From ESOblog (EU): “Spot the difference- Imaging Sagittarius A* and Messier 87*”
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12 May 2022
Science@ESO
Biography Anita Chandran
Anita Chandran is a science journalism intern at ESO. She recently completed her PhD in laser physics at Imperial College London (UK), where she also worked with the science communication department on the ethics of artificial intelligence. She is a writer and editor, having co-founding Tamarind, a literary magazine focusing on the intersections between the arts and sciences.
For decades, scientists have gotten closer and closer to the dark heart at the centre of our Milky Way galaxy, the supermassive black hole known as Sagittarius A* (Sgr A*). Now, as a result of the incredible work of the Event Horizon Telescope (EHT) Collaboration, we finally have a long-awaited image of Sagittarius A*, providing the first visual confirmation of the black hole at our galactic centre.
Sagittarius A* is the second black hole to be imaged by the EHT, with the first being Messier 87*, located at the centre of the Messier 87 galaxy, first imaged in 2019.
Messier 87*, The first image of the event horizon of a black hole. This is the supermassive black hole at the center of the galaxy Messier 87. Image via The Event Horizon Telescope Collaboration released on 10 April 2019 via National Science Foundation.
But how did we arrive at these images, and what can we learn from their similarities and differences?
Imaging a black hole is no easy task. Black holes are completely dark, meaning that taking a direct image of them is impossible. Instead, scientists must look at the glowing gas and dust that surround them. Even though supermassive black holes are intrinsically large, they’re so far away from us that their apparent size on the sky is extremely small, so small that no conventional telescope can discern their shape. To solve this problem, the EHT combines several radio telescopes located world-wide to form one huge, Earth-sized telescope, using a technique called Very Long Baseline interferometry.
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Event Horizon Telescope Array
EHT map.
The locations of the radio dishes that will be part of the Event Horizon Telescope array. Image credit: Event Horizon Telescope sites, via University of Arizona at https://www.as.arizona.edu/event-horizon-telescope.
Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)
Atacama Pathfinder EXperiment (APEX)
Combined Array for Research in Millimeter-wave Astronomy (CARMA)
Atacama Submillimeter Telescope Experiment (ASTE)
Caltech Submillimeter Observatory (CSO)
Institut de Radioastronomie Millimetrique (IRAM) 30m
James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
Large Millimeter Telescope Alfonso Serrano
Submillimeter Array Hawaii SAO
ESO/NRAO/NAOJ ALMA Array, Chile
Plateau de Bure interferometer
About the Event Horizon Telescope (EHT)
The EHT consortium consists of 13 stakeholder institutes; The Academia Sinica Institute of Astronomy & Astrophysics [中央研究院天文及天文物理研究所](TW) , The University of Arizona, The University of Chicago, The East Asian Observatory, Goethe University Frankfurt [Goethe-Universität](DE), Institut de Radioastronomie Millimétrique, Large Millimeter Telescope, The MPG Institute for Radio Astronomy[MPG Institut für Radioastronomie](DE), MIT Haystack Observatory, The National Astronomical Observatory of Japan[[国立天文台](JP), The Perimeter Institute for Theoretical Physics (CA), Radboud University [Radboud Universiteit](NL) and The Center for Astrophysics | Harvard & Smithsonian.
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Imaging Sagittarius A* proved challenging for scientists. In this blog, we spoke to three of the scientists in the EHT Collaboration to understand some of the similarities and differences between the images of Sagittarius A* and Messier 87*, as well as getting to the bottom of why imaging our galaxy’s central black hole was so difficult.
Why do the images of Sagittarius A* and Messier 87* look so similar, when we know the black holes are very different?
“There are two important things to note,” says Kazu Akiyama, an astrophysicist at MIT Haystack Observatory in the US. “Firstly, both images have a circular ring. Secondly, the rings are broadly speaking symmetric: they appear closed, without gaps.”
“The circularity is a beautiful result of the extremely strong gravity at the edge of a black hole. The size and shape of the black hole’s silhouette is governed simply by how its strong gravity bends the surrounding space-time. Einstein’s theory of relativity predicts that the shape of the silhouette is pretty circular no matter how fast the black hole is rotating or from where we are looking at it.”
