Scientists theorize that "supermassive black holes" lie at the center of all galaxies, including our own. "Sagittarius A*" is orders of magnitude more dense than the sun but it's also quite some distance from Earth.
A link of eight radio observatories form the virtual telescope that captured the first visual images of Sagittarius A*
An international team of astronomers on Thursday unveiled the first image of a supermassive black hole called Sagittarius A*, or Sgr A*, at the center of the Milky Way.
It comes three years after the very first image of a black hole from a distant galaxy was released.
Black holes are regions of space whose gravity pull is so strong that nothing can escape it, including light.
"For decades, we have known about a compact object that is at the heart of our galaxy that is four million times more massive than our Sun," Harvard University astronomer Sara Issaoun told a press conference in Garching, Germany.
"Today, right this moment, we have direct evidence that this object is a black hole," she added.
The image was captured by the Event Horizon Telescope Collaborative and is the first direct visual rendering of the presence of this object, which is invisible to the naked eye.
The black hole itself is not depicted by the telescope's image, but rather the glowing gas that encircles it in a bright ring of light.
Sagittarius A* is thought be several million times more dense than Earth's sun.
Although it is within our Milky Way galaxy, the black hole is located an estimated 27,000 light years from earth — by comparison, the sun is a little more than 8 light minutes away from Earth.
EHT captured the image
To capture the image from Sagittarius A*, scientists had to link eight giant radio observatories across the planet to form a single "Earth-sized" virtual telescope, the Event Horizon Telescope (EHT).
"The EHT can see three million times sharper than the human eye," German scientist Thomas Krichbaum of the Max Planck Institute for Radio Astronomy told reporters.
To capture the image, the EHT observed Sgr A* for multiple nights for many hours in a row, the same process used to produce the first image of a black hole in 2019.
Despite being closer to Earth, it was still difficult to capture the image. The brightness and pattern of the gas surrounding Sgr A* changed rapidly as the team observed it, "a bit like trying to take a clear picture of a puppy quickly chasing its tail," said EHT scientist Chi-kwan Chan of the University of Arizona.
jcg/msh (AFP, AP, dpa)
Issued on: 12/05/2022
Paris (AFP) – An international team of astronomers on Thursday unveiled the first image of a supermassive black hole at the centre of our own Milky Way galaxy -- a cosmic body known as Sagittarius A*.
The image -- produced by a global team of scientists known as the Event Horizon Telescope (EHT) Collaboration -- is the first, direct visual confirmation of the presence of this invisible object, and comes three years after the very first image of a black hole from a distant galaxy.
Black holes are regions of space where the pull of gravity is so intense that nothing can escape, including light.
The image thus depicts not the black hole itself, because it is completely dark, but the glowing gas that encircles the phenomenon -- which is four million times more massive than our Sun -- in a bright ring of bending light.
"These unprecedented observations have greatly improved our understanding of what happens at the very centre of our galaxy," said EHT project scientist Geoffrey Bower, of Taiwan's Academia Sinica.
Bower also said in a statement provided by the French National Centre for Scientific Research (CNRS) that the observations had offered "new insights on how these giant black holes interact with their surroundings".
The results are published in The Astrophysical Journal Letters.
Virtual telescope
Sagittarius A* -- abbreviated to Sgr A*, which is pronounced "sadge-ay-star" -- owes its name to its detection in the direction of the constellation Sagittarius.
Its existence has been assumed since 1974, with the detection of an unusual radio source at the centre of the galaxy.
In the 1990s, astronomers mapped the orbits of the brightest stars near the centre of the Milky Way, confirming the presence of a supermassive compact object there -- work that led to the 2020 Nobel Prize in Physics.
Though the presence of a black hole was thought to be the only plausible explanation, the new image provides the first direct visual proof.
Because it is 27,000 light years from Earth, it appears the same size in the sky as a donut on the Moon.
