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Shining a light on cosmic darkness

Speed read
  • Astrophysicist Debora Sijacki wins 2019 PRACE Ada Lovelace Award for HPC
  • Studying supermassive black holes may improve understanding of galaxy formation
  • High-resolution galaxy simulations reveal complex physics interactions on a vast scale

Carl Sagan once described the Earth as a “pale blue dot, a lonely speck in the great enveloping cosmic dark.”

The need to shine a light into that cosmic darkness has long inspired astronomers to investigate the wonders that lie beyond our lonely planet. For Debora Sijacki, a reader in astrophysics and cosmology at the University of Cambridge, her curiosity takes the form of simulating galaxies in order to understand their origins.

<strong>A supermassive black hole</strong> at the center of a young, star-rich galaxy. SMBHs distort space and light around them, as illustrated by the warped stars behind the black hole. Courtesy NASA/JPL-Caltech. “We human beings are a part of our Universe and we ultimately want to understand where we came from,” says Sijacki. “We want to know what is this bigger picture that we are taking part in.”

Sijacki is the winner of the 2019 PRACE Ada Lovelace Award for HPC for outstanding contributions to and impact on high-performance computing (HPC). Initiated in 2016, the award recognizes female scientists working in Europe who have an outstanding impact on HPC research and who provide a role model for other women.

Specifically, Sijacki wants to understand the role supermassive black holes (SMBH) play in galaxy formation. These astronomical objects are so immense that they contain mass on the order of hundreds of thousands to even billions of times the mass of the Sun.  At the same time they are so compact that, if the Earth were a black hole, it would fit inside a penny.

SMBHs are at the center of many massive galaxies—there’s even one at the center of our own galaxy, The Milky Way. Astronomers theorize that these SMBHs are important not just in their own right but because they affect the properties of the galaxies themselves.

“What we think happens is that when gas accretes very efficiently and draws close to the SMBH it eventually falls into the SMBH,” says Sijacki. “The SMBH then grows in mass, but at the same time this accretion process is related to an enormous release of energy that can actually change the properties of galaxies themselves.”

A big universe needs big computing

To investigate the interplay of these astronomical phenomena, Sijacki and her team create simulations where they can zoom into details of SMBHs while at the same time viewing a large patch of the Universe. This allows them to focus on the physics of how black holes influence galaxies and even larger environments.

<strong>Dark matter density</strong> (l) transitioning to gas density (r). Large-scale projection through the Illustris volume at z=0, centered on the most massive galaxy cluster of the Illustris cosmological simulation. Courtesy Illustris Simulation.But in order to study something as big as the Universe, you need a big computer. Or several. As a Hubble Fellow at Harvard University, Sijacki accessed HPC resources through XSEDE in the US and PRACE in Europe. She now uses the UK’s National Computing Service DiRAC in combination with PRACE.

According to Sijacki, in the 70s, 80s, and 90s, astrophysicists laid the foundations of galaxy formation and developed some of the key ideas that still guide our understanding. But it was soon recognized that these theories needed to be refined—or even refuted.

“There is only so much we can do with the pen-and-paper approach,” says Sijacki. “The equations we are working on are very complex and we have to solve them numerically. And it’s not just a single physical process, but many different mechanisms that we want to explain. Often when you put different bits of complex physics together, you can’t easily predict the outcome.”

The other motivation for high-performance computing is the need for higher resolution models. This is because the physics in the real Universe occurs on a vast range of scales.

“We’re talking about billions and trillions of resolution elements,” says Sijacki. “It requires massive parallel calculations on thousands of cores to evolve this really complex system with many resolution elements.”

In recent years, high-performance computing resources have become more powerful and more widely available. New architectures and novel algorithms promise even greater efficiency and optimized parallelization.

<strong>Jet feedback from active galactic nuclei.</strong> (A) Large-scale image of the gas density centered on a massive galaxy cluster. (B) High-velocity jet launched by the central supermassive black hole. (C) Cold disk-like structure around the SMBH from which black hole is accreting. (D) 2D Voronoi mesh reconstruction and (E) velocity streamline map of a section of the jet, illustrating massive increase in spatial resolution achieved by this simulation. Courtesy Bourne, Sijacki, and Puchwein. Given these advances, Sijacki projects a near-future where astrophysicists can, for the first time, perform simulations that can consistently track individual stars in a given galaxy and follow that galaxy within a cosmological framework.

“Full predictive models of the evolution of our Universe is our ultimate goal,” says Sijacki. “We would like to have a theory that is completely predictive, free of ill-constrained parameters, where we can theoretically understand how the Universe was built and how the structures in the Universe came about. This is our guiding star.”

Awards matter

When asked about the significance of the award, Sijacki says that she is proud to have her research recognized—and to be associated with the name of Ada Lovelace. 

Perhaps more importantly, the award has already had an immediate effect on the female PhD students and post-docs at Cambridge’s Institute of Astronomy. Sijacki says the recognition motivates the younger generations of female scientists, by showing them that this is a possible career path that leads to success and recognition.

“I have seen how my winning this award makes them more enthusiastic—and more ambitious,” says Sijacki. “I was really happy to see that.”

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