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Home , Anton Zensus, Messier 87, Ring singularity, Supermassive black hole

Overview_Black Holes

A picture of a dark monster. The image is the first direct visual evidence of a hole. This particularly massive specimen is at the center of the supergiant and was acquired by the Telescope (EHT), an array of eight ground-based radio telescopes distributed around the globe.

A portrait of a

Black holes swallow all , making them invisible. That’s what you’d think anyway, but astronomers thankfully know that this isn’t quite the case. They are, in fact, surrounded by a glowing disc of gas, which makes them visible against this bright background, like a black cat on a white sofa. And that’s how the has now succeeded in taking the first picture of a black hole. Researchers from the Max Planck Institute for Radio in and the Institute for in the Millimeter Range (IRAM) in Grenoble, France, were

among those making the observations. Photo: EHT Collaboration TEXT HELMUT HORNUNG

n spring 2017, scientists linked up objects at extremely small angles of less tion of the Max Planck Institute for Ra- eight telescopes spread out over than 20 microarcseconds. With that dio Astronomy. In the 2017 observing one face of the globe for the first resolution, our eyes would see individ- session alone, the telescopes recorded time, forming a virtual telescope ual molecules on our own hands. approximately four petabytes of data. with an effective aperture close to This is such a huge volume that it was the Idiameter of the entire . The THE DATA ARE PROCESSED IN A actually faster and more effective to technique is called Very Long Baseline SUPERCOMPUTER send the data by post than via the In- Interferometry (VLBI), which combines ternet. The data were calibrated and the signals of the individual antennas The Event Horizon Telescope (EHT), as analyzed at the Massachusetts Insti- into one image. This synchronization the network of observatories is known, tute of Technology (MIT) in the U.S. requires highly-precise atomic clocks, included the 30-meter IRAM dish in and at the Max Planck Institute for Ra- accurate to a billionth of a second. The Spain and the APEX telescope in Chile, dio Astronomy using supercomputers technique makes it possible to resolve which is operated with the participa- known as correlators. >

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Event horizon Left Beyond the horizon. The graphic depicts a . The is the area in which no particles can remain at rest. The event horizon can be regarded as the surface of the black hole; anything that moves beyond it, quite literally vanishes from the world. Right Space antenna. The 30 -meter IRAM dish is the most sensitive single telescope in the Event Horizon Telescope’s worldwide array.

Ergosphere Inner horizon

“The results have given us the first clear ed in the campaign with the 30-meter The heart of the supergiant galaxy M87 glimpse of a . telescope. At 2800 meters above sea lev- has two particular characteristics that They mark an important milestone in el on Mount Pico Veleta in the Spanish make it a good candidate for the pro­ our understanding of the fundamental Sierra Nevada, it is the most sensitive ject. It is both unusually massive and processes underlying the formation single telescope in the EHT array. “We relatively close to Earth, which makes and evolution of in the uni- can adjust the antenna’s surface with a it easier to see. This makes it a perfect verse,” says Anton Zensus, Director at precision on the scale of a human hair,” target for astronomers who, with the the Max Planck Institute in Bonn and says astronomer Pablo Torne. global telescope array, finally have an Chairman of the EHT Collaboration instrument capable of directly targeting Board. Zensus considers it remarkable IDEAL WEATHER CONDITIONS such an exotic object. that the project’s astronomical observa- AND FUNCTIONING TECHNOLOGY The regions around supermassive tions and theoretical interpretation black holes are subject to the most ex- were achieved faster than expected. Over four days in April 2017, Torne and treme conditions known in space. Black According to IRAM Director Karl his colleagues at IRAM simultaneously holes are fascinating cosmic objects Schuster, the success is based on “de- aligned their telescopes on the center that incorporate an incredible total cades of European expertise” in milli- of the galaxy M87 and its gigantic black within a tiny volume of space. meter astronomy: “Back in the 1990s, hole for the first time, along with the Their mass and thus their gravitational the Max Planck Institute for Radio As- other EHT stations located around the pull are so great that even light can’t es- tronomy and our Institute with its two globe. “We couldn’t have wished for cape them. That’s what makes them observatories was demonstrating tech- better weather conditions for the time black, making it impossible to directly nically and scientifically that our of year. Most importantly, the observa- observe them. high-resolution radio observations rep- tory’s equipment worked perfectly, The only way to actually observe resented a unique method for analyz- from its high-precision atomic clock such cosmic traps is to image ing the immediate vicinity of super- and receiver systems to its data record- their “shadow”. This is caused by the massive black holes.” ers,” says Torne. In total, the observa- extreme bending of light – shortly be- IRAM, a facility co-financed by the tions recorded at the 30-meter antenna fore it irretrievably vanishes into the

