Black Hole and its Shadow - first direct visual evidence of a supermassive black hole
A long standing goal in astrophysics is to directly observe the immediate environment of a black hole with an angular resolution comparable to the event horizon. Such observations could lead to images of strong gravity effects that are expected near a black hole, and to the direct detection of dynamics near the black hole as matter orbits at near light speeds. This capability would open a new window on the study of general relativity in the strong field regime, accretion and outflow processes at the edge of a black hole, the existence of event horizons, and fundamental black hole physics. 1)
The EHT (Event Horizon Telescope) is an international collaboration that has formed to continue the steady long-term progress on improving the capability of VLBI (Very Long Baseline Interferometry) at short wavelengths in pursuit of this goal. This technique of linking radio dishes across the globe to create an Earth-sized interferometer, has been used to measure the size of the emission regions of the two supermassive black holes with the largest apparent event horizons: SgrA* (Sagittarius A*) at the center of the Milky Way and M87 (Messier 87) in the center of the Virgo A galaxy. In both cases, the sizes match that of the predicted silhouette caused by the extreme lensing of light by the black hole. Addition of key millimeter and submillimeter wavelength facilities at high altitude sites has now opened the possibility of imaging such features and sensing the dynamic evolution of black hole accretion. The EHT project includes theoretical and simulation studies that are framing questions rooted at the black hole boundary that may soon be answered through observations.
By linking together existing telescopes using novel systems, the EHT leverages considerable global investment to create a fundamentally new instrument with an angular resolving power that is the highest possible from the surface of the Earth. Over the coming years, the international EHT team will mount observing campaigns of increasing resolving power and sensitivity, aiming to bring black holes into focus.
Black Hole events and discovery status
- When two black holes spiral around each other and ultimately collide, they send out gravitational waves - ripples in space and time that can be detected with extremely sensitive instruments on Earth. Since black holes and black hole mergers are completely dark, these events are invisible to telescopes and other light-detecting instruments used by astronomers. However, theorists have come up with ideas about how a black hole merger could produce a light signal by causing nearby material to radiate.
- Now, scientists using Caltech's Zwicky Transient Facility (ZTF) located at Palomar Observatory near San Diego may have spotted what could be just such a scenario. If confirmed, it would be the first known light flare from a pair of colliding black holes.
- The merger was identified on May 21, 2019, by two gravitational wave detectors - the National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO) and the European Virgo detector - in an event called GW190521g. That detection allowed the ZTF scientists to look for light signals from the location where the gravitational wave signal originated. These gravitational wave detectors have also spotted mergers between dense cosmic objects called neutron stars, and astronomers have identified light emissions from those collisions.
- The ZTF results are described in a new study published in the journal Physical Review Letters. The authors hypothesize that the two partner black holes, each several dozen times more massive than the Sun, were orbiting a third, supermassive black hole that is millions of times the mass of the Sun and surrounded by a disk of gas and other material. When the two smaller black holes merged, they formed a new, larger black hole that would have experienced a kick and shot off in a random direction. According to the new study, it may have plowed through the disk of gas, causing it to light up. 4)
- "This detection is extremely exciting," said Daniel Stern, coauthor of the new study and an astrophysicist at NASA's Jet Propulsion Laboratory in Southern California, which is a division of Caltech. "There's a lot we can learn about these two merging black holes and the environment they were in based on this signal that they sort of inadvertently created. So the detection by ZTF, coupled with what we can learn from the gravitational waves, opens up a new avenue to study both black hole mergers and these disks around supermassive black holes."
Figure 1: This artist's concept shows a supermassive black hole surrounded by a disk of gas. Embedded in this disk are two smaller black holes that may have merged together to form a new black hole [image credit: Caltech/R. Hurt, IPAC (Infrared Processing and Analysis Center)]
- The scientists attempted to get a more detailed look at the light of the supermassive black hole, called a spectrum, but by the time they looked, the flare had already faded. A spectrum would have offered more support for the idea that the flare came from merging black holes within the disk of the supermassive black hole. However, the researchers say they were able to largely rule out other possible causes for the observed flare, including a supernova or a tidal disruption event, which occurs when a black hole essentially eats a star (Ref. 3).
- What is more, the team says it is not likely that the flare came from the usual rumblings of the supermassive black hole, which regularly feeds off its surrounding disk. Using the Catalina Real-Time Transient Survey, led by Caltech, they were able to assess the behavior of the black hole over the past 15 years, and found that its activity was relatively normal until May of 2019, when it suddenly intensified.
- "Supermassive black holes like this one have flares all the time. They are not quiet objects, but the timing, size, and location of this flare was spectacular," says co-author Mansi Kasliwal (MS '07, PhD '11), an assistant professor of astronomy at Caltech. "The reason looking for flares like this is so important is that it helps enormously with astrophysics and cosmology questions. If we can do this again and detect light from the mergers of other black holes, then we can nail down the homes of these black holes and learn more about their origins."
- The newly formed black hole should cause another flare in the next few years. The process of merging gave the object a kick that should cause it to enter the supermassive black hole's disk again, producing another flash of light that ZTF should be able to see.
• 15 May 2020: Mergers between black holes and neutron stars in dense star clusters are quite unlike those that form in isolated regions where stars are few. Their associated features could be crucial to the study of gravitational waves and their source. Dr Manuel Arca Sedda of the Institute for Astronomical Computing at Heidelberg University came to this conclusion in a study that used computer simulations. The research may offer critical insights into the fusion of two massive stellar objects that astronomers observed in 2019. The findings were published in the journal “Communications Physics”. 5) 6)
- Stars much more massive than our sun usually end their lives as a neutron star or black hole. Neutron stars emit regular pulses of radiation that allow their detection. In August 2017, for example, when the first double neutron star merger was observed, scientists all around the globe detected light from the explosion with their telescopes. Black holes, on the other hand, usually remain hidden because their gravitational attraction is so strong that even light cannot escape, making them invisible to electromagnetic detectors.
- If two black holes merge, the event may be invisible but is nonetheless detectable from ripples in space-time in the form of so-called gravitational waves. Certain detectors, like the LIGO (Laser Interferometer Gravitational Waves Observatory) in the USA, are able to detect these waves. The first successful direct observation was made in 2015. The signal was generated by the fusion of two black holes. But this event may not be the only source of gravitational waves, which could also come from the merger of two neutron stars or a black hole with a neutron star. Discovering the differences is one of the major challenges in observing these events, according to Dr Arca Sedda.
- In his study, the Heidelberg researcher analyzed the fusion of pairs of black holes and neutron stars. He used detailed computer simulations to study the interactions between a system made up of a star and a compact object, such as a black hole, and a third massive roaming object that is required for a fusion. The results indicate that such three-body interactions can in fact contribute to black hole-neutron star mergers in dense stellar regions like globular star clusters. “A special family of dynamic mergers that is distinctly different from mergers in isolated areas can be defined”, explains Manuel Arca Sedda.
- The fusion of a black hole with a neutron star was first observed by gravitational wave observatories in August 2019. Yet optical observatories around the world were unable to locate an electromagnetic counterpart in the region from which the gravitational wave signal originated, suggesting that the black hole had completely devoured the neutron star without first destroying it. If confirmed, this could be the first observed black hole-neutron star merger detected in a dense stellar environment, as described by Dr Arca Sedda.
Figure 2: Invisible black hole-neutron star mergers, i.e. fusions without the emission of electromagnetic radiation, take place in dense stellar environments like in the globular cluster NGC 3201 seen here (image credit: ESO)
• 14 May 2020: Once you leave the majestic skies of Earth, the word “cloud” no longer means a white fluffy-looking structure that produces rain. Instead, clouds in the greater universe are clumpy areas of greater density than their surroundings. 7)
- Space telescopes have observed these cosmic clouds in the vicinity of supermassive black holes, those mysterious dense objects from which no light can escape, with masses equivalent to more than 100,000 Suns. There is a supermassive black hole in the center of nearly every galaxy, and it is called an “active galactic nucleus” (AGN) if it is gobbling up a lot of gas and dust from its surroundings. The brightest kind of AGN is called a "quasar." While the black hole itself cannot be seen, its vicinity shines extremely bright as matter gets torn apart close to its event horizon, its point of no return.
