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 centre 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 unprecedented Earth-sized virtual telescope. The EHT offers scientists a new way to study the most extreme objects in the Universe predicted by Einstein’s general relativity during the centenary 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 space time and superheating 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 has 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. The confrontation of theory with observations is always a dramatic moment for a theorist. It was a relief and a source of pride to realise that the observations matched our predictions so well, elaborated EHT Board member Luciano Rezzolla of Goethe Universität, Germany.
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 synchronises 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-arc seconds enough to read a newspaper in New York from a café in Paris. 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 specialised supercomputers hosted by the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory.