XSEDE and NICS Help Researchers Solve a Four-decades-long Mystery
Brought to life by a gas-gobbling supermassive black hole, powerful jets drill their way out of the galaxy with only minor bends and wiggles. (Simulations by Alexander Tchekhovskoy of the University of California at Berkeley, and Omer Bromberg of Hebrew University)
Alexander Tchekhovskoy, NASA Einstein postdoctoral fellow, University of California at Berkeley
Growing up in the small town of Dolgoprudny, Moscow Region, Russia, Alexander “Sasha” Tchekhovskoy never made a conscious decision to be a scientist. It just happened naturally.
The middle school he attended had no lab equipment, but it did have an enthusiastic teacher who devised kitchen-table physics demonstrations using bottles, forks, and matches that ignited the imaginations of the young students.
“It was fun to learn how the world works,” remembers Tchekhovskoy, whose heightened curiosity became the catalyst for his future in science.
In high school he won physics Olympiads, which in turn encouraged him to apply to the Moscow Institute of Physics and Technology, where he would earn, with honors, bachelor’s and master’s degrees in applied physics and mathematics.
“When I was an undergraduate student, the eye candy of seeing a movie of a simulated black hole gobbling up gas won me over,” he says. “It was so real that in an instant it was clear to me that this was what I wanted to do as a graduate student. What could be better than starting every day peeking into other worlds simulated on a computer, right there on my desk?”
Today, with a Ph.D. in astronomy from Harvard also to his credit and the distinction of being a NASA Einstein postdoctoral fellow at the University of California at Berkeley (UC Berkeley), Tchekhovskoy uses a variety of tools — from pencil and paper to supercomputers — to explore phenomena on the frontiers of science such as black holes.
Cosmic objects so massive and gravitationally strong that nothing, including light, can escape their clutches, black holes not only consume the gas that surrounds them but also spew gaseous outflows known as astrophysical jets.
“These jets can affect galaxy formation and evolution by, for example, heating up the surroundings and suppressing star formation, expelling the surrounding gas and thereby reducing the mass supply to the black hole,” Tchekhovskoy explains.
This ambient influence of the jets, often referred to as “black hole feedback,” is a mystery that to be solved requires black hole and full-galaxy simulations.
“By better understanding how the black holes affect the lives of galaxies, we will improve the understanding of how galaxies evolve and where we came from,” says Tchekhovskoy.
Delving into a Dichotomy
A particular enigma emerged in 1974 when researchers Bernie Fanaroff of South Africa and Julia Riley of the U.K. noticed that astrophysical jets appear to come in two main shapes: type 1, or FRI jets, which are short and wiggly, and tend to fall apart, often inside their host galaxies; and type 2, or FRII jets, which are long and nearly straight, and deposit their energy in a hot spot far outside their host galaxies. This dichotomy has presented astronomers with a formidable problem to solve.
“Whereas it was rather easy to reproduce stable FRI jets using hydrodynamic simulations in the 1980s, it turned out to be an extreme challenge to explain what causes the FRI jets to fall apart,” says Tchekhovskoy. “Researchers have tried a variety of things and even resorted to inserting red giant stars into the jets to make them fall apart. Most of such works, however, ignored large-scale magnetic fields that are known to be present in jets.”
Using supercomputing resources provided by the National Science Foundation’s eXtreme Science and Engineering Discovery Environment (XSEDE) and the Savio computer at UC Berkeley, Tchekhovskoy and co-researcher Omer Bromberg — formerly a Lyman Spitzer Jr. postdoctoral fellow at Princeton University, and currently at the Hebrew University of Jerusalem in Israel — resolved the 40-year puzzle of the astrophysical jet dichotomy. Altogether the simulation effort took about 500 hours on 2,000 computer cores, which, for perspective, would amount to about 1 million hours on a standard laptop.
“In our work, we show that magnetic instabilities naturally lead to jet disruption at low powers characteristic of FRI jets but leave the jets stable at higher powers characteristic of FRII jets,” says Tchekhovskoy, who led the computational aspect of the project. Details of the research are provided in a paper recently published in the journal Monthly Notices Letters of the Royal Astronomical Society.
Tchekhovskoy and Bromberg’s simulations took on the daunting challenge of going from where the jets are initiated to where they deposit their energy, which means a transition from a very small scale to one that is a thousand times larger.
“Darter [managed by the National Institute for Computational Sciences (NICS)], with its short-job turnaround times, was up to the challenge,” says Tchekhovskoy. “And the NICS team was very helpful in setting up the visualization tools so we could follow the simulations in real time. We have used the Texas Advanced Computing Center’s [TACC’s] Stampede to run some of these simulations, but the majority of this work was carried out on Darter. We also used TACC’s Ranch for backing up the simulation results.”
He credits NICS and TACC as having provided excellent support any time system issues arose. And he says, “We have been able to successfully use the analysis tools both on Stampede and Darter. Globus Online is how we backed up the data to TACC’s Ranch and also copied it to our local system for analysis. Globus Online helped to take away the hassle associated with baby-sitting the data transfers and enabled us to simplify our life in this respect dramatically.”
More Questions to Ask
Tchekhovskoy says the work done in this project so far represents the first steps in simulating the morphology, or form and structure, of the black hole jets. “There is a great variety of jets in the wild, and we have simulated only the spherical cow [highly simplified] version of them,” he says. “We do not expect our study to reproduce all of the observed jet shapes. The goal of the study is to capture the limiting jet behavior: the transition from stable to unstable jets.”
One of the compromises the researchers had to make because of computational expense was to start the jets not at the black hole, but farther away, where they begin interacting with the ambient gas of the galaxy. In future work, the researchers plan to design simulations with more realistic conditions that consider the smaller effects of gravity, buoyancy, and thermal pressure.
NASA supported this research through Einstein Postdoctoral Fellowship grant number PF3-140115 awarded by the Chandra X-ray Center, operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8-03060, and an XSEDE computational time allocation with project number TG-AST100040.
The study of black holes and their associated components such as astrophysical jets could reveal even more about the properties of gravity, one of physics’ fundamental concepts. And beyond that, as Albert Einstein said, “The important thing is not to stop questioning. Curiosity has its own reason for existing.” Lovers of science like Tchekhovskoy certainy agree.
Scott Gibson, science writer/communications specialist, NICS, JICS
Article posting date: 29 June 2016
About JICS and NICS: Established by the University of Tennessee and Oak Ridge National Laboratory, the Joint Institute for Computational Sciences (JICS) is a conduit and a nexus for research collaborations and a provider of advanced computing resources. It also is an educator in cutting-edge computing focused on solving the most difficult problems in science and technology. JICS operates the National Institute for Computational Sciences (NICS), a leading academic supercomputing center and a major partner in the National Science Foundation's eXtreme Science and Engineering Discovery Environment (XSEDE).