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First image of the black hole at the centre of our galaxy is revealed by astronomers

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 centre 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

At simultaneous press conferences around the globe, including one sponsored by the National Science Foundation at the U.S. National Press Club in Washington, D.C., astronomers unveiled the first image of the supermassive black hole at the centre of our Milky Way galaxy. This result provides conclusive evidence that the object is, in fact, a black hole and sheds light on the inner workings of these enormous objects, which are believed to reside at the centre of most galaxies. Using observations from a global network of radio telescopes, the image was created by the Event Horizon Telescope (EHT) Collaboration, a global research team.

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The image provides a long-awaited look at the massive object at the centre of our galaxy. Previously, scientists had observed stars orbiting something invisible, dense, and massive at the centre of the Milky Way. This strongly suggested that the object known as Sagittarius A* (Sgr A*, pronounced “sadge-star”) is a black hole, and the image released today is the first direct visual evidence of this.

We cannot see the black hole because it is completely dark, but glowing gas around it reveals a distinctive signature: a dark central region (called a “shadow”) surrounded by a bright, ring-like structure. The new image captures light deflected by the black hole’s powerful gravity, which is four million times more massive than the sun.

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“We were astounded by how well the size of the ring matched predictions from Einstein’s theory of general relativity,” said EHT Project Scientist Geoffrey Bower of Academia Sinica’s Institute of Astronomy and Astrophysics in Taipei. These observations have greatly increased our understanding of what occurs at the centre of our galaxy and shed new light on how these enormous black holes interact with their environment. Today, the results of the EHT team are published in a special issue of The Astrophysical Journal Letters.

Due to the black hole’s distance of approximately 27,000 light-years from Earth, it appears roughly the same size in the sky as a donut on the moon. To image it, the team developed the potent EHT by connecting eight existing radio observatories across the globe to form a single “Earth-sized” virtual telescope. The EHT observed Sgr A* over the course of multiple nights, collecting data for many hours in a row, much like a camera with a long exposure time.

And similar to a high-powered camera, imaging Sgr A* required the most sensitive radio astronomy instruments. This sensitivity is provided by the 1.3mm Band 6 receivers on the Atacama Large Millimeter/submillimeter Array (ALMA), which were designed by the Central Development Laboratory (CDL) at the National Radio Astronomy Observatory of the US National Science Foundation (NRAO).

Creation of the image of the central black hole of the Milky Way. EHT Collaboration Credit

Bert Hawkins, Director of CDL, explained the role of Band 6 and CDL in making the research and results possible, stating, “We are very proud at CDL to have provided some critical technology to support this astounding discovery by the EHT collaboration.” “Our team contributed by installing a custom-made atomic clock on ALMA and reprogramming the ALMA correlator to transform it into a phased-array telescope. This effectively transformed the telescope into a single 85-meter-diameter dish, 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 by CDL and the University of Virginia in collaboration.”

The breakthrough follows the 2019 release of the first image of a supermassive black hole, designated M87*, at the centre of the more distant Messier 87 galaxy by the EHT collaboration.

Even though our galaxy’s black hole is 1,000 times smaller and less massive than M87*, the similarities between the two black holes are striking. According to Sera Markoff, co-chair of the EHT Science Council and a professor of theoretical astrophysics at the University of Amsterdam in the Netherlands, “we have two completely different types of galaxies and two very different black hole masses, but close to the edge of these black holes, they look remarkably similar.” This indicates that general relativity governs these objects at close range, and that any differences observed at greater distances must be the result of differences in the material surrounding black holes.

This accomplishment was considerably more difficult than M87, despite the fact that Sgt. A is considerably closer to us. EHT researcher Chi-kwan ( “CK”) Chan, from the Steward Observatory and Department of Astronomy and the Data Science Institute at the University of Arizona in the United States, explains: “The gas in the vicinity of the black holes moves at the same speed—almost as fast as light—around both Sgr A* and M87. In contrast to the larger M87, where gas takes days to weeks to orbit, in the much smaller Sgr A, gas completes an orbit in mere minutes. This indicates that the brightness and pattern of the gas surrounding Sgr A were rapidly changing as the EHT Collaboration observed it, similar to trying to capture a clear image of a dog chasing its tail.”