Max Planck Society, actively participat- alone exceeded 500 terabytes of data. black hole. By making extremely pre- Graphic: designergold based on a draft from the MPI for Gravitational Physics

52 MaxPlanckResearch 2 | 19 Photo: IRAM/DiVertiCimes a journeyofaround55million light bent andamplifiedlikebya lens.After around themassivemonster isbeing light from the strongly heated matter ring-like structureintheimage: the and compactobject.Thisexplains the massive at highspeedsaroundthisvery black hole.Ahotgasplasmaismoving dark centralregion–theshadowof shows a ring-shaped structure with a quency of230gigahertz)andclearly 1.3 millimeters (correspondingtoafre leased, wasobtainedatawavelengthof to theedgesofblackholes. dense dustandgascloudsrightthrough range, astronomerscanseethroughthe inthemillimeter cise observations The image, which has now been re- » it is smaller and lies within the dark area. dark within the lies and smalleris it image; the visibleinnot is itself horizon event The - shoots outa“jet”ofmatter atleast active. Its nucleus Aandisvery known asastrongradiosource called under number87.Thegalaxy isalso Charles Messiercataloged the object ter. In1781,the Frenchastronomer near thecenterofVirgo galaxyclus pergiant ellipticalgalaxyknownasM87, dark area. age; itissmallerandlieswithinthe horizon itselfisnotvisibleintheim- than lightthatmovingaway. Theevent coming towardusappearsbrighter fect. Astheblackholeisrotating,light in theringareduetoarelativisticef- ray. Thevariationsinbrightnesswith- years, itarrivesattheEHTtelescopear- The originoftheblackholeisasu- - observations. tent withthatderivedfrom previous lion .Thefigureishighly consis- been calculatedasequivalent to6.5bil mass of the black hole in M87. This has accurately determinethe enormous andallowsthemto central machinery deal aboutthenatureofgalaxy’s The shadowtellstheresearchers agreat MACHINERY OFAGALAXY A GLIMPSEINTO THE CENTRAL perpendicular tothedisk. high velocityasatightlyfocusedbeam black holeatitscenterandemitted celerated in the disk of the 5000 light years long. This has been ac-

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A giant in the sky. The supergiant galaxy Messier 87 was one of the targets of the Event Horizon Telescope observation campaign. The jet visible in this optical image of the galactic nucleus clearly originates from the supermassive black hole in the center of the .

Anton Zensus considers the success to be a watershed moment in astronomy. “In the future, researchers both in and well beyond our field will clearly de- lineate periods before and after this discovery,” says the Max Planck re- searcher. He believes astronomers will learn more about galactic nuclei and obtain a complete picture of how ac- tive galaxies are formed and evolve. In addition, he anticipates the discovery will allow us to test Einstein’s general ever more rigorous- “For many decades, the only way to tivity range and gain even more fasci- ly. “Black holes, after all, are an ideal detect black holes was indirectly,” says nating insights,” says IRAM Director laboratory to test what happens in Michael Kramer, Director at the Max Karl Schuster. strong gravity.” Planck Institute for Radio Astronomy. Then, a few years ago, scientists suc-

