The Event Horizon Telescope (EHT) — a planet-scale array of eight ground-based radio telescopes forged through international collaboration — was designed to capture images of a black hole. Today, in coordinated press conferences around the globe, EHT researchers revealed that they have succeeded, unveiling the first direct visual evidence of a supermassive black hole and its shadow.
The image reveals the black hole at the center of Messier 87 (M87)1, a massive galaxy in the nearby Virgo galaxy cluster. This black hole resides 55 million light-years from Earth and has a mass 6.5 billion times that of the Sun2.
The EHT links telescopes around the globe to form an Earth-sized virtual telescope with unprecedented sensitivity and resolution3. The EHT is the result of years of international collaboration and offers scientists a new way to study the most extreme objects in the universe predicted by Einstein’s general relativity, during the centennial year of the historic experiment that first confirmed the theory4.
"We have taken the first picture of a black hole," said EHT project director Sheperd S. Doeleman of the Center for Astrophysics | Harvard & Smithsonian. "This is an extraordinary scientific feat accomplished by a team of more than 200 researchers."
This breakthrough was announced today in a series of six papers published in a special issue of Astrophysical Journal Letters. University of Illinois at Urbana-Champaign Professor of Physics and Astronomy Charles Gammie, a member of the EHT Science Council Board, co-led the theory working group and served as co-coordinator of Paper V, focused on the theoretical interpretation of the EHT data.
Black holes are extraordinary cosmic objects with enormous masses but extremely compact sizes. The presence of these objects affects their environment in extreme ways, warping spacetime and super-heating any surrounding material.
“If immersed in a bright region, like a disc of glowing gas, we expect a black hole to create a dark region similar to a shadow — something predicted by Einstein’s general relativity that we’ve never seen before,” explained chair of the EHT Science Council Heino Falcke of Radboud University, the Netherlands. “This shadow, caused by the gravitational bending and capture of light by the event horizon, reveals a lot about the nature of these fascinating objects and allowed us to measure the enormous mass of M87’s black hole.”
Multiple calibration and imaging methods have revealed a ring-like structure with a dark central region — the black hole’s shadow — that persisted over multiple independent EHT observations.
"Once we were sure we had imaged the shadow, we could compare our observations to extensive computer models that include the physics of warped space, superheated matter and strong magnetic fields. Many of the features of the observed image match our theoretical understanding surprisingly well,” remarked Paul T.P. Ho, EHT Board member and Director of the East Asian Observatory5. “This makes us confident about the interpretation of our observations, including our estimation of the black hole’s mass.”
The U of I research group led by Gammie made significant contributions to the collaboration’s theoretical analysis. Physics graduate students George N. Wong and Ben Prather and former Astronomy graduate student Ben R. Ryan worked with Gammie to generate an extensive library of 3-dimensional time-dependent numerical models, which they compared with the 5 petabytes of EHT data. The local effort was a massive computational undertaking, and Gammie’s team developed sophisticated computer code to make running and analyzing the simulations as efficient as possible.
The U of I team’s research allowed the EHT collaboration to constrain the spin of the black hole, which is related to the asymmetry of the ring in the EHT image. The U of I researchers’ work also shed light on the physical processes underlying the large radio jet emanating from M87’s black hole.
“M87 is the nearest galaxy with a supermassive black hole that’s generating a powerful jet — a beautiful streamer made of plasma travelling at close to the speed of light,” Gammie noted. “One of the great mysteries in astronomy has been how such jets are launched. Our simulations, which are based on the motion of magnetic fields and hot gas near the black hole, showed that the jets are powered by the black hole itself. Magnetic fields act to brake the rotation of the black hole and transfer its rotational energy to the jet."
Creating the EHT was a formidable challenge that required upgrading and connecting a worldwide network of eight pre-existing telescopes deployed at a variety of challenging high-altitude sites. These locations included volcanoes in Hawaii and Mexico, mountains in Arizona and the Spanish Sierra Nevada, the Chilean Atacama Desert, and Antarctica.
The EHT observations use a technique called very-long-baseline interferometry (VLBI), which synchronizes telescope facilities around the world and exploits the rotation of our planet to form one huge, Earth-size telescope observing at a wavelength of 1.3 mm. VLBI allows the EHT to achieve an angular resolution of 20 micro-arcseconds — enough to read a newspaper in New York from a sidewalk café in Paris6.
The telescopes contributing to this result were ALMA, APEX, the IRAM 30-meter telescope, the James Clerk Maxwell Telescope, the Large Millimeter Telescope Alfonso Serrano, the Submillimeter Array, the Submillimeter Telescope, and the South Pole Telescope7. Petabytes of raw data from the telescopes were combined by highly specialized supercomputers hosted by the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory.