Capturing images of such a faraway object required linking eight giant radio observatories across the planet to form a single "Earth-sized" virtual telescope called the EHT.
These included the Institute for Millimetre Radio Astronomy (IRAM) 30-meter telescope in Spain, the most sensitive single antenna in the EHT network.
The EHT gazed at Sgr A* across multiple nights for many hours in a row -- a similar idea to long-exposure photography and the same process used to produce the first image of a black hole, released in 2019.
That black hole is called M87* because it is in the Messier 87 galaxy.
Moving target
The two black holes bear striking similarities, despite the fact that Sgr A* is 2,000 times smaller than M87*.
"Close to the edge of these black holes, they look amazingly similar," said Sera Markoff, co-chair of the EHT Science Council, and a professor at the University of Amsterdam.
Both behaved as predicted by Einstein's 1915 theory of General Relativity, which holds that the force of gravity results from the curvature of space and time, and cosmic objects change this geometry.
Despite the fact Sgr A* is much closer to us, imaging it presented unique challenges.
Gas in the vicinity of both black holes moves at the same speed, close to the speed of light. But while it took days and weeks to orbit the larger M87*, it completed rounds of Sgr A* in just minutes.
The researchers had to develop complex new tools to account for the moving targets.
The resulting image -- the work of more than 300 researchers across 80 countries over a period of five years -- is an average of multiple images that revealed the invisible monster lurking at the centre of the galaxy.
Scientists are now eager to compare the two black holes to test theories about how gasses behave around them -- a poorly understood phenomenon thought to play a role in the formation of new stars and galaxies.
Probing black holes -- in particular their infinitely small and dense centers known as singularities, where Einstein's equations break down -- could help physicists deepen their understanding of gravity and develop a more advanced theory.
© 2022 AFP
The Event Horizon Project on Thursday, May 12, 2022, released the first image first look at the Milky Way black hole, Sagittarius A*, which required eight telescopes around the world and decades or work, according to researchers.
ORLANDO, Fla., May 12 (UPI) -- Astronomers who work on the Event Horizon Telescope project revealed the first-ever image of the supermassive black hole in the heart of the Milky Way galaxy.
Researchers presented the new findings at a multi-continent press conference with multiple live streams online.
"This is the first image of the black hole at the center of our galaxy," Sara Issaoun, an astronomer at Harvard's Center for Astrophysics, said during the press conference.
"For decades we have known about this compact object, but today, at this moment, we have direct evidence of its existence," Issaoun said.
The telescope, a series of eight synchronized radio telescopes spread across the globe, in 2019 produced the first-ever close-up image of a black hole.
Decades of data indicate that a cosmic monster -- a super massive black hole named Sagittarius A* -- lurks at the heart of the Milky Way, which follows the expectation that most other galaxies across the universe have them, too, the astronomers said.
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The telescope has been peering into center of the galaxy for several weeks, with many scientists speculating that the news could be the first-ever image of the Milky Way's galactic center.
"We've combined eight of the world's greatest telescopes to take this picture," José L. Gómez, a research scientist of the Institute of Astrophysics of Andalucía, said during the press conference.
"It was like trying to take a picture of a child running at night," Gómez said.
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The image released in 2019 was of a different supermassive black hole, 53 million light-years from the Milky Way, in a galaxy known as Messier 87, or M87.
From the images, astronomers can compare the two objects. The images look strikingly similar, but the black holes are very different. For instance, SgrA* is approximately a million times less massive than the one in M87, and it also consumes gas at a much slower rate.
"Despite all these differences, the images look very similar," Issaoun said. "This reveals to us a key aspect of black holes: no matter their size or environment, once you arrive at the edge of a black hole, gravity takes over."
Researchers said that these ground-breaking mages are only possible with a telescope like the EHT. By combining the power of multiple radio telescopes around the globe, the team has created one super telescope that they can use to learn more about black holes.
