Milky Way's Black Hole Revealed

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Astronomers have unveiled the first image of the supermassive black hole at the center of our own Milky Way galaxy. This result provides overwhelming evidence that the object is indeed a black hole and yields valuable clues about the workings of such giants, which are thought to reside at the center of most galaxies. The image was produced by a global research team, which includes UMass astronomers, called the Event Horizon Telescope (EHT) Collaboration, using observations from a worldwide network of radio telescopes — including the Large Millimeter Telescope Alfonso Serrano (LMT), which is operated jointly by UMass Amherst and the country of Mexico.

The image is a long-anticipated look at the massive object that sits at the very center of our galaxy. Scientists had previously seen stars orbiting around something invisible, compact and very massive at the center of the Milky Way. This strongly suggested that this object — known as Sagittarius A* (Sgr A*, pronounced “sadge-ay-star”) — is a black hole, and today’s image provides the first direct visual evidence of it.

"This new image, of Sgr A*, provides a closer view that helps to confirm some of the basics of Einstein’s general theory of relativity. It’s a very exciting moment for science," says Peter Schloerb, professor of astronomy at UMass Amherst, director of the Five College Radio Astronomy Observatory and UMass director of the LMT.

Although we cannot see the black hole itself, because it is completely dark, glowing gas around it reveals a tell-tale 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.

“We were stunned by how well the size of the ring agreed with predictions from Einstein’s theory of general relativity,” said EHT Project Scientist Geoffrey Bower from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei. “These unprecedented observations have greatly improved our understanding of what happens at the very center of our galaxy, and offer new insights on how these giant black holes interact with their surroundings.” The EHT team's results were recently published in a special issue of The Astrophysical Journal Letters.

“The exciting thing about this result,” says Gopal Narayanan research professor in astronomy at UMass Amherst, “is that it’s in our own back yard, in our own galaxy.”

UMass Amherst astronomers Gopal Narayanan and Peter Schloerb were part of the team of researchers who unveiled the first image of the black hole at the center of the Milky Way known as Sagittarius A*.

Peter Schloerb, professor of astronomy at UMass Amherst, director of the Five College Radio Astronomy Observatory and UMass director of the LMT, adds, “in 2019 the EHT made the first-ever picture of a black hole in a distant galaxy called M87; this new image, of Sgr A*, provides a closer view that helps to confirm some of the basics of Einstein’s general theory of relativity. It’s a very exciting moment for science.”

Because the black hole is about 27,000 light years away from Earth, it appears to us to have about the same size in the sky as a donut on the Moon. To image it, the team created the powerful EHT, which linked together eight existing radio observatories across the planet to form a single “Earth-sized” virtual telescope.

One of these telescopes was the LMT, the world’s largest single-dish, steerable radio telescope, itself a remarkable engineering feat. Radio telescopes “see” a wavelength that isn’t visible to the human eye, and it takes a telescope with incredible precision to do so. The LMT’s dish is 50 meters in diameter and forms a nearly perfect parabola. “It’s accurate to about the width of a human hair, across its length and breadth,” says Schloerb. “It’s an open-air instrument,” adds Narayanan. “It's out in the elements and has to maintain its surface and pointing accuracy in varying ambient temperature and wind conditions.”

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 analyse their data, all while compiling an unprecedented library of simulated black holes to compare with the observations.

“It has been an honor to be part of such an amazing group,” says Sandra Bustamante, a member of the UMass EHT team, graduate student in astronomy at UMass and an international student hailing originally from Mexico. “In a collaboration this big, it is a major feat to be able to coordinate such a big group of people from very different backgrounds, countries, and languages (since for most, English is not their first language), and agree on a consolidated message that represents the EHT collaboration.”

Scientists are particularly excited to finally have images of two black holes of very different sizes, which offers the opportunity to understand how they compare and contrast. They have also begun to use the new data to test theories and models of how gas behaves around supermassive black holes. This process is not yet fully understood but is thought to play a key role in shaping the formation and evolution of galaxies.

Progress on the EHT continues: a major observation campaign in March 2022 included more telescopes than ever before. The ongoing expansion of the EHT network and significant technological upgrades will allow scientists to share even more impressive images as well as movies of black holes in the near future.

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