- But black holes aren’t truly like vacuum cleaners; they don’t just suck up everything that gets too close. While some material around a black hole will fall directly in, never to be seen again, some of the nearby gas will be flung outward, creating a shell that expands over thousands of years. That’s because the area near the event horizon is extremely energetic; the high-energy radiation from fast-moving particles around the black hole can eject a significant amount of gas into the vastness of space.
- Scientists would expect that this outflow of gas would be smooth. Instead, it is clumpy, extending well beyond 1 parsec (3.3 light-years) from the black hole. Each cloud starts out small, but can expand to be more than 1 parsec wide — and could even cover the distance between Earth and the nearest star beyond the Sun, Proxima Centauri.
- Astrophysicist Daniel Proga at the University of Nevada, Las Vegas, likens these clumps to groups of cars waiting at a highway onramp with stoplights designed to regulate the influx of new traffic. “Every now and then you have a bunch of cars,” he said.
- What explains these clumps in deep space? Proga and colleagues have a new computer model that presents a possible solution to this mystery, published in the Astrophysical Journal Letters, led by doctoral student Randall Dannen. Scientists show that extremely intense heat near the supermassive black hole can allow the gas to flow outward really fast, but in a way that can also lead to clump formation. If the gas accelerates too quickly, it will not cool off enough to form clumps. The computer model takes these factors into account and proposes a mechanism to make the gas travel far, but also clump. 8)
Figure 3: This illustration depicts a quasar, a type of active galactic nucleus, surrounded by a dusty donut shape (torus) and clumps called “clouds.” These clouds start small but can expand to be more than 1 parsec (3.3 light-years) wide. In this diagram, the clouds are at least 1 parsec from the torus (image credit: Illustration by Nima Abkenar)
• 06 May 2020: ESO Instrument Finds Closest Black Hole to Earth. A team of astronomers from the European Southern Observatory (ESO) and other institutes has discovered a black hole lying just 1000 light-years from Earth. The black hole is closer to our Solar System than any other found to date and forms part of a triple system that can be seen with the naked eye. The team found evidence for the invisible object by tracking its two companion stars using the MPG/ESO 2.2-meter telescope at ESO’s La Silla Observatory in Chile. They say this system could just be the tip of the iceberg, as many more similar black holes could be found in the future. 9)
Figure 4: This artist’s impression shows the orbits of the objects in the HR 6819 triple system (a Milky Way star). This system is made up of an inner binary with one star (orbit in blue) and a newly discovered black hole (orbit in red), as well as a third object, another star, in a wider orbit (also in blue). The team originally believed there were only two objects, the two stars, in the system. However, as they analyzed their observations, they were stunned when they revealed a third, previously undiscovered body in HR 6819: a black hole, the closest ever found to Earth. The black hole is invisible, but it makes its presence known by its gravitational pull, which forces the luminous inner star into an orbit. The objects in this inner pair have roughly the same mass and circular orbits. The observations, with the FEROS spectrograph on the 2.2 m telescope at ESO’s La Silla, showed that the inner visible star orbits the black hole every 40 days, while the second star is at a large distance from this inner pair (image credit: ESO/L. Calçada)
- "We were totally surprised when we realized that this is the first stellar system with a black hole that can be seen with the unaided eye,” says Petr Hadrava, Emeritus Scientist at the Academy of Sciences of the Czech Republic in Prague and co-author of the research. Located in the constellation of Telescopium, the system is so close to us that its stars can be viewed from the southern hemisphere on a dark, clear night without binoculars or a telescope. “This system contains the nearest black hole to Earth that we know of,” says ESO scientist Thomas Rivinius, who led the study published today in Astronomy & Astrophysics. 10)
- Dietrich Baade, Emeritus Astronomer at ESO in Garching and co-author of the study, says: “The observations needed to determine the period of 40 days had to be spread over several months. This was only possible thanks to ESO’s pioneering service-observing scheme under which observations are made by ESO staff on behalf of the scientists needing them.”
- The hidden black hole in HR 6819 is one of the very first stellar-mass black holes found that do not interact violently with their environment and, therefore, appear truly black. But the team could spot its presence and calculate its mass by studying the orbit of the star in the inner pair. “An invisible object with a mass at least 4 times that of the Sun can only be a black hole,” concludes Rivinius, who is based in Chile.
- Astronomers have spotted only a couple of dozen black holes in our galaxy to date, nearly all of which strongly interact with their environment and make their presence known by releasing powerful X-rays in this interaction. But scientists estimate that, over the Milky Way’s lifetime, many more stars collapsed into black holes as they ended their lives. The discovery of a silent, invisible black hole in HR 6819 provides clues about where the many hidden black holes in the Milky Way might be. “There must be hundreds of millions of black holes out there, but we know about only very few. Knowing what to look for should put us in a better position to find them,” says Rivinius. Baade adds that finding a black hole in a triple system so close by indicates that we are seeing just “the tip of an exciting iceberg.”
- Already, astronomers believe their discovery could shine some light on a second system. “We realized that another system, called LB-1, may also be such a triple, though we'd need more observations to say for sure,” says Marianne Heida, a postdoctoral fellow at ESO and co-author of the paper. "LB-1 is a bit further away from Earth but still pretty close in astronomical terms, so that means that probably many more of these systems exist. By finding and studying them we can learn a lot about the formation and evolution of those rare stars that begin their lives with more than about 8 times the mass of the Sun and end them in a supernova explosion that leaves behind a black hole."
- The discoveries of these triple systems with an inner pair and a distant star could also provide clues about the violent cosmic mergers that release gravitational waves powerful enough to be detected on Earth. Some astronomers believe that the mergers can happen in systems with a similar configuration to HR 6819 or LB-1, but where the inner pair is made up of two black holes or of a black hole and a neutron star. The distant outer object can gravitationally impact the inner pair in such a way that it triggers a merger and the release of gravitational waves. Although HR 6819 and LB-1 have only one black hole and no neutron stars, these systems could help scientists understand how stellar collisions can happen in triple star systems.
• 16 April 2020: Observations made with ESO's Very Large Telescope (VLT) have revealed for the first time that a star orbiting the supermassive black hole at the center of the Milky Way moves just as predicted by Einstein's general theory of relativity. Its orbit is shaped like a rosette and not like an ellipse as predicted by Newton's theory of gravity. This long-sought-after result was made possible by increasingly precise measurements over nearly 30 years, which have enabled scientists to unlock the mysteries of the behemoth lurking at the heart of our galaxy. 11)
- “Einstein’s General Relativity predicts that bound orbits of one object around another are not closed, as in Newtonian Gravity, but precess forwards in the plane of motion. This famous effect — first seen in the orbit of the planet Mercury around the Sun — was the first evidence in favor of General Relativity. One hundred years later we have now detected the same effect in the motion of a star orbiting the compact radio source Sagittarius A* at the center of the Milky Way. This observational breakthrough strengthens the evidence that Sagittarius A* must be a supermassive black hole of 4 million times the mass of the Sun,” says Reinhard Genzel, Director at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany and the architect of the 30-year-long program that led to this result.
- Located 26,000 light-years from the Sun, Sagittarius A* and the dense cluster of stars around it provide a unique laboratory for testing physics in an otherwise unexplored and extreme regime of gravity. One of these stars, S2, sweeps in towards the supermassive black hole to a closest distance less than 20 billion kilometers (one hundred and twenty times the distance between the Sun and Earth), making it one of the closest stars ever found in orbit around the massive giant. At its closest approach to the black hole, S2 is hurtling through space at almost three percent of the speed of light, completing an orbit once every 16 years. “After following the star in its orbit for over two and a half decades, our exquisite measurements robustly detect S2’s Schwarzschild precession in its path around Sagittarius A*,” says Stefan Gillessen of the MPE, who led the analysis of the measurements published today in the journal Astronomy & Astrophysics. 12)
- Most stars and planets have a non-circular orbit and therefore move closer to and further away from the object they are rotating around. S2’s orbit precesses, meaning that the location of its closest point to the supermassive black hole changes with each turn, such that the next orbit is rotated with regard to the previous one, creating a rosette shape. General Relativity provides a precise prediction of how much its orbit changes and the latest measurements from this research exactly match the theory. This effect, known as Schwarzschild precession, had never before been measured for a star around a supermassive black hole.