You may think that the circular shadow that we see in the EHT images corresponds to the event horizon –– the surface beyond which not even light can escape. But that’s not the case. As the video below shows, the black hole bends the trajectory of light around it, casting a shadow that is about 2.5 times larger than the event horizon itself.
This artist’s impression zooms into a black hole and depicts the paths of photons in its vicinity. The gravitational bending and capture of light by the event horizon is the cause of the shadow captured by the Event Horizon Telescope.
Credit: Nicolle R. Fuller/The National Science Foundation.
Hot matter orbits the black hole in a disc, and the orientation of this disc will determine whether the bright ring looks symmetric or not. “If the rotational axis of the matter around the black hole is tilted relative to our line of sight,” Kazu says, “the bright ring enclosing the shadow of the black hole will be asymmetric: the side where matter is approaching us will be brighter while the receding side will be dimmer. If the tilt is very pronounced we would see an unclosed ring where only a single side is visible, like a waxing crescent moon.”
“I was indeed surprised and didn’t even expect to see such a closed ring for Sgr A*,” says Kazu. “There was no guarantee that it should be. This second ring was basically a gift from nature.”
Black holes can also emit strong jets of material perpendicularly to the disc, which give astronomers a clue as to the orientation of the disc. Unfortunately, Sgr A* has been relatively quiet on this front.
“In contrast with Messier 87*, which has a prominent jet indicating the orientation of the system, Sagittarius A* has never shown us a clearly visible jet,” says Katie Bouman, an assistant professor at The California Institute of Technology (Caltech). “For this reason, Sagittarius A* has been hiding its orientation from us for nearly 50 years until it was finally imaged with the EHT. As a symmetric ring appears in our image, we found that the theoretical models can explain EHT data only when the rotational axis is tilted within 30 degrees from our line of sight.”
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So, what made imaging Sagittarius A* so difficult compared with Messier 87*?
“There were two primary challenges in imaging Sagittarius A*, arising from its unique properties,” says José L. Gómez, a Research Scientist at The Institute of Astrophysics of Andalusia [Instituto de Astrofísica de Andalucía] – CSIC (ES). “Firstly, Sagittarius A* is about 1600 times lighter and smaller than Messier 87*. Material takes days or weeks to travel around Messier 87* but just minutes or hours to get around Sagittarius A*. The EHT fills up a gigantic, planet-sized virtual mirror using the Earth’s rotation. Each individual EHT telescope is like a segment of this mirror, gradually obtaining data over a night. M87* was easier to image, as it was steady when we were observing: the data obtained by all individual telescopes came from the same stable image.”
“However, Sagittarius A* is like a toddler who can’t stay standing still while we are taking its photo over the course of a night,” continues Katie. “The glowing gas around Sagittarius A* was dancing while we were taking the data. We needed to reconstruct how Sagittarius A* appears in the sky from a series of sparse information obtained while the gas was quickly moving around.”
José L. continues: “The second challenge is that radio signals from Sagittarius A* become somewhat distorted and blurred by turbulent gas located between Earth and the Galactic centre. The image of Sagittarius A* ripples like a distant mountain seen from a window of an airplane through its hot jet exhaust fumes. To see a true image of Sagittarius A*, we need to reconstruct the image as it was before such turbulent gas scattered the radio waves.”
“This was not the case for Messier 87*,” adds Kazu, “which is located far away from the plane of the Milky Way, and therefore we did not have to image it through the dense material in our galaxy.”
Meet Sagittarius A*: Zooming into the black hole at the centre of our galaxy.
Watch as this video sequence zooms into the black hole (Sgr A*) at the centre of our galaxy.
Credit: L. Calçada, N. Risinger (skysurvey.org)/ESO, DSS, VISTA, VVV Survey/D. Minniti DSS, Nogueras-Lara et al., Schoedel, NACO, GRAVITY Collaboration, EHT Collaboration (Music: Azul Cobalto)
How did you address these problems?
“For the scattering effects, we carefully examined the optics of propagation of radio waves from Sagittarius A* through the turbulent plasma that scatters them,” says Kazu. “Over the last several years, EHT scientists have built up a sophisticated model of the scattering for Sagittarius A* with an extensive series of observations at different wavelengths, and used it to assess how the distortion and blurring of Sagittarius A* appears in the EHT data. This allows us to reconstruct the image of Sagittarius A*.”