To account for the gas movement around Sgr A, the researchers had to develop sophisticated new tools. While M87 was a simpler, more stable target with nearly identical images, this was not the case for Sgt. A. The image of the Sgr A black hole is an average of the images extracted by the team, revealing for the first time the giant lurking at the centre of our galaxy.

The effort was made possible by the ingenuity of more than 300 researchers from 80 institutes across the globe who comprise the EHT Collaboration. In addition to developing complex tools to overcome the difficulties of imaging Sgr A*, the team laboured for five years using supercomputers to combine and analyse their data while compiling an unprecedented library of simulated black holes for comparison with the observations.

Sgr A, pronounced sadge-ay-star, is a complex radio source in the centre of the Milky Way Galaxy that contains a supermassive black hole (SMBH). More than 300 researchers from 80 institutions across the globe collaborated to image SgrA using the Event Horizon Telescope (EHT), a global telescope comprised of multiple radio arrays operating in tandem. Visually, SgrA* resembles M87*, the first black hole ever captured on camera. However, the latest results demonstrate that they are as dissimilar as can be. Credit: NRAO/AUI/NSF Collaboration on the EHT

“This research clearly demonstrates the critical importance of using radio, millimetre, and submillimeter frequencies to understand the universe’s most extreme environments,” said Tony Remijan, Director of the North American ALMA Science Center (NAASC) at the National Radio Astronomy Observatory (NRAO). “Other frequency ranges are incapable of revealing the unique environment surrounding the black hole. ALMA was essential to the observations because it provided the necessary sensitivity to make this observation unambiguously. The sensitivity and resolution required for these types of discoveries were provided by combining data from facilities all over the world, with ALMA serving as the anchor for all these facilities. And this is just the start. ALMA is planning a significant increase in its sensitivity within the next decade, which will lead to even more profound cosmic discoveries.”

Scientists are ecstatic to finally have images of two black holes of vastly different sizes, which allows them to compare and contrast their characteristics. They have also begun testing theories and models of how gas behaves around supermassive black holes using the new data. This process is not yet fully understood, but it is believed to play a crucial role in galaxies’ formation and evolution.

EHT scientist Keiichi Asada from the Institute of Astronomy and Astrophysics, Academia Sinica in Taipei stated, “Now, we can study the differences between these two supermassive black holes to gain valuable new insights into how this important process works.” We have images of two black holes, one at the large end and one at the small end of supermassive black holes in the universe, allowing us to test the behaviour of gravity in these extreme environments more thoroughly than ever before.

An unprecedented number of telescopes participated in a major observation campaign in March 2022, as the EHT continues to make progress. In the near future, the ongoing expansion of the EHT network and significant technological advancements will enable scientists to share even more impressive images and movies of black holes.

Through the North American ALMA Development Program, the NSF and the ALMA Board approved a multimillion-dollar upgrade for the Observatory’s Band 6 receivers in 2021. The upgrade will increase the quantity and quality of science measured at wavelengths between 1.4mm and 1.1mm, which will provide research projects such as those at EHT with greater sensitivity than ever before, as well as more precise and effective science results. In addition, the Astro2020 decadal survey supported the Next Generation Very Large Array (ngVLA) of the National Radio Astronomy Observatory. The Next-Generation Very Large Array (ngVLA) will achieve high-priority goals in astronomy and astrophysics and is expected to become the ultimate black hole hunting instrument.

Dr. Tony Beasley, Director of NRAO, stated, “These new EHT results are exciting because they demonstrate how far astronomy has already come, and because they confirm that there’s still so much out there we haven’t seen and haven’t yet been able to observe and study.” “The antennas and instrumentation we design and develop at NRAO make this progress possible, and we look forward to continuing to lead advancements in radio astronomy that will reveal black holes and other phenomena hiding in the galaxy’s and universe’s outskirts.”

Further information:The Astrophysical Journal Lettersiopscience.iop.org/journal/2041-8205

Journal information: Astrophysical Journal Letters

Source: National Radio Astronomy Observatory

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