“The Shadow Knows” –EHT Image Reveals Anatomy of M87’s Gargantuan Black Hole

M87 Black Hole


“Paradoxical, Intriguing, Frightening” was how scientists described the monster elliptical galaxy that harbors the now iconic black hole the size of our solar system at the center of M87, the largest, most massive galaxy in the nearby universe. The supermassive object was imaged for the first time ever by the Event Horizon Telescope (EHT) on April 10, 2019. Today, reports the Institute for Advanced Study, a team of observational astronomers, theoretical physicists, and astrophysicists have published new calculations that predict a striking and intricate substructure within black hole images from extreme gravitational light bending.

The EHT image marked the culmination of years of work undertaken by a team of 200 scientists in 59 institutes across 18 countries. The project drew on data collected by eight telescopes whose locations range from Hawaii to the South Pole. In order to construct this image digitally, the team of astronomers at EHT created the equivalent of a lens the size of planet Earth by integrating data from all the telescopes that were part of the project that’s 4,000 times more powerful than the Hubble Space Telescope.

A Nested Series of Rings

“The image of a black hole actually contains a nested series of rings,” explains Michael Johnson of the Center for Astrophysics, Harvard and Smithsonian (CfA). “Each successive ring has about the same diameter but becomes increasingly sharper because its light orbited the black hole more times before reaching the observer. With the current EHT image, we’ve caught just a glimpse of the full complexity that should emerge in the image of any black hole.”

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The image below has a bright ring of emission surrounding a “shadow” cast by the black hole. This ring is composed of a stack of increasingly sharp subrings that correspond to the number of orbits that photons took around the black hole before reaching the observer. George Wong (UIUC) and Michael Johnson (CfA)


M87 Black Hole Shadows


Because black holes trap any photons that cross their event horizon, they cast a shadow on their bright surrounding emission from hot infalling gas. A “photon ring” encircles this shadow, produced from light that is concentrated by the strong gravity near the black hole. This photon ring carries the fingerprint of the black hole—its size and shape encode the mass and rotation or “spin” of the black hole. With the EHT images, black hole researchers have a new tool to study these extraordinary objects.

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“This is an extremely exciting time to be thinking about the physics of black holes,” says Daniel Kapec at the Institute for Advanced Study. “Einstein’s theory of general relativity makes a number of striking predictions for the types of observations that are finally coming within reach, and I think we can look forward to lots of advances in the coming years. As a theorist, I find the rapid convergence between theory and experiment especially rewarding, and I hope we can continue to isolate and observe more universal predictions of general relativity as these experiments become more sensitive.”



“Bringing together experts from different fields enabled us to really connect a theoretical understanding of the photon ring to what is possible with observation,” notes George Wong, a physics graduate student at the University of Illinois at Urbana-Champaign. Wong developed software to produce simulated black hole images at higher resolutions than had previously been computed and to decompose these into the predicted series of sub-images. “What started as classic pencil-and-paper calculations prompted us to push our simulations to new limits.”

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“What really surprised us was that while the nested subrings are almost imperceptible to the naked eye on images—even perfect images—they are strong and clear signals for arrays of telescopes called interferometers,” says Johnson. “While capturing black hole images normally requires many distributed telescopes, the subrings are perfect to study using only two telescopes that are very far apart. Adding one space telescope to the EHT would be enough.”

“Black hole physics has always been a beautiful subject with deep theoretical implications, but now it has also become an experimental science,” says Alex Lupsasca from the Harvard Society of Fellows. “As a theorist, I am delighted to finally glean real data about these objects that we’ve been abstractly thinking about for so long.”

More information: M.D. Johnson el al., “Universal interferometric signatures of a black hole’s photon ring,” Science Advances (2020). DOI: 10.1126/sciadv.aaz1310 , https://advances.sciencemag.org/content/6/12/eaaz1310

The Daily Galaxy, Max Goldberrg, via Lee Sandberg at the Institute for Advanced Study

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