At the center of the Milky Way, the galaxy our planet is in, resides a supermassive black hole (SMBH) called Sagittarius A* (pronounced A-star). While this might fill you with existential dread, Sagittarius A* (Sgr A*) is a surprisingly quiet black hole, almost dormant, so it won’t be consuming our solar system any time soon. Researchers at the University of Illinois Urbana-Champaign (U. of I.) have been using their U.S. National Science Foundation ACCESS allocation with the National Center for Supercomputing Applications (NCSA) to run simulations of Sgr A* that would be nearly impossible to complete without the assistance of high-performance computing resources like Delta and DeltaAI.
Vedant Dhruv is a graduate student fellow at U. of I. working with data and images from the Event Horizon Telescope (EHT), an array that links eight telescopes around the world to create one powerful virtual telescope. One of the major successes of the EHT was capturing the first image of Sgr A*, a black hole four million times larger than our sun.
Dhruv worked with his advisor and other collaborators on this project, and they published their findings in The Astrophysical Journal Letters. He’s been able to gain a number of insights from using this data to simulate Sgr A*.
“In recent years, the EHT has produced high-resolution images of two supermassive black holes: Sgr A* at the center of the Milky Way and the black hole in Messier 87 (M87*), a giant elliptical galaxy in the Virgo constellation,” explained Dhruv. “These images provide insight into the plasma accreting onto the black hole.”
One of the images that Dhruv mentions might be one you’ve already seen. When M87* was first captured in an image, it made global news, especially because it was the first image of a black hole. Sgr A* took longer to get a clear image, but scientists have theorized for a while that it was there, even capturing a time-lapse of stars orbiting Sgr A* at the center of the Milky Way.
It’s impossible to take direct physical measurements of something as far away as the center of our galaxy, but fluid simulations of black hole accretion on machines like NCSA’s Delta can bring the sky down to the earth for researchers to study.
“The first horizon-scale image of Sgr A* revealed a bright ring surrounding a central brightness depression, the black hole ‘shadow.’ Comparison with theoretical models suggests that these observations are consistent with a hot, dilute plasma accreting onto a spinning black hole with a mass of roughly four million times the mass of the sun,” said Dhruv.
These theoretical models had to make some assumptions because designing and simulating highly realistic models for these phenomena is challenging. As Dhruv explains, most contemporary studies assume the gas behaves like an “ideal” fluid, where particles crash into each other constantly. “In reality, however, the hot plasma accreting onto a black hole is largely collisionless,” said Dhruv. “Charged particles can travel long distances before significantly interacting with one another through collisions.”
These older models also predicted that the black hole’s light should “flicker” in a violent and chaotic way. But the photo of the black hole indicates something different: a steadiness in the light curve.
“In our paper, we attempt to address these issues using ‘weakly collisional’ fluid models that include the leading-order corrections associated with particles having long mean free paths, namely viscosity and heat conduction,” said Dhruv. “In particular, we ask whether these ‘nonideal’ effects play a significant role in the synthetic observations produced by the model. Answering this helps determine whether such effects should be included in future modeling efforts.”
NCSA’s Delta system provided the GPU capability, scale and support we needed to survey physically realistic simulations of the galactic center. Access to these resources made an otherwise computationally prohibitive study feasible.
–Vedant Dhruv, Graduate Research Fellow, University of Illinois
“We found that, when viewed in a time-averaged sense, the weakly collisional fluid models are remarkably similar to the ideal models,” said Dhruv. “However, the low-collisionality physics included in our model tends to make the simulations ‘quieter,’ reducing variability and bringing them closer to observations of the galactic center.”
If black holes can capture humanity’s imagination, supermassive black holes must inspire awe. But studying these phenomena brings us more than just pretty pictures and interesting settings for science fiction cinema. The science derived from accurate modeling of fluid mechanics eventually works its way into many other domains that benefit from the research.
“The same modeling ideas we use here, namely viscosity and heat conduction, also show up in many familiar settings,” explains Dhruv. “Viscosity is what makes honey flow more ‘thickly’ than water, and heat conduction is what spreads heat through a metal spoon or a cooking pan. In a plasma, these processes control how momentum and heat are transported through a hot, magnetized gas. In this work, we ask how these effects change how the flow evolves over time, which is exactly the kind of physics that becomes increasingly important when the goal is to interpret the light curves of astronomical sources.”
You can find a deeper dive into this science in the original story here: Delta Helps Smooth the Chaos at Galaxy’s Center
Resource Provider Institution(s): National Center for Supercomputing Applications (NCSA)
Resources Used: Delta, DeltaAI
Affiliations: University of Illinois
Funding Agency: NSF
Grant or Allocation Number(s): AST170024
The science story featured here was enabled by the U.S. National Science Foundation’s ACCESS program, which is supported by National Science Foundation grants #2138259, #2138286, #2138307, #2137603, and #2138296.