment compared to the history of the universe-but with the help of a $900,000 computer, U of T astronomers at the Canadian Institute for Theoretical Astrophysics (CITA) can watch things evolve on cosmic time-scales right before their eyes.
CITA’s McKenzie cluster computer-a network of 512 processors working in tandem-was used early this year to run simulations of interstellar gas movement and galaxy collisions.
Astronomy professors Chris Matzner and Ue-Li Pen, along with a graduate student, simulated gas movement surrounding a black hole. Black holes are objects with such intense gravitational fields that even light can’t escape them. They are often surrounded by clouds of gas, including the one likely to be at the center of our galaxy.
The researchers wanted to shed light on an outstanding problem: Why are these gas clouds so faint when their proximity to black holes should make them hot and bright?
“Since [gas] is falling toward the black hole, you can predict that the gas should heat up and radiate part of that heat,” said Matzner. The problem is if you look at the X-rays, you don’t see anything close to the predicted amount of radiation.
It’s thought in one convection-like model that as colder, heavier gas is heated when it approaches the black hole, the resulting pressure forces most of the gas to shoot off before radiating light. This explains the low energy levels, but satellite X-rays don’t show much escaping gas.
Because magnetic fields are almost always a factor in gas movement within galaxies, says Matzner, the team postulated it was preventing the gas from breaking free. The researchers decided to test the convection model but with magnetic fields as an added parameter.
The simulation modeled a cube of space with a black hole at its center. After a week of running the simulation, results looked promising. When the researchers included a magnetic field in the simulations, the gas was confined near the black hole. When they turned the field off, the gas started to escape.
“Energy is going [into the black hole], but very slowly,” said Pen.
But there are concerns. The black hole was modeled to have no rotation and other simplifications were made-simplifications the researchers say is a source of worry. “It’s one of the things we need to check with further,” said Matzner.
McKenzie was also used to run galaxy collision experiments. John Dubinski, another U of T astronomy professor, wanted to better understand the processes involved.
“I’m trying to understand how the black hole in the two [colliding] galaxies affects the central structures of the galaxies,” he said.
The simulation depicts the colliding and merging of two spiral galaxies. The time between each still image is approximately 100 million years. The galaxies collide only to explode into a violent, yet beautiful whirling of gas and stars, eddies and spirals.
But beneath this austere beauty is an ambitious project. By embedding the currently understood cosmological parameters in these simulations, astrophysicists are testing the very parameters that governed the universe’s history.
Astronomers, of course, can’t watch collisions and star formations unfolding in the real universe because they happen so slowly. But they do have the scattered snapshots of the universe’s past.
“It’s like having a photo album of different stages of different people’s lives and we’re trying to reconstruct the life of a human being,” said Dubinski. “We’re trying to make a story.”
