Astronomers Capture First-ever Image of a Black Hole
Scientists have obtained the first image of a black hole using Event Horizon Telescope (EHT) observations of the center of the galaxy Messier 87 (M87). The image shows a bright ring formed as light bends in the intense gravity around a black hole that is 6.5 billion times more massive than the Sun. This long-sought image provides the strongest evidence to date for the existence of supermassive black holes and opens a new window onto the study of black holes, their event horizons, and gravity. (Image Credit: Event Horizon Telescope Collaboration)
Astronomers Capture First-ever Image of a Black Hole
M87 (also known as Virgo A or NGC 4486) is one of the most massive galaxies in the local Universe. To give you an idea of its size, M87 has a large population of globular clusters (about 12,000) compared with the 150 to 200 orbiting our Milky Way galaxy. It also has a jet of energetic plasma traveling at relativistic speed that originates at the core and extends at least 4900 light-years. It is one of the brightest radio sources in the sky and a popular target for both amateur and professional astronomers. As in most, if not all, spiral galaxies, M87 has a supermassive black hole at its center. Black holes are extraordinary cosmic objects with enormous masses but extremely compact sizes. The extreme density of these objects affects their immediate environment in peculiar ways, warping space-time and super-heating any surrounding material. To date, no one has ever imaged a black hole. But that has now changed with the Event Horizon Telescope (EHT), a planet-scale array of eight ground-based radio telescopes forged through an international collaboration.
The EHT was designed specifically to capture images of a black hole. In coordinated press conferences across the globe, EHT researchers revealed the first direct visual evidence of a supermassive black hole and its shadow. The image shows the black hole at the center of M87.
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. This black hole resides 55 million light-years from Earth and has a mass 6.5 billion times that of the Sun.
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.
The EHT links telescopes around the globe to form an Earth-sized virtual telescope with unprecedented sensitivity and resolution. Although the telescopes are not physically connected, they are able to synchronize their recorded data with atomic clocks known as 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 the MIT Haystack Observatory to be combined. They were then painstakingly converted into an image using novel computational tools developed by the collaboration.
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. One hundred years ago, two expeditions set out for the island of Principe 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.
"We have taken the first picture of a black hole," said EHT project director Sheperd S. Doeleman of the Harvard and Smithsonian Center for Astrophysics. "This is an extraordinary scientific feat accomplished by a team of more than 200 researchers."
"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 in 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 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-arc-seconds -- enough to read a newspaper in New York from a sidewalk cafe 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 Sub-millimeter Array, the Sub-millimeter 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 the MIT Haystack Observatory.
The construction of the EHT and the observations 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, 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."
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