• This international study represents a paradigm shift in observations of the supermassive black hole located in the center of the Messier 87 galaxy
• Spanish astronomers from the Institute of Astrophysics of Andalusia (IAA-CSIC), the National Geographic Institute, the Millimeter Radioastronomy Institute and the University of Valencia participated of the study
• The Event Horizon Telescope, designed to capture images of a black hole succeeded in obtaining the first direct visual evidence of a supermassive black hole and its shadow

A black hole is a region of space-time where gravity reaches such high magnitudes that it does not allow even light to escape its attraction force. 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.

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 supermassive black hole. These are relatively tiny astronomical objects, which in the past made them impossible to observe directly. EHT succeeded in its objective.

The announcement of the first image of a black hole

The first captured image of a black hole was published together with a series of six papers published in a special issue of The Astrophysical Journal Letters. This breakthrough image revealed the black hole at the centre of Messier 87, a massive galaxy in the nearby Virgo galaxy cluster. In coordinated press conferences across the globe, EHT researchers revealed that they had succeeded, unveiling the first direct visual evidence of a supermassive black hole and its shadow.

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, Messier 87’s black hole was predicted to be one of the largest viewable black holes from Earth — making it a perfect target for the EHT. The shadow of a black hole is the closest one can come to obtain an image it, otherwise a completely dark object from which light cannot escape.

The captured black hole resides 55 million light-years away from Earth and has a mass of 6.5-billion times that of the Sun. Its 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. “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.”

“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 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 challenges behind the EHT

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 links telescopes around the globe to form an Earth-sized virtual telescope with unprecedented sensitivity and resolution. The EHT, result of years of international collaboration, 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.

While the EHT telescopes are not physically connected, they synchronize their recorded data with atomic clocks (hydrogen masers) which precisely time their observations. 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-arcseconds, which would be enough to read a newspaper in New York from a sidewalk café in Paris.

The 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. The 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. There, they were laboriously converted into the announced image using novel computational tools developed by the collaboration.

An international effort with Spanish contribution

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. Future EHT observations will see substantially increased sensitivity with the participation of the IRAM NOEMA Observatory, the Greenland Telescope and the Kitt Peak Telescope.

The construction of the EHT and the observations announced 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.

Several Spanish astronomers have participated in this scientific milestone. José Luis Gómez, researcher of the Superior Council of Scientific Research (CSIC) in the Institute of Astrophysics of Andalusia (IAA-CSIC), has developed one of the three algorithms used for the reconstruction of the images of the shadow of the black hole in M87. In addition, Gómez is one of the coordinators of the published scientific article where these images are presented and analyzed.

Antxon Alberdi, director of the IAA-CSIC, leads the research on the formation of relativistic jets from the accretion around supermassive black holes. Iván Martí-Vidal, from the Spanish National Geographic Institute (IGN), designed the algorithms that allowed the combination of the ALMA data (the most sensitive element of the EHT) with the rest of the radio telescopes; he is also coordinator of the polarimetry group (whose main objective is to study the role of magnetic fields in the vicinity of the black hole).

Miguel Sánchez-Portal (director of IRAM-Granada), Salvador Sánchez and Ignacio Ruíz (engineers), and Pablo Torné (researcher) also from the Millimeter Radioastronomy Institute (IRAM), and Rebecca Azulay (University of Valencia) have participated in the configuration of the technical equipment and the observations from the IRAM 30 meter telescope in Sierra Nevada, Granada.

“The Event Horizon Telescope has transformed our vision of black holes from a mathematical concept to something real that can be studied through repeated astronomical observations,” said Gómez.”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.”

Additional notes about the participant observatories:

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.

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.

Image credits:

Mesier 87 galaxy picture by NASA is in the public domain and was downloaded from Wikimedia Commons.

All other pictures kindly provided by IAA-CSIC and re-used with permission.