Figure 5: This artist’s rendition illustrates the Schwarzschild precession of the star’s orbit, with the effect exaggerated for easier visualization (image credit: ESO/L. Calçada)
- The study with ESO’s VLT also helps scientists learn more about the vicinity of the supermassive black hole at the center of our galaxy. “Because the S2 measurements follow General Relativity so well, we can set stringent limits on how much invisible material, such as distributed dark matter or possible smaller black holes, is present around Sagittarius A*. This is of great interest for understanding the formation and evolution of supermassive black holes,” say Guy Perrin and Karine Perraut, the French lead scientists of the project.
- This result is the culmination of 27 years of observations of the S2 star using, for the best part of this time, a fleet of instruments at ESO’s VLT, located in the Atacama Desert in Chile. The number of data points marking the star’s position and velocity attests to the thoroughness and accuracy of the new research: the team made over 330 measurements in total, using the GRAVITY, SINFONI and NACO instruments. Because S2 takes years to orbit the supermassive black hole, it was crucial to follow the star for close to three decades, to unravel the intricacies of its orbital movement.
- The research was conducted by an international team led by Frank Eisenhauer of the MPE with collaborators from France, Portugal, Germany and ESO. The team make up the GRAVITY collaboration, named after the instrument they developed for the VLT Interferometer, which combines the light of all four 8-meter VLT telescopes into a super-telescope (with a resolution equivalent to that of a telescope 130 meters in diameter). The same team reported in 2018 another effect predicted by General Relativity: they saw the light received from S2 being stretched to longer wavelengths as the star passed close to Sagittarius A*. “Our previous result has shown that the light emitted from the star experiences General Relativity. Now we have shown that the star itself senses the effects of General Relativity,” says Paulo Garcia, a researcher at Portugal’s Center for Astrophysics and Gravitation and one of the lead scientists of the GRAVITY project.
- With ESO’s upcoming Extremely Large Telescope, the team believes that they would be able to see much fainter stars orbiting even closer to the supermassive black hole. “If we are lucky, we might capture stars close enough that they actually feel the rotation, the spin, of the black hole,” says Andreas Eckart from Cologne University, another of the lead scientists of the project. This would mean astronomers would be able to measure the two quantities, spin and mass, that characterize Sagittarius A* and define space and time around it. “That would be again a completely different level of testing relativity," says Eckart.
Figure 6: This simulation shows the orbits of stars very close to the supermassive black hole at the heart of the Milky Way. One of these stars, named S2, orbits every 16 years and is passing very close to the black hole in May 2018. This is a perfect laboratory to test gravitational physics and specifically Einstein's general theory of relativity (image credit: ESO/L. Calçada/spaceengine.org)
• 07 April 2020: One year ago, the Event Horizon Telescope (EHT) Collaboration published the first image of a black hole in the nearby radio galaxy M 87. Now the collaboration has extracted new information from the EHT data on the distant quasar 3C 279: they observed the finest detail ever seen in a jet produced by a supermassive black hole. New analyses, led by Jae-Young Kim from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, enabled the collaboration to trace the jet back to its launch point, close to where violently variable radiation from across the electromagnetic spectrum arises. 13)
- The EHT collaboration continues extracting information from the groundbreaking data collected in its global campaign in April 2017. One target of the observations was a galaxy 5 billion light-years away in the constellation Virgo that scientists classify as a quasar because an ultra-luminous source of energy at its center shines and flickers as gas falls into a giant black hole. The target, 3C 279, contains a black hole about one billion times more massive than our Sun. Twin fire-hose-like jets of plasma erupt from the black hole and disk system at velocities close to the speed of light: a consequence of the enormous forces unleashed as matter descends into the black hole’s immense gravity.
- To capture the new image, the EHT uses a technique called very long baseline interferometry (VLBI), which synchronizes and links radio dishes around the world. By combining this network to form one huge virtual Earth-size telescope, the EHT is able to resolve objects as small as 20 micro-arcseconds (µas) on the sky — the equivalent of someone on Earth identifying an orange on the Moon. Data recorded at all the EHT sites around the world is transported to special supercomputers at MPIfR and at MIT’s Haystack Observatory, where they are combined. The combined data set is then carefully calibrated and analyzed by team of experts, which then enables EHT scientists to produce images with the finest detail possible from the surface of the Earth.
- For 3C 279, the EHT can measure features finer than a light-year across, allowing astronomers to follow the jet down to the accretion disk and to see the jet and disk in action. The newly analyzed data show that the normally straight jet has an unexpected twisted shape at its base and, revealing features perpendicular to the jet that could be interpreted as the poles of the accretion disk where the jets are ejected. The fine details in the images change over consecutive days, possibly due to rotation of the accretion disk, and shredding and infall of material, phenomena expected from numerical simulations but never before observed.
- Jae-Young Kim, researcher at MPIfR and lead author of the paper, is enthusiastic and at the same time puzzled: "We knew that every time you open a new window to the Universe you can find something new. Here, where we expected to find the region where the jet forms by going to the sharpest image possible, we find a kind of perpendicular structure. This is like finding a very different shape by opening the smallest Matryoshka doll."
Figure 7: Illustration of multiwavelength 3C 279 jet structure in April 2017. The observing epochs, arrays, and wavelengths are noted at each panel (image credit: J. Y. Kim (MPIfR), Boston University Blazar Program (VLBA and GMVA), and Event Horizon Telescope Collaboration)
- Avery Broderick, an astrophysicist working at the Perimeter Institute, explains "For 3C 279, the combination of the transformative resolution of the EHT and new computational tools for interpreting its data have proved revelatory. What was a single radio 'core' is now resolved into two independent complexes. And they move — even on scales as small as light-months, the jet in 3C 279 is speeding toward us at more than 99.5% of light speed!"
- Because of this rapid motion, the jet in 3C 279 appears to move at about 20 times the speed of light. "This extraordinary optical illusion arises because the material is racing toward us, chasing down the very light it is emitting and making it appear to be moving faster than it is," clarifies Dom Pesce, a postdoctoral fellow at the Center for Astrophysics | Harvard & Smithsonian (CfA). The unexpected geometry suggests the presence of traveling shocks or instabilities in a bent, rotating jet, which might also explain emission at high energies such as gamma-rays.
- Anton Zensus, Director at the MPIfR and Chair of the EHT Collaboration Board, stresses the achievement as a global effort: "Last year we could present the first image of the shadow of a black hole. Now we see unexpected changes in the shape of the jet in 3C 279, and we are not done yet. As we told last year: this is just the beginning."
- "The EHT array is always improving," explains Shep Doeleman ot the CfA, EHT Founding Director. "These new quasar results demonstrate that the unique EHT capabilities can address a wide range of science questions, which will only grow as we continue to add new telescopes to the array. Our team is now working on a next-generation EHT array that will greatly sharpen the focus on black holes and allow us to make the first black hole movies."
- Opportunities to conduct EHT observing campaigns occur once a year in early Northern springtime, but the March/April 2020 campaign had to be cancelled in response to the CoViD-19 global outbreak. In announcing the cancellation Michael Hecht, astronomer from the MIT/Haystack Observatory and EHT Deputy Project Director, concluded that: "We will now devote our full concentration to completion of scientific publications from the 2017 data and dive into the analysis of data obtained with the enhanced EHT array in 2018. We are looking forward to observations with the EHT array expanded to eleven observatories in the spring of 2021." 14)
The international collaboration announced the first-ever image of a black hole at the heart of the radio galaxy Messier 87 on April 10, 2019 by creating a virtual Earth-sized telescope. Supported by considerable international investment, the EHT links existing telescopes using novel systems — creating a new instrument with the highest angular resolving power that has yet been achieved.