In the image, radio waves create the glow around the dark heart of the black hole, which is called its shadow, which can provide astronomers with details about the black hole's properties.
"The size of a black hole shadow is proportional to its mass," Issaoun said. "We've determined that the size of SgrA* is indeed four million times larger than the size of the sun."
Issaoun said that this discovery is exciting because it confirms predictions that are based on stellar orbits.
In 2019, the team who captured the image of the M87 black hole, were able to make similar measurements, gleaning information on its magnetic field and its surrounding environment.
More recently, the suite of telescopes has increased in number, increasing to 11, which will help improve future images of SgrA* and other black holes.
Caltech researchers help generate first image of black hole at the center of our galaxy
IMAGE: THIS IS THE FIRST-EVER IMAGE OF SAGITTARIUS A* (OR SGR A* FOR SHORT), THE SUPERMASSIVE BLACK HOLE AT THE CENTER OF OUR MILKY WAY GALAXY. IT IS THE FIRST DIRECT VISUAL EVIDENCE OF THE PRESENCE OF THIS BLACK HOLE AND WAS CAPTURED BY THE EVENT HORIZON TELESCOPE (EHT), AN ARRAY THAT LINKED TOGETHER EIGHT 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, 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 DIFFERENT IMAGES EXTRACTED FROM THE EHT'S 2017 OBSERVATIONS. view more
CREDIT: EHT COLLABORATION
A multi-institution collaboration that includes a Caltech-led imaging team has generated the first image of the supermassive black hole at the center of the Milky Way galaxy.
This result provides conclusive evidence that the body, known as Sagittarius A* (Sgr A*, pronounced "sadge-ay-star"), is indeed a black hole and yields valuable clues about the workings of such massive objects, which are thought to reside at the center of most galaxies.
The Sgr A* image was produced by an international research team, called the Event Horizon Telescope (EHT) Collaboration, consisting of more than 300 researchers from 80 institutions around the world. The result includes key contributions from an imaging team led by Caltech's Katherine L. (Katie) Bouman together with Kazunori Akiyama of MIT Haystack Observatory and José L. Gómez of Instituto de Astrofísica de Andalucía in Spain.
So far, the most convincing evidence that Sgr A* is a supermassive black hole has been provided by Caltech alumna Andrea Ghez (MS '89, PhD '92) of the University of California, Los Angeles, and Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Germany and University of California, Berkeley, whose work revealed that Sgr A* is a compact object that has a mass nearly four million times that of the sun. The discovery earned Ghez and Genzel—together with Roger Penrose of the University of Oxford for related advances in the understanding of black holes—the 2020 Nobel Prize in Physics.
The new EHT image of Sgr A* shows that the 4 million solar masses are constrained within a diameter smaller than Mercury's orbit, providing clearer evidence that the object is indeed a black hole.
This is the second-ever image taken of a black hole; in 2019, the EHT collaboration released an image of a black hole named M87*, at the center of the more distant Messier 87 galaxy.
Taking an image of Sgr A* at 27,000 light-years away from Earth is akin to taking a photo of a single grain of salt in New York City using a camera in Los Angeles. Taking an image of an object that appears that small requires an Earth-sized telescope—or data from many telescopes tiled evenly across the entire Earth. Since the EHT was not able to accomplish this impossible feat, it instead collected data from eight radio observatories scattered across the globe to form a single "Earth-sized" virtual telescope.
"This image from the Event Horizon Telescope required more than just snapping a picture from telescopes on high mountaintops. It is the product of both technically challenging telescope observations and innovative computational algorithms," Bouman said at a press conference announcing the new image. "Taking this picture of our black hole proved even more challenging than imaging the M87 black hole." While working on the M87 image, Bouman joined Caltech's faculty. She arrived on campus in 2019, shortly after the image's publication.
Joining Bouman, who is an assistant professor of computing and mathematical sciences, electrical engineering and astronomy; a Rosenberg Scholar; and an investigator at the Heritage Medical Research Institute, were former Caltech postdoctoral researcher He Sun and current Caltech postdoctoral researchers Aviad Levis and Junhan Kim.
Imaging A Black Hole
Although we cannot see the black hole itself because it is completely dark, we can see a telltale ring of glowing gas surrounding a dark central region called a "shadow." The size of the shadow observed, which theory says depends primarily on the black hole mass, precisely matched the mass estimated by prior observations. 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 team collected entire nights' worth of data over the course of multiple days -- a bit like a traditional camera with a long exposure time. Members of the EHT went to the ends of the earth, literally, to collect these measurements; Caltech's Kim, for example, helped prepare a telescope near the South Pole to observe with the EHT.
Because the data came from only a small number of telescopes peering at an object far away, the EHT team was left with incomplete data to use to construct the image of Sgr A*. That sparse and noisy data creates uncertainty, meaning that more than one image that could explain the data EHT gathered. And to make imaging the black hole even more challenging, Sgr A* is far more dynamic than M87*, with a bright ring of swirling gases that changes on a minute-by-minute basis. "With EHT, we're using telescopes scattered across the globe, each capturing data from the target at various points during the night as the earth rotates," Levis says. "When the target we are observing changes during that time—as it did for Sgr A*—it makes the problem much more complicated."
To reconstruct an image, the EHT team developed computational imaging algorithms capable of making inferences to fill in the blanks in the data that had been gathered. The team discovered that multiple ring geometries could explain the Sgr A* data, as well as some geometries that do not look ring-like at all. To better understand how the imaging algorithms worked on data from an evolving target, Levis built a synthetic model of variable structure (akin to water swirling around a drain) that was based on statistics taken from the real data about Sgr A* that EHT had gathered. The model was then used to exhaustively test EHT's imaging methods and identify the best settings to recover an accurate image.
"Through literally years of exhaustive tests on both real and synthetic data we are now confident that there is compelling evidence that the true underlying source has a ring structure," Bouman says.
Although non-ring geometries cannot be fully discarded, the statistical analysis performed by the team suggests that they are very unlikely -- the EHT results provide compelling evidence that the Sgr A* image is dominated by a bright emission ring about 50 microarcseconds in diameter, Bouman says.
After determining that Sgr A*'s image geometry is likely a ring, the EHT used a number of cutting-edge tools to estimate the uncertainty in the recovered structure. Former postdoc Sun, now starting a faculty position at Peking University, used a deep-learning method developed at Caltech to analyze the data and quantify various aspects of the black hole, including its diameter and the amount of asymmetry in the ring of bright gas surrounding it.
"We not only recovered an image of Sgr A*, but also characterized the uncertainty of features in the image," says Sun. "This analysis helped the team deliver scientific results with some guarantees."
Next Steps
The next step in future work is to make a movie of the black hole, showing it as it changes over time, which could yield insight into the way gas behaves as it swirls around a black hole, and would also help estimate the spin of the black hole itself. (Static images give an estimate of the mass, but if scientists know both the mass and the spin of the black hole they can better test Einstein's theory of general relativity and the theory that a black hole's spin and mass should fully describe how it affects its environment.)
In October, EHT member Antonio Fuentes, currently at Instituto de Astrofísica de Andalucía-CSIC in Spain, will join Caltech as a postdoctoral researcher. Fuentes and Bouman are currently developing methods that will allow them to piece together snapshots of Sgr A* to form a movie by imposing frame-to-frame continuity.
That work has already begun to yield results: By running these methods on 100-minute chunks of the Sgr A* data from April 6 and 7 of 2017, the researchers were able to create a number of different movies that could possibly fit the observed data. This analysis revealed that, although Sgr A* appeared fairly static on April 6 during these 100 minutes, during the same time period on April 7 it showed possible signs of significant evolution. However, while the EHT data shows detectable signs of image variability, the current data does not reliably constrain the underlying image evolution.
