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Astronomers reveal Milky Way's giant black hole

Sagittarius A* (Sgr A*) at the center of our galaxy was observed by earth-sized Multiple telescopes, Multiple Radio astronomy telescopes combined EHT (Event Horizon telescope) and simultaneously with X-ray Observatories

First image of the black hole at the center of the Milky Way This is the first image of Sagittarius A* (or Sgr A* for short), the supermassive black hole at the center of our galaxy. It’s the first direct visual evidence of the presence of this black hole. It was captured by the Event Horizon Telescope (EHT), an array which linked together eight existing radio observatories across the planet to form a single “Earth-sized” virtual telescope. The telescope is named after the “event horizon”, the boundary of the black hole beyond which no light can escape. Although we cannot see the event horizon itself, because it cannot emit light, glowing gas orbiting around the black hole reveals a telltale signature: a dark central region (called a “shadow”) surrounded by a bright ring-like structure. The new view captures light bent by the powerful gravity of the black hole, which is four million times more massive than our Sun. The image of the Sgr A* black hole is an average of the different images the EHT Collaboration has extracted from its 2017 observations. Credit: EHT Collaboration

May 12, 2022 At simultaneous press conferences around the world, including at a National Science Foundation-sponsored press conference at the US National Press Club in Washington, D.C., astronomers have unveiled the first image of the supermassive black hole at the center of our own Milky Way galaxy.

"Until now, we didn't have the direct picture to prove that this gentle giant in the center of our galaxy is a black hole," Feryal Özel, an astrophysicist at the University of Arizona, said during a National Science Foundation news conference held Thursday (May 12). "It shows a bright ring surrounding the darkness, and the telltale sign of the shadow of the black hole."

The effort was made possible through the ingenuity of more than 300 researchers from 80 institutes around the world that together make up the EHT Collaboration. In addition to developing complex tools to overcome the challenges of imaging Sgr A*, the team worked rigorously for five years, using supercomputers to combine and analyze their data, all while compiling an unprecedented library of simulated black holes to compare with the observations.

This image of Sagittarius A*, and of the black hole in M87 before it, has been made possible through the magic of a technique known as Very Long Baseline Interferometry, which allows astronomers to combine data from radio telescopes all across the world as though they were one large telescope, effectively making the EHT the largest telescope on Earth.

At the time when the observations were made, the network consisted of eight telescopes (including one, the South Polar Telescope, that was too far south to study M87), although three more have since been added to the network. The eight-telescope configuration means that the EHT's maximum baseline — which is equivalent to a telescope's aperture — for observing Sagittarius A* was 10,700 kilometers (6,650 miles) across for an earth's hemisphere.

A comparison of Event Horizon Telescope views of the black holes at the center of the galaxy M87, on the left, and of the one in the Milky Way, at right. (Image credit: EHT Collaboration)

In 2019, the EHT made history in headlines when it succeeded in producing the first-ever image of the event horizon of a black hole, specifically the black hole at the center of the active elliptical galaxy Messier 87. At the same time as it gathered the data that became that image, the EHT also performed observations of Sagittarius A*, which is the name given to the Milky Way's supermassive black hole. However, producing an image of Sagittarius A* proved more difficult than for M87.

“We are very proud at CDL to have provided some critical technology to support this amazing discovery by the EHT collaboration,” said Bert Hawkins, Director of CDL, who explained the role of Band 6 and CDL in making the research and the results possible. “Our team contributed by installing a custom-built atomic clock on ALMA and reprogramming the ALMA correlator to make the telescope a phased array. This effectively turned the telescope into a single dish with an effective diameter of 85 meters– the largest component on the EHT. In addition, the mixers at the heart of the receivers on ALMA, the Submillimeter Telescope (SMT) in Arizona, the Large Millimeter Telescope (LMT) in Mexico, and the South Pole Telescope (SPT) in Antarctica were developed at CDL along with our partners at the University of Virginia.”

For one thing, Earth's moisture-laden atmosphere can absorb the submillimeter radio waves that the EHT observatories rely on. That sensitivity comes from the 1.3mm Band 6 receivers on the Atacama Large Millimeter/submillimeter Array (ALMA),