The individual telescopes involved in the EHT collaboration are: the Atacama Large Millimeter Telescope (ALMA), the Atacama Pathfinder EXplorer (APEX), the Greenland Telescope (since 2018), the IRAM 30-meter Telescope, the IRAM NOEMA Observatory (expected 2021), the Kitt Peak Telescope (expected 2021), the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope (LMT), the Submillimeter Array (SMA), the Submillimeter Telescope (SMT), and the South Pole Telescope (SPT).
The EHT consortium consists of 13 stakeholder institutes: the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA, Taiwan), the University of Arizona, the University of Chicago, the East Asian Observatory (EAO, located on Hilo, Hawaii), Goethe-Universität Frankfurt, Institut de Radioastronomie Millimétrique (IRAM, located on Pico Veleta in the Spanish Sierra Nevada), Large Millimeter Telescope (LMT located on top of the Sierra Negra, Mexico), Max-Planck-Institut für Radioastronomie (MPIfR, Bonn), MIT Haystack Observatory, National Astronomical Observatory of Japan (NAOJ), Perimeter Institute for Theoretical Physics (Waterloo, Ontario, Canada), Radboud University (Nijmegen, the Netherlands) and the Smithsonian Astrophysical Observatory (SAO, Cambridge, Massachusetts).
• 18 March 2020: Last April, the Event Horizon Telescope (EHT) sparked international excitement when it unveiled the first image of a black hole. Today, a team of researchers have published new calculations that predict a striking and intricate substructure within black hole images from extreme gravitational light bending. 15)
Figure 8: The image of a black hole has a bright ring of emission surrounding a "shadow" cast by the black hole. This ring is composed of a stack of increasingly sharp subrings that correspond to the number of orbits that photons took around the black hole before reaching the observer (image credit: CFA, Harvard & Smithsonian)
- "The image of a black hole actually contains a nested series of rings," explains Michael Johnson of the Center for Astrophysics | Harvard and Smithsonian (CfA). "Each successive ring has about the same diameter but becomes increasingly sharper because its light orbited the black hole more times before reaching the observer. With the current EHT image, we've caught just a glimpse of the full complexity that should emerge in the image of any black hole."
- Because black holes trap any photons that cross their event horizon, they cast a shadow on their bright surrounding emission from hot infalling gas. A "photon ring" encircles this shadow, produced from light that is concentrated by the strong gravity near the black hole. This photon ring carries the fingerprint of the black hole—its size and shape encode the mass and rotation or "spin" of the black hole. With the EHT images, black hole researchers have a new tool to study these extraordinary objects.
- "Black hole physics has always been a beautiful subject with deep theoretical implications, but now it has also become an experimental science," says Alex Lupsasca from the Harvard Society of Fellows. "As a theorist, I am delighted to finally glean real data about these objects that we've been abstractly thinking about for so long."
- The research team included observational astronomers, theoretical physicists, and astrophysicists.
- "Bringing together experts from different fields enabled us to really connect a theoretical understanding of the photon ring to what is possible with observation," notes George Wong, a physics graduate student at the University of Illinois at Urbana-Champaign. Wong developed software to produce simulated black hole images at higher resolutions than had previously been computed and to decompose these into the predicted series of sub-images. "What started as classic pencil-and-paper calculations prompted us to push our simulations to new limits."
- The researchers also found that the black hole's image substructure creates new possibilities to observe black holes. "What really surprised us was that while the nested subrings are almost imperceptible to the naked eye on images—even perfect images—they are strong and clear signals for arrays of telescopes called interferometers," says Johnson. "While capturing black hole images normally requires many distributed telescopes, the subrings are perfect to study using only two telescopes that are very far apart. Adding one space telescope to the EHT would be enough."
- The results were published in Science Advances. 16) This research was supported by grants from the National Science Foundation, the Gordon and Betty Moore Foundation, the John Templeton Foundation, the Jacob Goldfield Foundation, the Department of Energy, and NASA.
Astronomers Capture First Image of a Black Hole
• 10 April 2019: The Event Horizon Telescope (EHT) — a planet-scale array of eight ground-based radio telescopes forged through international collaboration — was designed to capture images of a black hole. Today, in coordinated press conferences across the globe, EHT researchers reveal that they have succeeded, unveiling the first direct visual evidence of a supermassive black hole and its shadow. 17)
This breakthrough was announced today in a series of six papers published in a special issue of The Astrophysical Journal Letters. The image reveals the black hole at the center of Messier 87 , a massive galaxy in the nearby Virgo galaxy cluster. This black hole resides 55 million light-years from Earth and has a mass 6.5 billion times that of the Sun .
The EHT links telescopes around the globe to form an Earth-sized virtual telescope with unprecedented sensitivity and resolution . The EHT is the result of years of international collaboration, and offers scientists a new way to study the most extreme objects in the Universe predicted by Einstein’s general relativity during the centennial year of the historic experiment that first confirmed the theory .
"We have taken the first picture of a black hole," said EHT project director Sheperd S. Doeleman of the Center for Astrophysics, Harvard & Smithsonian. "This is an extraordinary scientific feat accomplished by a team of more than 200 researchers."
Black holes are extraordinary cosmic objects with enormous masses but extremely compact sizes. The presence of these objects affects their environment in extreme ways, warping spacetime and super-heating any surrounding material.
"If immersed in a bright region, like a disc of glowing gas, we expect a black hole to create a dark region similar to a shadow — something predicted by Einstein’s general relativity that we’ve never seen before, explained chair of the EHT Science Council Heino Falcke of Radboud University, the Netherlands. "This shadow, caused by the gravitational bending and capture of light by the event horizon, reveals a lot about the nature of these fascinating objects and allowed us to measure the enormous mass of M87’s black hole."
Multiple calibration and imaging methods have revealed a ring-like structure with a dark central region — the black hole’s shadow — that persisted over multiple independent EHT observations.
"Once we were sure we had imaged the shadow, we could compare our observations to extensive computer models that include the physics of warped space, superheated matter and strong magnetic fields. Many of the features of the observed image match our theoretical understanding surprisingly well," remarks Paul T. P. Ho, EHT Board member and Director of the East Asian Observatory . "This makes us confident about the interpretation of our observations, including our estimation of the black hole’s mass."
Figure 9: Scientists have obtained the first image of a black hole, using Event Horizon Telescope observations of the center of the galaxy M87. The image shows a bright ring formed as light bends in the intense gravity around a black hole that is 6.5 billion times more massive than the Sun. This long-sought image provides the strongest evidence to date for the existence of supermassive black holes and opens a new window onto the study of black holes, their event horizons, and gravity (image credit: Event Horizon Telescope Collaboration)
Creating the EHT was a formidable challenge which required upgrading and connecting a worldwide network of eight pre-existing telescopes deployed at a variety of challenging high-altitude sites. These locations included volcanoes in Hawai`i and Mexico, mountains in Arizona and the Spanish Sierra Nevada, the Chilean Atacama Desert, and Antarctica.
The EHT observations use a technique called VLBI which synchronizes telescope facilities around the world and exploits the rotation of our planet to form one huge, Earth-size telescope observing at a wavelength of 1.3 mm. VLBI allows the EHT to achieve an angular resolution of 20 µas (micro-arcseconds) — enough to read a newspaper in New York from a sidewalk café in Paris .
The telescopes contributing to this result were ALMA (Atacama Large Millimeter/submillimeter Array), APEX (Atacama Pathfinder EXperiment), the IRAM (Institute for Radio Astronomy in the Millimeter Range) 30 m telescope, the James Clerk Maxwell Telescope, the Large Millimeter Telescope Alfonso Serrano, the Submillimeter Array, the Submillimeter Telescope, and the South Pole Telescope . Petabytes of raw data from the telescopes were combined by highly specialised supercomputers hosted by the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory.
The construction of the EHT and the observations announced today represent the culmination of decades of observational, technical, and theoretical work. This example of global teamwork required close collaboration by researchers from around the world. Thirteen partner institutions worked together to create the EHT, using both pre-existing infrastructure and support from a variety of agencies. Key funding was provided by the US National Science Foundation (NSF), the EU's European Research Council (ERC), and funding agencies in East Asia.
"We have achieved something presumed to be impossible just a generation ago," concluded Doeleman. "Breakthroughs in technology, connections between the world's best radio observatories, and innovative algorithms all came together to open an entirely new window on black holes and the event horizon."
