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The first image of the shadow of the black hole in the center of M87 taken with EHT. The size of the ring is only about 40 microarcseconds across - equivalent to measuring the diameter of a baseball on the surface of the moon from the Earth. (Credit: EHT Collaboration)(Original size:643KB)
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 across the globe, EHT researchers reveal that they have succeeded, unveiling the first direct visual evidence of a supermassive black hole and its shadow.
This breakthrough was announced today in a series of six papers published in a special issue of The Astrophysical Journal Letters. The image reveals the black hole at the center of Messier 87 , 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 Sun .
The EHT links telescopes around the globe to form an Earth-sized virtual telescope with unprecedented sensitivity and resolution . 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 theory .
"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."
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," remarks Paul T.P. Ho, EHT Board member and Director of the East Asian Observatory . "This makes us confident about the interpretation of our observations, including our estimation of the black hole’s mass."
Creating the EHT was a formidable challenge which 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 Hawai`i 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 1.3mm. VLBI allows the EHT to achieve an angular resolution of 20 micro-arcseconds — enough to read a newspaper in Singapore from a sidewalk café in Tokyo .
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 Telescope . 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.
Locations of the telescopes making up the EHT in the 2017 observing campaign. (Credit: NRAO/AUI/NSF)(Original size:2.3MB)
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.
Japanese researchers contributed to various aspects of this research. ALMA, operated through international collaboration between East Asia, Europe, and North America, is the most sensitive element in the EHT Array and greatly enhanced the array sensitivity. The National Astronomical Observatory of Japan (NAOJ) developed the data transmission instrument for ALMA to deliver the massive data taken with the array to its base facility. Japan, together with Asian partners, established the East Asian Observatory, which operates the James Clark Maxwell Telescope at Maunakea, Hawai`i.
"We also contributed in software," said Mareki Honma, Professor and Director of NAOJ Mizusawa VLBI Observatory. "We adopted a novel method called ‘sparse modeling’ in the imaging software. It enabled us to obtain a reliable image from the limited data. The EHT collaboration established four independent teams for imaging and the teams employed three different methods to generate the final images. In the end, we confirmed that all the teams obtained nearly identical images of the black hole."
“Furthermore, we carefully checked the reliability of the image features of the black hole shadow by changing the input parameters of the imaging software,” said Kotaro Moriyama, a researcher at NAOJ Mizusawa VLBI Observatory (now a researcher at MIT Haystack Observatory). “In total, we performed the imaging process with 50000 sets of different parameters.”
“I was really excited when we succeed to obtain the very first image of the black hole with the novel imaging technique we developed,” said Fumie Tazaki, a researcher at NAOJ Mizusawa VLBI Observatory. “I couldn’t be more delighted than to share the images obtained by the whole team.”
“We applied three independent algorithms in the data reduction process before imaging and confirmed that the results are identical,” commented Shoko Koyama, a researcher at the Academia Sinica Institute of Astronomy and Astrophysics, Taiwan. “Usually this kind of processing is not spotlighted, but this wonderful result is the culmination of many small steps done by each team member.”
"We have achieved something presumed to be impossible just a generation ago," concluded Doeleman. "Breakthroughs in technology, connections between the world's best radio observatories, and innovative algorithms all came together to open an entirely new window on black holes and the event horizon."
 The 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.
 Supermassive 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.
 Although 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 specialised 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.
 100 years ago, two expeditions set out for the island of Príncipe off the coast of Africa and Sobral 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.
 The 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.
 Future EHT observations will see substantially increased sensitivity with the participation of the IRAM NOEMA Observatory, the Greenland Telescope and the Kitt Peak Telescope.
 ALMA 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.
This research was presented in the following six papers published today in a special issue of The Astrophysical Journal Letters.
- First M87 Event Horizon Telescope Results I: The Shadow of the Supermassive Black Hole
- First M87 Event Horizon Telescope Results II: Array and Instrumentation
- First M87 Event Horizon Telescope Results III: Data Processing and Calibration
- First M87 Event Horizon Telescope Results IV: Imaging the Central Supermassive Black Hole
- First M87 Event Horizon Telescope Results V: Physical Origin of the Asymmetric Ring
- First M87 Event Horizon Telescope Results VI: The Shadow and Mass of the Central Black Hole
The EHT collaboration involves more than 200 researchers from Africa, Asia, Europe, North and South America. The international collaboration is working to capture the most detailed black hole images ever by creating a virtual Earth-sized telescope. Supported by considerable international investment, the EHT links existing telescopes using novel systems - creating a fundamentally new instrument with the highest angular resolving power that has yet been achieved.
The individual telescopes involved are; ALMA, APEX, the IRAM 30-meter Telescope, the IRAM NOEMA Observatory, the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope Alfonso Serrano (LMT), the Submillimeter Array (SMA), the Submillimeter Telescope (SMT), the South Pole Telescope (SPT), the Kitt Peak Telescope, and the Greenland Telescope (GLT).
The EHT Collaboration consists of 13 stakeholder institutes; the Academia Sinica Institute of Astronomy and Astrophysics, the University of Arizona, the University of Chicago, the East Asian Observatory, Goethe-Universitaet Frankfurt, Institut de Radioastronomie Millimétrique, Large Millimeter Telescope, Max Planck Institute for Radio Astronomy, MIT Haystack Observatory, National Astronomical Observatory of Japan, Perimeter Institute for Theoretical Physics, Radboud University and the Smithsonian Astrophysical Observatory.
This research was supported by MEXT/JSPS KAKENHI (No. 18K13594, 18K03656, 18H01245, 18H03721, 18KK0090, 25120007, 25120008), MEXT as “Priority Issue on post-K computer” (Elucidation of the Fundamental Laws and Evolution of the Universe) and JICFuS, NINS Project of Formation of International Scientific Base and Network, Toray Science Foundation, The Sumitomo Foundation, and other funding agencies worldwide.
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Assistant Professor Mahito Sasada