The construction of the EHT and the observations announced today represent the culmination of decades of observational, technical, and theoretical work. This example of global teamwork required close collaboration by researchers from around the world. Thirteen partner institutions worked together to create the EHT, using both pre-existing infrastructure and support from a variety of agencies. Key funding was provided by the US National Science Foundation (NSF), the EU's European Research Council (ERC), and funding agencies in East Asia.
“We have achieved something presumed to be impossible just a generation ago,” concluded Doeleman. “Breakthroughs in technology and the completion of new radio telescopes over the past decade enabled our team to assemble this new instrument — designed to see the unseeable.”
Research contributing to the Event Horizon Telescope done at the University of Illinois at Urbana-Champaign was funded by the National Science Foundation. The conclusions presented are those of the researchers and not necessarily those of the funding agencies.
More information about the Illinois effort: U of I Professor of Physics and Astronomy Charles Gammie was interviewed for an “Expert Viewpoints” article about the importance of the new EHT findings, published on the website of the Illinois News Bureau here: https://news.illinois.edu/view/6367/772550.
For more information, contact:
U of I science contact, Professor Charles Gammie, email gammie@illinois.edu
U of I media contact, Siv Schwink, Communications Coordinator, Department of Physics
(217) 300-2201; email sschwink@illinois.edu
EHT Outreach contact, Mislav Baloković, Fellow, Black Hole Initiative, Harvard-Smithsonian,
email mislav.balokovic@cfa.harvard.edu
Article Text by EHT Collaboration and Siv Schwink for Illinois Physics
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Notes
1The shadow of a black hole is the closest we can come to an image of the black hole itself, a completely dark object from which light cannot escape. The black hole’s boundary — the event horizon from which the EHT takes its name — is around 2.5 times smaller than the shadow it casts and measures just under 40 billion km across.
2Supermassive black holes are relatively tiny astronomical objects — which has made them impossible to directly observe until now. As a black hole’s size is proportional to its mass, the more massive a black hole, the larger the shadow. Thanks to its enormous mass and relative proximity, M87’s black hole was predicted to be one of the largest viewable from Earth — making it a perfect target for the EHT.
3Although the telescopes are not physically connected, they are able to synchronize their recorded data with atomic clocks — hydrogen masers — which precisely time their observations. These observations were collected at a wavelength of 1.3 mm during a 2017 global campaign. Each telescope of the EHT produced enormous amounts of data — roughly 350 terabytes per day — which was stored on high-performance helium-filled hard drives. These data were flown to highly specialized supercomputers — known as correlators — at the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory to be combined. They were then painstakingly converted into an image using novel computational tools developed by the collaboration.
4100 years ago, two expeditions set out for the island of Príncipe off the coast of Africa and Sobra in Brazil to observe the 1919 solar eclipse, with the goal of testing general relativity by seeing if starlight would be bent around the limb of the sun, as predicted by Einstein. In an echo of those observations, the EHT has sent team members to some of the world's highest and isolated radio facilities to once again test our understanding of gravity.
5The East Asian Observatory (EAO) partner on the EHT project represents the participation of many regions in Asia, including China, Japan, Korea, Taiwan, Vietnam, Thailand, Malaysia, India and Indonesia.
6Future EHT observations will see substantially increased sensitivity with the participation of the IRAM NOEMA Observatory, the Greenland Telescope and the Kitt Peak Telescope.
7ALMA is a partnership of the European Southern Observatory (ESO; Europe, representing its member states), the U.S. National Science Foundation (NSF), and the National Institutes of Natural Sciences(NINS) of Japan, together with the National Research Council (Canada), the Ministry of Science and Technology (MOST; Taiwan), Academia Sinica Institute of Astronomy and Astrophysics (ASIAA; Taiwan), and Korea Astronomy and Space Science Institute (KASI; Republic of Korea), in cooperation with the Republic of Chile. APEX is operated by ESO, the 30-meter telescope is operated by IRAM (the IRAM Partner Organizations are MPG (Germany), CNRS (France) and IGN (Spain)), the James Clerk Maxwell Telescope is operated by the EAO, the Large Millimeter Telescope Alfonso Serrano is operated by INAOE and UMass, the Submillimeter Array is operated by SAO and ASIAA and the Submillimeter Telescope is operated by the Arizona Radio Observatory (ARO). The South Pole Telescope is operated by the University of Chicago with specialized EHT instrumentation provided by the University of Arizona.