Note : The shadow of a black hole is the closest we can come to an image of the black hole itself, a completely dark object from which light cannot escape. The black hole’s boundary — the event horizon from which the EHT takes its name — is around 2.5 times smaller than the shadow it casts and measures just under 40 billion km across.
Note : Supermassive black holes are relatively tiny astronomical objects — which has made them impossible to directly observe until now. As a black hole’s size is proportional to its mass, the more massive a black hole, the larger the shadow. Thanks to its enormous mass and relative proximity, M87’s black hole was predicted to be one of the largest viewable from Earth — making it a perfect target for the EHT.
Note : Although the telescopes are not physically connected, they are able to synchronize their recorded data with atomic clocks — hydrogen masers — which precisely time their observations. These observations were collected at a wavelength of 1.3 mm during a 2017 global campaign. Each telescope of the EHT produced enormous amounts of data — roughly 350 terabytes per day — which was stored on high-performance helium-filled hard drives. These data were flown to highly specialised supercomputers — known as correlators — at the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory to be combined. They were then painstakingly converted into an image using novel computational tools developed by the collaboration.
Note : 100 years ago, two expeditions set out for the island of Príncipe off the coast of Africa and Sobra in Brazil to observe the 1919 solar eclipse, with the goal of testing general relativity by seeing if starlight would be bent around the limb of the sun, as predicted by Einstein. In an echo of those observations, the EHT has sent team members to some of the world's highest and isolated radio facilities to once again test our understanding of gravity.
Note : The East Asian Observatory (EAO) partner on the EHT project represents the participation of many regions in Asia, including China, Japan, Korea, Taiwan, Vietnam, Thailand, Malaysia, India and Indonesia.
Note : Future EHT observations will see substantially increased sensitivity with the participation of the IRAM NOEMA Observatory, the Greenland Telescope and the Kitt Peak Telescope.
Note : 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. APEX 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 James Clerk Maxwell Telescope is operated by the EAO, the Large Millimeter Telescope Alfonso Serrano is operated by INAOE and UMass, the Submillimeter Array is operated by SAO and ASIAA and the Submillimeter Telescope is operated by the Arizona Radio Observatory (ARO). The South Pole Telescope is operated by the University of Chicago with specialized EHT instrumentation provided by the University of Arizona.
Focus on the First Event Horizon Telescope Results
This research was presented in a series of six papers published today in a special issue of The Astrophysical Journal Letters, along with a Focus Issue: 18)
Figure 10: EHT images of M87 on four different observing nights. In each panel, the white circle shows the resolution of the EHT. All four images are dominated by a bright ring with enhanced emission in the south (image credit: Shep Doeleman & EHT Collaboration)
We report the first image of a black hole.
This Focus Issue shows ultra-high angular resolution images of radio emission from the supermassive black hole believed to lie at the heart of galaxy M87 (Figure 10). A defining feature of the images is an irregular but clear bright ring, whose size and shape agree closely with the expected lensed photon orbit of a 6.5 billion solar mass black hole. Soon after Einstein introduced general relativity, theorists derived the full analytic form of the photon orbit, and first simulated its lensed appearance in the 1970s. By the 2000s, it was possible to sketch the "shadow" formed in the image when synchrotron emission from an optically thin accretion flow is lensed in the black hole's gravity. During this time, observational evidence began to build for the existence of black holes at the centers of active galaxies, and in our own Milky Way. In particular, a steady progression in radio astronomy enabled very long baseline interferometry (VLBI) observations at ever-shorter wavelengths, targeting supermassive black holes with the largest apparent event horizons: M87, and Sgr A* in the Galactic Center. The compact sizes of these two sources were confirmed by studies at 1.3mm, first exploiting baselines that ran from Hawai'i to the mainland US, then with increased resolution on baselines to Spain and Chile.
Over the past decade, the EHT extended these first measurements of size to mount the more ambitious campaign of imaging the shadow itself. During 5-11 April 2017, the Event Horizon Telescope (EHT) observed M87 and calibrators on four separate days using an array that included eight radio telescopes at six geographic locations: Arizona (USA), Chile, Hawai'i (USA), Mexico, the South Pole, and Spain (Figure 2). Years of preparation (and an astonishing spate of planet-wide good weather) paid off with an extraordinary multi-petabyte yield of data. The results presented here, from observations through images to interpretation, issue from a team of instrument, algorithm, software, modeling, and theoretical experts, following a tremendous effort by a group of scientists that span all career stages, from undergraduates to senior members of the field. More than 200 members from 59 institutes in 20 countries and regions have devoted years to the effort, all unified by a common scientific vision.
Figure 11: A map of the EHT. Stations active in 2017 and 2018 are shown with connecting lines and labeled in yellow, sites in commission are labeled in green, and legacy sites are labeled in red. From Paper II (Figure 10) image credit: EHT Collaboration
The sequence of Letters in this issue provides the full scope of the project and the conclusions drawn to date. Paper II opens with a description of the EHT array, the technical developments that enabled precursor detections, and the full range of observations reported here. Through the deployment of novel instrumentation at existing facilities, the collaboration created a new telescope with unique capabilities for black hole imaging. Paper III details the observations, data processing, calibration algorithms, and rigorous validation protocols for the final data products used for analysis. Paper IV gives the full process and approach to image reconstruction. The final images emerged after a rigorous evaluation of traditional imaging algorithms and new techniques tailored to the EHT instrument — alongside many months of testing the imaging algorithms through the analysis of synthetic data sets. Paper V uses newly assembled libraries of general relativistic magnetohydrodynamic (GRMHD) simulations and advanced ray-tracing to analyze the images and data in the context of black hole accretion and jet-launching. Paper VI employs model fits, comparison of simulations to data, and feature extraction from images to derive formal estimates of the lensed emission ring size and shape, black hole mass, and constraints on the nature of the black hole and the space-time surrounding it. Paper I is a concise summary.
Our image of the shadow confines the mass of M87 to within its photon orbit, providing the strongest case for the existence of supermassive black holes. These observations are consistent with Doppler brightening of relativistically moving plasma close to the black hole lensed around the photon orbit. They strengthen the fundamental connection between active galactic nuclei and central engines powered by accreting black holes through an entirely new approach. In the coming years, the EHT Collaboration will extend efforts to include full polarimetry, mapping of magnetic fields on horizon scales, investigations of time variability, and increased resolution through shorter wavelength observations.
In short, this work signals the development of a new field of research in astronomy and physics as we zero in on precision images of black holes on horizon scales. The prospects for sharpening our focus even further are excellent.
Table 1: Overview of EHT publications in The Astrophysical Journal
Some context and background on Black Holes
Astronomers have finally glimpsed the blackness of a black hole. By stringing together a global network of radio telescopes, they have for the first time produced a picture of an event horizon — a black hole’s perilous edge — against a backdrop of swirling light. 19)
“We have seen the gates of hell at the end of space and time,” said astrophysicist Heino Falcke of Radboud University in Nijmegen, the Netherlands, at a press conference in Brussels. “What you’re looking at is a ring of fire created by the deformation of space-time. Light goes around, and looks like a circle.”
The images — of a glowing, ring-like structure — show the supermassive black hole at the center of the galaxy M87, which is around 16 megaparsecs (55 million light years) away and 6.5 billion times the mass of the Sun. They reveal, in greater detail than ever before, the event horizon — the surface beyond which gravity is so strong that nothing that crosses it, even light, can ever climb back out.
The highly anticipated results, comparable to recognizing a doughnut on the Moon’s surface, were unveiled today by the Event Horizon Telescope (EHT) collaboration in seven simultaneous press conferences on four continents. The findings were also published in a suite of papers in Astrophysical Journal Letters on 10 April.
The image is a “tremendous accomplishment”, says astrophysicist Roger Blandford at Stanford University in California, who was not involved with the work. “When I was a student, I never dreamt that anything like this would be possible,” he says. “It is yet another confirmation of general relativity as the correct theory of strong gravity.”