"This analysis provides a promising starting point for future studies of evolution seen in Sgr A* EHT observations when we have the opportunity to observe with more telescopes," Bouman says.
Moving forward, a portion of the EHT team, dubbed ngEHT (for Next Generation Event Horizon Telescope), has been awarded funding from the National Science Foundation (NSF) to determine the best locations for telescopes to create an Earth-sized lens through which to view the universe. Already, Caltech's Owens Valley Radio Observatory (OVRO) has joined the EHT team, with Vikram Ravi, assistant professor of astronomy at Caltech, leading the effort to include one of OVRO's 10-meter radio dishes in the EHT network by 2024. EHT is funded by multiple agencies, including the NSF.
METHOD OF RESEARCH
Observational study
ARTICLE TITLE
There are 10 articles, all under the heading "Focus on First Sgr A* Results from the Event Horizon Telescope"
ARTICLE PUBLICATION DATE
12-May-2022
Sagittarius A* revealed
Astronomers reveal first image of the black hole at the heart of our galaxy
Reports and ProceedingsIMAGE: 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. view more
CREDIT: EVENT HORIZON TELESCOPE COLLABORATION
UC Santa Barbara and Las Cumbres Observatory graduate student Joseph Farah participated this morning in the press conference in Washington D.C. this morning, where astronomers 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 called the Event Horizon Telescope (EHT) Collaboration, using observations from a worldwide network of radio telescopes.
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.
Although we cannot see the black hole itself, because it is completely dark, glowing gas around it 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.
“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 are being published today in a special issue of The Astrophysical Journal Letters.
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 [1]. The EHT observed Sgr A* on multiple nights, collecting data for many hours in a row, similar to using a long exposure time on a camera.
Farah devised a new technique for producing a dynamical movie representation of the black hole Sgr A*. He is the lead author on the paper released today, Selective dynamical imaging of interferometric data, in the special issue of The Astrophysical Journal Letters. Now a member in the lab of UCSB/LCO astronomy professor Andy Howell, Farah conducted much of the work for this project as an undergrad at University of Massachusetts, Boston.
The breakthrough follows the EHT collaboration’s 2019 release of the first image of a black hole, called M87*, at the center of the more distant Messier 87 galaxy.
The two black holes look remarkably similar, even though our galaxy’s black hole is more than a thousand times smaller and less massive than M87* [2]. "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 amazingly similar,” says Sera Markoff, co-chair of the EHT Science Council and a professor of theoretical astrophysics at the University of Amsterdam, the Netherlands. "This tells us that General Relativity governs these objects up close, and any differences we see further away must be due to differences in the material that surrounds the black holes.”
This achievement was considerably more difficult than for M87*, even though Sgr A* is much closer to us. EHT scientist Chi-kwan (‘CK’) Chan, from Steward Observatory and Department of Astronomy and the Data Science Institute of the University of Arizona, explains: “The gas in the vicinity of the black holes moves at the same speed — nearly as fast as light — around both Sgr A* and M87*. But where gas takes days to weeks to orbit the larger M87*, in the much smaller Sgr A* it completes an orbit in mere minutes. This means the brightness and pattern of the gas around Sgr A* was changing rapidly as the EHT Collaboration was observing it — a bit like trying to take a clear picture of a puppy quickly chasing its tail.”
The researchers had to develop sophisticated new tools that accounted for the gas movement around Sgr A*. While M87* was an easier, steadier target, with nearly all images looking the same, that was not the case for Sgr A*. The image of the Sgr A* black hole is an average of the different images the team extracted, finally revealing the giant lurking at the center of our galaxy for the first time.
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. Mr. Farah was part of the team that produced the image of the black hole M87* and he applied his knowledge from that work to developing the tools to generate dynamic images of Sgr A*.
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.