“As for the rapid motion of the gas around Sagittarius A*,” says José L., “we indeed saw in the EHT data that the glowing gas around Sagittarius A* is moving and dancing around during our observations. It is truly stunning that the EHT could see such complicated dynamics of the gas around a black hole, giving us a very important clue about how black holes interact with their surrounding environment.”
“We did a careful statistical analysis of the data to see how the appearance of Sagittarius A* changes from night to night,” explains Katie, “providing a statistical model of the dynamical structure in Sagittarius A*. This allows us to image the common structure of Sagittarius A* seen across the nights.”
As evidenced from the above answers, it is clear that a huge amount of analysis was undertaken to obtain an image of Sagittarius A*. And some very well developed algorithms were required for this analysis.
Could you describe how the different algorithms that you used work?
“The EHT creates the sharpest eye on the universe by computationally forming a planet-sized virtual telescope with radio telescopes spread across the Earth,” says Katie. “But we have only eight telescopes on six geographic sites, which provide a sparse coverage of telescopes that can fill up only a tiny fraction of this gigantic virtual mirror. It’s like having a fraction of the pieces of a jigsaw puzzle, from which we need to reconstruct what the entire picture of the puzzle looks like. Or like listening to a song from a piano where many of the keys are missing.”
“Because we have such limited information, you can imagine that there are an infinite number of ways to fill up the missing pieces of the puzzle,” continues Kazu, “though most of them wouldn’t make any sense, just like randomly adding a piece to a puzzle or a note to a song results in something awkward and very unlikely.”
“Imaging algorithms are mathematical detectives that we use to find the most reasonable images among the many possibilities that can explain the EHT’s measurements,” Katie explains. “Each algorithm has its own method for determining which image is most likely. It is a bit like hiring Sherlock Holmes, Hercule Poirot, Jane Marple, and Jules Maigret simultaneously to see what they commonly conclude and what they don’t.”
How accurately can the algorithms recover the true shape of Sagittarius A* or any other source?
“We vetted and evaluated each way of imaging EHT data according to whether it can distinguish and recover different shapes,” says Kazu. “including non-ring structures that may provide data similar to EHT data. Thousands of Sagittarius A* images were made from well-vetted methods that can distinguish different structures, and the vast majority show a ring shape.“
“The rapidly varying emission from Sagittarius A* and the limited amount of data introduced some uncertainty in where the brightest parts of the ring were,” continues José L. “The ongoing expansion of the EHT network and significant technological upgrades will allow us to better constrain the emission around the ring, and even obtain the first movies of black holes.”
The Event Horizon Telescope (EHT) Collaboration has created a single image (top frame) of the supermassive black hole at the centre 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.
In addition to other facilities, the EHT network of radio observatories that made this image possible includes the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder EXperiment (APEX) in the Atacama Desert in Chile, co-owned and co-operated by ESO is a partner on behalf of its member states in Europe. Credit: EHT Collaboration.
I can’t help but ask, how confident are you that the image is a ring? What could lead to a non-ring image?
“We have spent years developing sophisticated new tools to account for the challenges in imaging Sagittarius A*,” José L. told us. “Tens of millions of images from test data (created to best resemble Sagittarius A*) have been produced in supercomputers around the world to refine our algorithms, much like searching for the best lens and filter in a camera to obtain the sharpest snapshot.”
“We found that our different algorithms can reliably distinguish between ring and non-ring images,” Katie adds. “Most importantly, as we found when imaging Sagittarius A*, only a very small fraction of non-ring images appear. Based on this detailed study we conclude that the non-ring images are caused by the limited amount of data and rapidly varying emission from Sagittarius A*, rather than being intrinsic to the black hole itself.”
“The image of Sagittarius A* now holds the record (previously set by Messier 87*) of the most thoroughly vetted interferometric image that has ever been made,” José L. concludes.
Now that we have imaged Sagittarius A*, what’s next for the EHT?
“This is just the dawn of the exciting era opened by the EHT,” José L. tells us. “The EHT has been getting more powerful since 2017 when we obtained data providing these first images of black holes. Now, the EHT has a more sensitive and sharper eye, with new additional telescopes and upgraded instruments. We will work on new data from the upgraded EHT in the coming years, which will provide a sharper and more dynamic view of black holes.”