Figure 12: The first image of a black hole: A three minute guide. Astronomers from the Event Horizon Telescope Collaboration have taken the first ever image of a black hole - at the heart of the galaxy M87 (video credit: Nature, published 10 April 2019)
The image is a “tremendous accomplishment”, says astrophysicist Roger Blandford at Stanford University in California, who was not involved with the work. “When I was a student, I never dreamt that anything like this would be possible,” he says. “It is yet another confirmation of general relativity as the correct theory of strong gravity.”
“I was so delighted,” says Andrea Ghez, an astronomer at the University of California, Los Angeles. The images provide “clear evidence” of a ‘photon ring’ around a black hole, she says.
Figure 13: Six press conferences around the world revealed the black-hole images (image credit: Nature)
Nearly a century ago, physicists first deduced that black holes should exist from Albert Einstein’s general theory of relativity, but most of the evidence so far has been indirect. The EHT (Event Horizon Telescope) has now made a new, spectacular confirmation of those predictions.
The team observed two supermassive black holes — M87’s and Sagittarius A*, the void at the Milky Way’s center — over five nights in April 2017. They mustered enough resolution to capture the distant objects by linking up eight radio observatories across the globe — from Hawaii to the South Pole — and each collected more data than the Large Hadron Collider does in a year (see ‘Global effort’). The data set is likely to be the largest ever collected by a science experiment, and it took two years of work to produce the pictures.
After combining the observatories’ data, the team started analysis in mid-2018. They quickly realized that they could get a first, clean picture from M87. “We focused all our attention on M87 when we saw our first results because we saw this is going to be awesome,” says Falcke.
At the Brussels press conference, astrophysicist and collaboration member Monika Moscibrodzka, also at Radboud, said that the measurements so far are not precise enough to measure how fast the M87 hole spins — a crucial feature for a black hole. But it indicates the direction in which it’s spinning, which is clockwise in the sky, she said. Further studies could also help researchers understand how the black hole produces its gigantic jets.
The teams will also now turn their attention to the Sagittarius A* data. Because Sagittarius A* is nearly 1,000 times smaller than the M87 black hole, matter orbited it many times during each observing session, producing a rapidly changing signal rather than a steady one, says Luciano Rezzolla, a theoretical astrophysicist at the Goethe University of Frankfurt in Germany and a member of the EHT team. That makes the data more complicated to interpret, but also potentially richer in information.
Figure 14: Nik Spencer/Nature; Avery Broderick/University of Waterloo (IMAGES bottom)
Event horizons are the defining feature of black holes. To a nearby observer, an event horizon should appear as a spherical surface shrouding its interiors from view. Because light can cross the surface only one way — inwards — the globe should look completely black (see ‘Power of the dark’).
A black hole’s event horizon should appear five times larger than it is, because the hole warps the surrounding space and bends the paths of light. The effect, discovered by physicist James Bardeen at the University of Washington in Seattle in 1973, is similar to the way that a spoon looks larger when dipped in a glass of water. Moreover, Bardeen showed that the black hole would cast an even larger ‘shadow’. This is because within a certain distance of the event horizon, most light rays bend so much that they effectively orbit the black hole.
To actually resolve details on the scale of the event horizon, radio astronomers calculated that they would need a telescope the size of Earth (a telescope’s resolution is also proportional to its size). Fortunately, a technique called interferometry could help. It involves multiple telescopes, located far apart from one another and pointed at the same object simultaneously. Effectively, the telescopes work as if they were shards of one big dish.
Various teams around the world refined their techniques, and retrofitted some major observatories so that they could add them to a network. In particular, a group led by Shep Doeleman, now at Harvard University in Cambridge, Massachusetts, adapted the 10-meter South Pole Telescope and the US $1.4-billion ALMA (Atacama Large Millimeter/submillimeter Array) in Chile to do the work.
In 2014, Falcke, Doeleman and groups from around the world joined forces to form the EHT collaboration. They did their first Earth-spanning observation campaign in 2017. They observed Sagittarius A* and M87 during a two-week window in April when the locations of the observatories are most likely to get good weather simultaneously.
The raw data, which ran into petabytes, were collected on hard disks and travelled by air, sea and land to be compiled at the Max Planck Institute for Radio Astronomy in Bonn, Germany and the Massachusetts Institute of Technology’s Haystack Observatory in Westford.
Last year, while the data were still being processed, Falcke told Nature that he expected the experiment to gather a wealth of information about the structure of the black holes, but not yet a pretty picture. At best, it would resemble “an ugly peanut”, he said. “Or maybe, the first image will be just a few blots. It may not even resemble a peanut.”
Figure 15: How to hunt for a black hole with a telescope the size of Earth (image credit: ESA advanced concepts team; S. Brunier /ESO) 20)
The EHT ran another observing campaign in 2018 — the analysis of those data is still in the works — but cancelled a planned observing campaign this year because of security issues near one of its most important sites, the 50 meter LMT (Large Millimeter Telescope) in Puebla, Mexico. They plan to continue to do observations once a year starting in 2020.
The collaboration is now looking for funding to establish a foothold in Africa, which would fill in a major gap in the network. The plan is to relocate a 15 meter dish — a decommissioned Swedish telescope — from Chile to the Gamsberg Table Mountain in Namibia. For now, the network has already secured two major additions: a dish in Greenland and an array in the French Alps.
An expanded EHT network could provide detail on what happens inside the voids — “how the world behaves inside black holes, and if it is as we expected it to be”, says David Sánchez Argüelles, a physicist at the LMT.
“It was a great sense of relief to see this, but also surprise,” says Doeleman of the results. “You know what I was really expecting to see? A blob. To see this ring is probably the best outcome that we could have had.”
Chilean Senate Honors ALMA For 1st Image of Black Hole
18 April 2019: The Honorable Senate of Chile invited representatives from the ALMA observatory to its session held today, 17 April , in recognition of its role in obtaining the first image of a black hole published by the Event Horizon Telescope (EHT) last week. In addition to congratulating the observatory, the five scientists from the ALMA team who were directly involved in the process were recognized individually: Alejandro Sáez, Violette Impellizzeri, Hugo Messias, Rubén Herrero-Illana and Akihiko Hirota. In addition, Neil Nagar and Venkatessh Ramakrishnan from the Universidad de Concepción were also distinguished. 21)
After the successful revelation of the first image of a black hole on 10 April by the Event Horizon Telescope consortium, which had a global impact, the Honorable Senate of Chile decided to award a silver medal to the ALMA Observatory and to the seven scientists who participated from Chile as coauthors in the investigation. The ceremony was held in the Senate Hall.
“I thank all of the ALMA staff for their extraordinary efforts in enabling ALMA’s vital participation in this very successful, iconic experiment to obtain the first image of a black hole” said ALMA Director, Sean Doguherty.
After the ceremony, the President of the Senate, Jaime Quintana, said to be “very proud that they have accepted the invitation to give them the Silver Medal of the Senate. A research team that has made a big contribution to Science with this great discovery. Furthermore, taking into account that this is what we have been talking about from the Senate in the last decade: to strengthen links between Science and Politics, an important matter, and a key objective of Congreso del Futuro“.
Figure 16: From left: Paulina Bocaz, NRAO/AUI legal representative in Chile; Hugo Messias; Alejandro Sáez; Akihiko Hirota (image credit: N. Lira – ALMA (ESO/NAOJ/NRAO)
The Giant Galaxy Around the Giant Black Hole
On April 10, 2019, the Event Horizon Telescope (EHT) unveiled the first-ever image of a black hole's event horizon, the area beyond which light cannot escape the immense gravity of the black hole. That giant black hole, with a mass of 6.5 billion Suns, is located in the elliptical galaxy Messier 87 (M87). EHT is an international collaboration whose support in the U.S. includes the National Science Foundation. 22)
This image from NASA's Spitzer Space Telescope shows the entire M87 galaxy in infrared light. The EHT image, by contrast, relied on light in radio wavelengths and showed the black hole's shadow against the backdrop of high-energy material around it.