“Now we can study the differences between these two supermassive black holes to gain valuable new clues about how this important process works,” said EHT scientist Keiichi Asada from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei. “We have images for two black holes — one at the large end and one at the small end of supermassive black holes in the Universe — so we can go a lot further in testing how gravity behaves in these extreme environments than ever before.”
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.
Farah is excited about the success of this endeavor, “I can’t believe how quickly the project grew from analyzing the exciting-but-preliminary initial datasets to seeing the beautiful shadow reconstruct before our eyes. It’s the only thing that seems to evolve faster than Sgr A*! What a privilege it has been to be a part of this wonderful project, with all of our wonderful collaborators.” He is grateful to the National Science Foundation for supporting his work through a graduate research fellowship.
Making of the image of the black hole at the center of the Milky Way
The Event Horizon Telescope (EHT) Collaboration has created a single image (top frame) of the supermassive black hole at the center of our galaxy, called Sagittarius A* (or Sgr A* for short), by combining images extracted from the EHT observations.
The main image was produced by averaging together thousands of images created using different computational methods — all of which accurately fit the EHT data. This averaged image retains features more commonly seen in the varied images, and suppresses features that appear infrequently.
The images can also be clustered into four groups based on similar features. An averaged, representative image for each of the four clusters is shown in the bottom row. Three of the clusters show a ring structure but, with differently distributed brightness around the ring. The fourth cluster contains images that also fit the data but do not appear ring-like.
The bar graphs show the relative number of images belonging to each cluster. Thousands of images fell into each of the first three clusters, while the fourth and smallest cluster contains only hundreds of images. The heights of the bars indicate the relative "weights," or contributions, of each cluster to the averaged image at top.
Notes
[1] The individual telescopes involved in the EHT in April 2017, when the observations were conducted, were: the Atacama Large Millimeter/submillimeter Array (ALMA), the Atacama Pathfinder Experiment (APEX), the IRAM 30-meter Telescope, the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope Alfonso Serrano (LMT), the Submillimeter Array (SMA), the UArizona Submillimeter Telescope (SMT), the South Pole Telescope (SPT). Since then, the EHT has added the Greenland Telescope (GLT), the NOrthern Extended Millimeter Array (NOEMA) and the UArizona 12-meter Telescope on Kitt Peak to its network.
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. The Joint ALMA Observatory is operated by ESO, the Associated Universities, Inc./National Radio Astronomy Observatory (AUI/NRAO) and the National Astronomical Observatory of Japan (NAOJ). APEX, a collaboration between the Max Planck Institute for Radio Astronomy (Germany), the Onsala Space Observatory (Sweden) and ESO, 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 JCMT is operated by the East Asian Observatory on behalf of the Center for Astronomical Mega-Science of the Chinese Academy of Sciences, NAOJ, ASIAA, KASI, the National Astronomical Research Institute of Thailand, and organizations in the United Kingdom and Canada. The LMT is operated by INAOE and UMass, the SMA is operated by Center for Astrophysics | Harvard & Smithsonian and ASIAA and the UArizona SMT is operated by the University of Arizona. The SPT is operated by the University of Chicago with specialized EHT instrumentation provided by the University of Arizona.
The Greenland Telescope (GLT) is operated by ASIAA and the Smithsonian Astrophysical Observatory (SAO). The GLT is part of the ALMA-Taiwan project, and is supported in part by the Academia Sinica (AS) and MOST. NOEMA is operated by IRAM and the UArizona 12-meter telescope at Kitt Peak is operated by the University of Arizona.
[2] Black holes are the only objects we know of where mass scales with size. A black hole a thousand times smaller than another is also a thousand times less massive.