“Of course challenges lie ahead too,” Kazu explains. “As the EHT becomes more powerful, we will have much richer data. The sheer volume of EHT data, which already totaled 5 petabytes in 2017, has grown even more since then. How we handle this and hunt for treasures hiding in such enormous datasets will be our next exciting challenge—with accordingly enormous rewards.”
“The EHT is also looking at other supermassive black holes, investigating the hot plasma in their vicinity. This could one day lead to an eventual third image of a black hole — but only if we have underestimated the sizes of these black holes; if they are too small, we won’t be able to discern their shadows.”
Of course, just the story behind the image of Sagittarius A* is one for the history books, and we asked our three interviewees what advice they’d give to future astronomers.
“Dream big, stay curious, and don’t let anyone tell you that you can’t succeed,” says Katie. “If you set your mind to it, and work hard, oftentimes you can achieve what originally sounded impossible. Remember that you can be your own worst enemy –– once you start doubting yourself then the game is over. It’s normal to feel unsure of yourself, but just put those blinders on and plow ahead!”
José L. agrees: “Follow your dreams! It is your imagination, and the fascination to understand how the Universe works. These are the only things you need to make every day of your life a journey of discovery.”
“Don’t be afraid of the risk of failure,” Kazu adds. “With the last ten years of my life at the EHT, I learned that whatever happens and however it ends up, the entire experience you will get will be a very fruitful element of your life.”
About the EHT scientists
Now iconic image of Katie Bouman-Harvard Smithsonian Astrophysical Observatory after the image of Messier 87* was achieved. Headed from Harvard to Caltech as an Assistant Professor. On the committee for the next iteration of the EHT .
Katherine L. (Katie) Bouman is an assistant professor in the Computing and Mathematical Sciences, Electrical Engineering, and Astronomy Departments at the California Institute of Technology (Caltech). Before joining Caltech, she was a postdoctoral fellow in the Harvard-Smithsonian Center for Astrophysics. She received her Ph.D. in The Computer Science and Artificial Intelligence Laboratory (CSAIL) at MIT in EECS, and her bachelor’s degree in Electrical Engineering from The University of Michigan. She is a Rosenberg Scholar, Heritage Medical Research Institute Investigator, recipient of the Royal Photographic Society Progress Medal and IST Electronic Imaging Scientist of the Year Award. As part of the Event Horizon Telescope Collaboration, she is co-organizer of the Imaging Working Group and acted as coordinator for papers concerning the first imaging of the M87* and Sagittarius A* black holes. She is also co-organizer of the Algorithms and Inference Working Group for the next-generation Event Horizon Telescope (ngEHT) effort.
Kazu Akiyama
Kazu Akiyama is a research scientist at MIT Haystack Observatory. He joined the team of the Event Horizon Telescope (EHT) in 2010, and has worked on various aspects of EHT observations, from the development of new imaging algorithms to the calibration, imaging, and scientific interpretation of EHT data. He co-founded the Imaging Working Group of the EHT Collaboration in 2017, and has been a co-leader of the group since its establishment. As a lead of the EHT’s imaging team, he has coordinated the first imaging of two primary targets: M87* and Sgr A*. He also served as a coordinator of the EHT Sgr A* data calibration team. He developed one of the imaging software package, SMILI, used to create the first images of M87* and Sgr A*.
José L. Gómez is a Research Scientist, and head of the EHT group at the Instituto de Astrofísica de Andalucía (CSIC), in Granada, Spain. His research career has been focused on the study of relativistic jets often seen emanating as a byproduct of black hole accretion. He is a member of the EHT Science Council, and one of the coordinators of the EHT Imaging Working Group. He developed the scripts based on the classical CLEAN method for imaging interferometric data used for imaging M87* and Sgr A*, being one of the coordinators of the imaging papers for these two EHT primary targets. He also participates actively in other EHT scientific activities, in particular in the Polarisation, AGN, and Multi-wavelength Working Groups.
See the full article here.
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European Southern Observatory(EU) , Very Large Telescope at Cerro Paranal in the Atacama Desert •ANTU (UT1; The Sun ) •KUEYEN (UT2; The Moon ) •MELIPAL (UT3; The Southern Cross ), and •YEPUN (UT4; Venus – as evening star). Elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo.
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