Figure 17: The galaxy M87, imaged here by NASA's Spitzer Space Telescope, is home to a supermassive black hole that spews two jets of material out into space at nearly the speed of light. The inset shows a close-up view of the shockwaves created by the two jets (image credit: NASA/JPL-Caltech/IPAC)
Located about 55 million light-years from Earth, M87 has been a subject of astronomical study for more than 100 years and has been imaged by many NASA observatories, including the Hubble Space Telescope, the Chandra X-ray Observatory and NuSTAR. In 1918, astronomer Heber Curtis first noticed "a curious straight ray" extending from the galaxy's center. This bright jet of high-energy material, produced by a disk of material spinning rapidly around the black hole, is visible in multiple wavelengths of light, from radio waves through X-rays. When the particles in the jet impact the interstellar medium (the sparse material filling the space between stars in M87), they create a shockwave that radiates in infrared and radio wavelengths of light but not visible light. In the Spitzer image, the shockwave is more prominent than the jet itself.
The brighter jet, located to the right of the galaxy's center, is traveling almost directly toward Earth. Its brightness is amplified due to its high speed in our direction, but even more so because of what scientists call "relativistic effects," which arise because the material in the jet is traveling near the speed of light. The jet's trajectory is just slightly offset from our line of sight with respect to the galaxy, so we can still see some of the length of the jet. The shockwave begins around the point where the jet appears to curve down, highlighting the regions where the fast-moving particles are colliding with gas in the galaxy and slowing down.
The second jet, by contrast, is moving so rapidly away from us that the relativistic effects render it invisible at all wavelengths. But the shockwave it creates in the interstellar medium can still be seen here.
Located on the left side of the galaxy's center, the shockwave looks like an inverted letter "C." While not visible in optical images, the lobe can also be seen in radio waves, as in this image from the National Radio Astronomy Observatory's Very Large Array.
By combining observations in the infrared, radio waves, visible light, X-rays and extremely energetic gamma rays, scientists can study the physics of these powerful jets. Scientists are still striving for a solid theoretical understanding of how gas being pulled into black holes creates outflowing jets.
Infrared light at wavelengths of 3.6 and 4.5 µm are rendered in blue and green, showing the distribution of stars, while dust features that glow brightly at 8.0 µm are shown in red. The image was taken during Spitzer's initial "cold" mission.
The Jet Propulsion Laboratory in Pasadena, California, manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate in Washington. Science operations are conducted at the Spitzer Science Center at Caltech in Pasadena. Space operations are based at Lockheed Martin Space Systems in Littleton, Colorado. Data are archived at the Infrared Science Archive housed at IPAC (Infrared Processing and Analysis Center) at Caltech. Caltech manages JPL for NASA.
Figure 18: The galaxy M87 looks like a hazy, blue space-puff in this image from NASA's Spitzer Space Telescope. At the galaxy's center is a supermassive black hole that spews two jets of material out into space (image credit: NASA/JPL-Caltech/IPAC)
Figure 19: This wide-field image of the galaxy M87 was taken by NASA's Spitzer Space Telescope. The top inset shows a close-up of two shockwaves, created by a jet emanating from the galaxy's supermassive black hole. The Event Horizon Telescope recently took a close-up image of the silhouette of that black hole, show in the second inset (image credit: NASA/JPL-Caltech/Event Horizon Telescope Collaboration)
Black holes are among the most fascinating objects in the Universe. Enclosing huge amounts of matter in relatively small regions, these compact objects have enormous densities that give rise to some of the strongest gravitational fields in the cosmos, so strong that nothing can escape – not even light. 23)
Figure 20: Two merging black holes (image credit: ESA)
This artistic impression shows two black holes that are spiralling towards each other and will eventually coalesce. A black hole merger was first detected in 2015 by LIGO, the Laser Interferometer Gravitational-Wave Observatory, which detected the gravitational waves – fluctuations in the fabric of spacetime – created by the giant collision.
Black holes and gravitational waves are both predictions of Albert Einstein’s general relativity, which was presented in 1915 and remains to date the best theory to describe gravity across the Universe.
Karl Schwarzschild derived the equations for black holes in 1916, but they remained rather a theoretical curiosity for several decades, until X-ray observations performed with space telescopes could finally probe the highly energetic emission from matter in the vicinity of these extreme objects. The first ever image of a black hole’s dark silhouette, cast against the light from matter in its immediate surrounding, was only captured recently by the Event Horizon Telescope and published just last month.
As for gravitational waves, it was Einstein himself who predicted their existence from his theory, also in 1916, but it would take another century to finally observe these fluctuations. Since 2015, the ground-based LIGO and Virgo observatories have assembled over a dozen detections, and gravitational-wave astronomy is a burgeoning new field of research.
But another of Einstein’s predictions found observational proof much sooner: the gravitational bending of light, which was demonstrated only a few years after the theory had appeared, during a total eclipse of the Sun in 1919.
In the framework of general relativity, any object with mass bends the fabric of spacetime, deflecting the path of anything that passes nearby – including light. An artistic view of this distortion, also known as gravitational lensing, is depicted in this representation of two merging black holes.
One hundred years ago, astronomers set out to test general relativity, observing whether and by how much the mass of the Sun deflects the light of distant stars. This experiment could only be performed by obscuring the Sun’s light to reveal the stars around it, something that is possible during a total solar eclipse.
On 29 May 1919, Sir Arthur Eddington observed the distant stars around the Sun during an eclipse from the island of Príncipe, in West Africa, while Andrew Crommelin performed similar observations in Sobral, in the north east of Brazil. Their results, presented six months later, indicated that stars observed near the solar disc during the eclipse were slightly displaced, with respect to their normal position in the sky, roughly by the amount predicted by Einstein’s theory for the Sun’s mass to have deflected them.
“Lights All Askew in the Heavens,” headlined the New York Times in November 1919 to announce the triumph of Einstein’s new theory. This inaugurated a century of exciting experiments investigating gravity on Earth and in space and proving general relativity more and more precisely.
Supermassive black holes, with masses ranging from millions to billions of Suns, sit at the core of most massive galaxies across the Universe. We don’t know exactly how these huge, enormously dense objects took shape, nor what triggers a fraction of them to start devouring the surrounding matter at extremely intense rates, radiating copiously across the electromagnetic spectrum and turning their host galaxies into ‘active galactic nuclei’. 24)
Tackling these open questions in modern astrophysics is among the main goals of two future missions in ESA’s space science program: Athena, the Advanced Telescope for High-ENergy Astrophysics, and LISA, the Laser Interferometer Space Antenna. Currently in the study phase, both missions are scheduled for launch in the early 2030s.
We have made giant leaps over the past hundred years, but there is still much for us to discover Athena, ESA’s future X-ray observatory, will investigate in unprecedented detail the supermassive black holes that sit at the center of galaxies. LISA, another future ESA mission, will detect gravitational waves from orbit, looking for the low-frequency fluctuations that are released when two supermassive black holes merge and can only be detected from space.
If Athena and LISA could operate jointly for at least a few years, they could perform a unique experiment: observing the merger of supermassive black holes both in gravitational waves and X-rays, using an approach known as multi-messenger astronomy.
We have never observed such a merger before: we need LISA to detect gravitational waves at the onset of the merger and tell us where to look in the sky, then we need Athena to observe it at high precision in X-rays to see how the mighty collision affects the gas surrounding the black holes. We don’t know what happens during such a cosmic clash so this experiment, much like the eclipse of 1919 that first proved Einstein’s theory, is set to shake our understanding of gravity and the Universe.
Figure 21: Artist's impression of the merger of two supermassive black holes during a galaxy collision. Simulations predict that their mergers, unlike those of their stellar-mass counterparts, emit both gravitational waves and radiation – the latter originating in the hot, interstellar gas of the two colliding galaxies stirred by the black holes pair when they fall towards one another. As the two spiralling black holes modulate the motion of the surrounding gas, it is likely that the X-ray signature will have a frequency commensurate to that of the gravitational wave signal. — Combining the observing power of two future ESA missions, Athena and LISA, would allow us to study these cosmic clashes and their mysterious aftermath for the first time. After the merger, we could see the emergence of a new X-ray source, and perhaps witness the birth of an active galactic nucleus, with jets of high-energy particles being launched at close to the speed of light above and beyond the newly formed black hole (image credit: ESA)
On Earth, we deal with gravity every day. We feel it, we fight it, and – more importantly – we investigate it. Space agencies such as ESA routinely launch spacecraft against our planet’s gravity, and sometimes these spacecraft borrow the gravity of Earth or other planets to reach interesting places in the Solar System. We study the gravity field of Earth from orbit, and fly experiments on parabolic flights, sounding rockets and the International Space Station to examine a variety of systems under different gravitational conditions. On the grandest scales, our space science missions explore how gravity affects planets, stars and galaxies across the cosmos and probe how matter behaves in the strong gravitational field created by some of the Universe’s most extreme objects like black holes.