Astronomers reveal first image of the black hole at the heart of our galaxy
First direct visual evidence – ring-like structure like M87* - Theoretical Physicists of Goethe University Frankfurt instrumental in interpreting the data
Peer-Reviewed PublicationIMAGE: EXAMPLE OF A SIMULATION OF HOW THE GAS ORBITS THE BLACK HOLE IN THE CENTER OF OUR MILKY WAY AND EMITS RADIO WAVES AT 1.3 MM. view more
CREDIT: YOUNSI, FROMM, MIZUNO & REZZOLLA (UNIVERSITY COLLEGE LONDON, GOETHE UNIVERSITY FRANKFURT
FRANKFURT. The image is a long-anticipated look at the massive object that sits at the very centre of our galaxy. Scientists had previously seen stars orbiting around something invisible, compact, and very massive at the centre 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.
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,” says 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 centre of our galaxy and offer new insights on how these giant black holes interact with their surroundings.”
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 [1]. The EHT observed Sgr A* on multiple nights, collecting data for many hours in a row, similar to using a long exposure time on a camera.
The enormous amount of observational data collected had to be interpreted theoretically. For this, a research team led by theoretical astrophysicist Luciano Rezzolla from Goethe University Frankfurt used supercomputers to simulate how a black hole could look like when observed by the EHT – based on what had already been known about Sgr A*. In this way, the scientists created a library of millions of images. Then, they compared this image library with the thousands of different images of the EHT to deduce the properties of Sgr A*.
The breakthrough follows the EHT Collaboration’s 2019 release of the first image of a black hole, called M87*, at the centre of the more distant Messier 87 galaxy.
The two black holes look remarkably similar, even though our galaxy’s black hole is more than a thousand times smaller and less massive than M87* [2]. “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 amazingly similar,” says Sera Markoff, Vice Chair of the EHT Science Council and a professor of theoretical astrophysics at the University of Amsterdam, the Netherlands. “This tells us that general relativity governs these objects up close, and any differences we see further away must be due to differences in the material that surrounds the black holes.”
This achievement was considerably more difficult than for M87*, even though Sgr A* is much closer to us. EHT scientist Chi-kwan (‘CK’) Chan, from Steward Observatory, the Department of Astronomy and the Data Science Institute at the University of Arizona, US, explains: “The gas in the vicinity of the black holes moves at the same speed — nearly as fast as light — around both Sgr A* and M87*. But where gas takes days to weeks to orbit the larger M87*, in the much smaller Sgr A* it completes an orbit in mere minutes. This means the brightness and pattern of the gas around Sgr A* was changing rapidly as the EHT Collaboration was observing it — a bit like trying to take a clear picture of a puppy quickly chasing its tail.”
The researchers had to develop sophisticated new tools that accounted for the gas movement around Sgr A*. While M87* was an easier, steadier target, with nearly all images looking the same, that was not the case for Sgr A*. The image of the Sgr A* black hole is an average of the different images the team extracted, finally revealing the giant lurking at the centre of our galaxy for the first time.
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.
Luciano Rezzolla, professor of Theoretical Astrophysics at Goethe University Frankfurt, explains: “The mass and distance of the object were known very precisely before our observations. We thus used these tight constraints on the size of the shadow to rule out other compact objects – such as boson stars or wormholes – and conclude that: ‘What we're seeing definitely looks like a black hole!’”
Using advanced numerical codes, theorists in Frankfurt have performed extensive calculations on the properties of the plasma accreting onto the black hole. Rezzolla: “We managed to calculate three million synthetic images varying the accretion and radiation emission models, and considering the variations seen by observers at different inclinations with respect to the black hole.”
This last operation was necessary because the image of a black hole can be radically different when seen by observers at different inclinations. “Indeed, a reason why our images of Sgr A* and M87* are rather similar is because we’re seeing the two black holes from an almost identical angle,” Rezzolla explains.
“To understand how the EHT has produced an image of Sgr A* one can think of producing a picture of a mountain peak based on a time-lapse video. While most of the time the peak will be visible in the time-lapse video, there are times when it is not because it is obscured by clouds. On average, however, the peak is clearly there. Something similar is true also for Sgr A*, whose observations lead to thousands of images which have been collected in four classes and then averaged according to their properties. The end result is a clear first image of the black hole at the centre of the Milky Way.” Rezzolla concludes.