Figure 22: One hundred years ago this month, observations performed during a total solar eclipse proved for the first time the gravitational bending of light predicted by Albert Einstein’s new theory of gravity, general relativity. In this video, Günther Hasinger, ESA Director of Science, reflects on this historic measurement that inaugurated a century of exciting experiments, investigating gravity on Earth and in space and proving general relativity in ever greater detail [video credit: ESA/CESAR (solar eclipse sequence); ESO/M. Kornmesser (black hole); Royal Astronomical Society (negative photo of the 1919 solar eclipse); ESA/Hubble, NASA (gravitationally lensed quasar); ESO/Gravity Consortium/L. Calçada (black hole simulation)] 25)
Einstein’s general theory of relativity tested by star orbiting a black hole
July 26, 2019: More than 100 years after Albert Einstein published his iconic theory of general relativity, it is beginning to fray at the edges, said Andrea Ghez, UCLA professor of physics and astronomy. Now, in the most comprehensive test of general relativity near the monstrous black hole at the center of our galaxy, Ghez and her research team report July 25 in the journal Science that Einstein’s theory of general relativity holds up. 26) 27)
“Einstein’s right, at least for now,” said Ghez, a co-lead author of the research. “We can absolutely rule out Newton’s law of gravity. Our observations are consistent with Einstein’s theory of general relativity. However, his theory is definitely showing vulnerability. It cannot fully explain gravity inside a black hole, and at some point we will need to move beyond Einstein’s theory to a more comprehensive theory of gravity that explains what a black hole is.”
Einstein’s 1915 theory of general relativity holds that what we perceive as the force of gravity arises from the curvature of space and time. The scientist proposed that objects such as the sun and the Earth change this geometry. Einstein’s theory is the best description of how gravity works, said Ghez, whose UCLA-led team of astronomers has made direct measurements of the phenomenon near a supermassive black hole — research Ghez describes as “extreme astrophysics.”
Figure 23: Testing Einstein's theory of relativity near a black hole (video credit: UCLA, Published on 25 July 2019)
The laws of physics, including gravity, should be valid everywhere in the universe, said Ghez, who added that her research team is one of only two groups in the world to watch a star known as S0-2 make a complete orbit in three dimensions around the supermassive black hole at the center of the Milky Way. The full orbit takes 16 years, and the black hole’s mass is about four million times that of the sun.
The researchers say their work is the most detailed study ever conducted into the supermassive black hole and Einstein’s theory of general relativity.
Figure 24: Detailed UCLA-led analysis of the star’s orbit near supermassive black hole gives a look into how gravity behaves. SO-2 and SO-38 circle SGR A* (image credit: UCLA Galactic Center Groupe via S. Sakai and Andrea Ghez at Keck Observatory)
The key data in the research were spectra that Ghez’s team analyzed this April, May and September as her “favorite star” made its closest approach to the enormous black hole. Spectra, which Ghez described as the “rainbow of light” from stars, show the intensity of light and offer important information about the star from which the light travels. Spectra also show the composition of the star. These data were combined with measurements Ghez and her team have made over the last 24 years.
Spectra — collected at the W. M. Keck Observatory in Hawaii using a spectrograph built at UCLA by a team led by colleague James Larkin — provide the third dimension, revealing the star’s motion at a level of precision not previously attained (images of the star the researchers took at the Keck Observatory provide the two other dimensions). Larkin’s instrument takes light from a star and disperses it, similar to the way raindrops disperse light from the sun to create a rainbow, Ghez said.
“What’s so special about S0-2 is we have its complete orbit in three dimensions,” said Ghez, who holds the Lauren B. Leichtman and Arthur E. Levine Chair in Astrophysics. “That’s what gives us the entry ticket into the tests of general relativity. We asked how gravity behaves near a supermassive black hole and whether Einstein’s theory is telling us the full story. Seeing stars go through their complete orbit provides the first opportunity to test fundamental physics using the motions of these stars.”
Figure 25: How a star's orbit helps us see a black hole (video credit: UCLA, Published on 25 July 2019)
Ghez’s research team was able to see the co-mingling of space and time near the supermassive black hole. “In Newton’s version of gravity, space and time are separate, and do not co-mingle; under Einstein, they get completely co-mingled near a black hole,” she said.
“Making a measurement of such fundamental importance has required years of patient observing, enabled by state-of-the-art technology,” said Richard Green, director of the National Science Foundation’s division of astronomical sciences. For more than two decades, the division has supported Ghez, along with several of the technical elements critical to the research team’s discovery.
Keck Observatory Director Hilton Lewis called Ghez “one of our most passionate and tenacious Keck users.” “Her latest groundbreaking research,” he said, “is the culmination of unwavering commitment over the past two decades to unlock the mysteries of the supermassive black hole at the center of our Milky Way galaxy.”
The researchers studied photons — particles of light — as they traveled from S0-2 to Earth. S0-2 moves around the black hole at blistering speeds of more than 16 million miles per hour at its closest approach. Einstein had reported that in this region close to the black hole, photons have to do extra work. Their wavelength as they leave the star depends not only on how fast the star is moving, but also on how much energy the photons expend to escape the black hole’s powerful gravitational field. Near a black hole, gravity is much stronger than on Earth.
Ghez was given the opportunity to present partial data last summer, but chose not to so that her team could thoroughly analyze the data first. “We’re learning how gravity works. It’s one of four fundamental forces and the one we have tested the least,” she said. “There are many regions where we just haven’t asked, how does gravity work here? It’s easy to be overconfident and there are many ways to misinterpret the data, many ways that small errors can accumulate into significant mistakes, which is why we did not rush our analysis.”
Figure 26: An artist visualization of the star S0-2 getting closer to the supermassive black hole at the center of the Milky Way and causing a gravitational redshift that is predicted by Einstein’s General Relativity. By observing this redshift, we can test Einstein’s theory of gravity (image credit: Nicolle R. Fuller, National Science Foundation)
Ghez, a 2008 recipient of the MacArthur “Genius” Fellowship, studies more than 3,000 stars that orbit the supermassive black hole. Hundreds of them are young, she said, in a region where astronomers did not expect to see them.
It takes 26,000 years for the photons from S0-2 to reach Earth. “We’re so excited, and have been preparing for years to make these measurements,” said Ghez, who directs the UCLA Galactic Center Group. “For us, it’s visceral, it’s now — but it actually happened 26,000 years ago!”
This is the first of many tests of general relativity Ghez’s research team will conduct on stars near the supermassive black hole. Among the stars that most interest her is S0-102, which has the shortest orbit, taking 11 1/2 years to complete a full orbit around the black hole. Most of the stars Ghez studies have orbits of much longer than a human lifespan.
Ghez’s team took measurements about every four nights during crucial periods in 2018 using the Keck Observatory — which sits atop Hawaii’s dormant Mauna Kea volcano and houses one of the world’s largest and premier optical and infrared telescopes. Measurements are also taken with an optical-infrared telescope at Gemini Observatory and Subaru Telescope, also in Hawaii.
Figure 27: Keck Observatory, operated by Caltech and the University of California, MaunaKea Hawaii USA, altitude of 4,207 m
Figure 28: NOAO (National Optical Astronomy Observatory) Gemini North on MaunaKea, Hawaii, USA, altitude of 4,213 m (image credit: UCLA)
Figure 29: The Japanese NAOJ/Subaru Telescope at MaunaKea Hawaii, USA, altitude of 4,207 m
Andrea Ghez and her team have used these telescopes both on site in Hawaii and remotely from an observation room in UCLA’s department of physics and astronomy.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).Status Focus on results Context and background Einsteins Theory Tested
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