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.
“Now we can study the differences between these two supermassive black holes to gain valuable new clues about how this important process works,” says EHT scientist Keiichi Asada from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei. “We have images for two black holes — one at the large end and one at the small end of supermassive black holes in the Universe — so we can go a lot further in testing how gravity behaves in these extreme environments than ever before.”
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 videos of black holes in the near future.
To Goethe University are associated a number of scientists in the EHT Collaboration. Together with Professor Luciano Rezzolla, Dr Alejandro Cruz Orsorio, Dr Prashant Kocherlakota and Kotaro Moriyama, also Prof Mariafelicia De Laurentis (University of Naples), Dr Christian Fromm (University of Würzburg), Prof Roman Gold (University of Southern Denmark), Dr Antonios Nathanail (University of Athens), and Dr Ziri Younsi (University College London) have provided essential contributions to the theoretical research in the EHT Collaboration.
This work has been supported by the European Research Council.
Notes:
[1] The individual telescopes involved in the EHT in April 2017, when the observations were conducted, were: the Atacama Large Millimeter/submillimeter Array (ALMA), the Atacama Pathfinder Experiment (APEX), the IRAM 30-meter Telescope, the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope Alfonso Serrano (LMT), the Submillimeter Array (SMA), the UArizona Submillimeter Telescope (SMT), the South Pole Telescope (SPT). Since then, the EHT has added the Greenland Telescope (GLT), the NOrthern Extended Millimeter Array (NOEMA) and the UArizona 12-meter Telescope on Kitt Peak to its network.
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. The Joint ALMA Observatory is operated by ESO, the Associated Universities, Inc./National Radio Astronomy Observatory (AUI/NRAO) and the National Astronomical Observatory of Japan (NAOJ). APEX, a collaboration between the Max Planck Institute for Radio Astronomy (Germany), the Onsala Space Observatory (Sweden) and ESO, 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 JCMT is operated by the East Asian Observatory on behalf of the Center for Astronomical Mega-Science of the Chinese Academy of Sciences, NAOJ, ASIAA, KASI, the National Astronomical Research Institute of Thailand and organizations in the United Kingdom and Canada. The LMT is operated by INAOE and UMass, the SMA is operated by Center for Astrophysics | Harvard & Smithsonian and ASIAA, and the UArizona SMT is operated by the University of Arizona. The SPT is operated by the University of Chicago with specialized EHT instrumentation provided by the University of Arizona.
The Greenland Telescope (GLT) is operated by ASIAA and the Smithsonian Astrophysical Observatory (SAO). The GLT is part of the ALMA-Taiwan project, and is supported in part by the Academia Sinica (AS) and MOST. NOEMA is operated by IRAM and the UArizona 12-meter telescope at Kitt Peak is operated by the University of Arizona.
[2] Black holes are the only objects we know of where mass scales with size. A black hole a thousand times smaller than another is also a thousand times less massive.
Youtube-Playlist Black Hole
Find further animations on how the picture of the black hole in the center of our galaxy was made on the Goethe University’s playlist „Black Hole“
https://youtube.com/playlist?list=PLn5gYfEKIag8nps1GKLqUW35AOgQY7aM2
Further pictures and video clips provided by EHT Collaboration:
https://eventhorizontelescope.teamwork.com/#notebooks/240600 (Animationen)
https://eventhorizontelescope.teamwork.com/#notebooks/240540 (Bilder)
Websites
https://eventhorizontelescope.org/ EHT Website
https://blackholecam.org/ Black Hole Cam-Project
JOURNAL
The Astrophysical Journal Letters
METHOD OF RESEARCH
Observational study
ARTICLE TITLE
First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way
ARTICLE PUBLICATION DATE
12-May-2022
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