ceeded in detecting gravitational waves for the first time, allowing us to “hear” the effect of merging black SUMMARY holes on space-time. l Astronomers have taken the first picture of a black hole. It resides at the center “Now we can finally see them as well, of the supergiant elliptical galaxy M87, about 55 million light years away. giving us the fascinating chance to l The discovery was made by combining observations made in April 2017 from an study in a unique way these exotic array of radio antennas distributed around the Earth to form a virtual telescope. ­objects and the extreme curvature l This was done using very-long-baseline interferometry (VLBI), a technique that they impose on space-time,” says combines the signals of the individual antennas. ­Michael Kramer, a leading scientist in l Researchers hope that images of black holes will help them better under­stand the BlackHoleCam project, which is galactic nuclei and provide insights into the formation and evolution of active galaxies. part of the Event Horizon Telescope consortium totaling roughly 200 re- searchers. GLOSSARY THE OBSERVATION REPRESENTS Accretion disc: A disk of matter rotating and coalescing towards (accreting) a central ob - A WATERSHED MOMENT ject, for instance a black hole. It can be composed of atomic gas, (ionized gas) or interstellar dust. Toward the center, the speed of rotation and temperature rise sharply. The observations are ongoing. At the Arc second: The term arc second is often employed as an expression of a telescope’s re- end of 2018, NOEMA in the French solving power, the smallest angle it can successfully resolve. An arc second corresponds th Alps joined the global array. This sec- to one 3600 of a degree. A microarcsecond is one millionth of an arcsecond. ond IRAM observatory has twelve : French astronomer (1730–1817) whose career began in the French Navy, before leading him to France’s Bureau of Longitude. He discovered 20 and created highly sensitive antennas, making it a catalog of 103 diffuse astronomical objects, which have since been discovered to be gas the most powerful in the EHT in the nebulae, clusters, and galaxies. northern hemisphere. “NOEMA will

allow us to venture into a new sensi- Photo: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

54 MaxPlanckResearch 2 | 19 Interview_Black Holes

“An astounding coincidence with theory”

Max Planck Director Anton Zensus on the first observation of the shadow of a black hole

What sounds paradoxical is reality: Black antennas via software. This “phasing” was The key word is very long baseline interfer- holes have a shadow! In the Messier 87 an enormous technical challenge for us and ometry (VLBI). We aim several radio tele- galaxy, astronomers were able to observe was essential for the EHT. The weather con- scopes, which are far away from each oth- such a phenomenon for the first time with ditions also played into our hands – they er, at the same celestial object at the same the Event Horizon Telescope (EHT). The Max were really good right from the start. time. The signals collected are combined in Planck Institute for Radio Astronomy in a special computer – the correlator. In this Bonn had a first row seat. Anton Zensus What does an observation with eight way, a virtual telescope is created. This pro- heads the “Very-Long-Baseline Interferome- telescopes look like? vides an image sharpness corresponding to try” Department. The Department deals with a technique that enabled discovery in the first place. We talked to Anton Zensus, This marks the culmination of a long period Chairman of the EHT Collaboration Board, » about how the successful observation came of development. about and what the results mean.

Mr Zensus, how long has the Event Horizon Telescope project been running? Anton Zensus: Officially, it started two years ago. But the preparations have been going on for a decade. And if you factor in the preparatory and pioneering work, it’s actually 20 years. During this time, we have fundamentally improved the quality of our measurements and have already investi- gated important questions about active galaxies like M87 – such as the of the gigantic matter jets from their central re- gions. In this sense, we have now reached the peak of a long development.

You say that EHT had only been conducting observations for two years. Were you surprised by your success after this relatively short time? Yes indeed! It was also astonishing that so many things worked right away. After all, the Event Horizon Telescope consists of a combination of eight different telescopes. One of these telescopes – which is called ALMA – is located at an altitude of 5000 m in the Chilean Atacama Desert and com- prises 66 individual antennas. In order to be able to integrate this system into the Anton Zensus, Director at the Max Planck Institute for Radio Astronomy and Chairman of the EHT Collaboration Council. Photo: Helge Horn Photography EHT, we had to interconnect all individual

2 | 19 MaxPlanckResearch 55 that of a single antenna with the diameter glish astronomer Arthur Eddington mea- therefore appear black to us. According to of the distance between the most distant sured this phenomenon during a total so- theory, outside the event horizon, attract- antennas – in the case of the Event Horizon lar eclipse when he observed a small shift ed by the enormous mass, there is a huge Telescope, this is about 8000 km. Just of the star positions near the solar disk. By amount of gas swirling around in a vor- imagine: if your eyes were as sharp as the the way, that was on 29th of May 1919 – ex- tex-like disk structure at tremendous EHT, you could theoretically read a news- actly one hundred years ago. This com- speeds. The gas heats up and begins to paper in New York from Bonn. However, pletes the circle. glow. Relativistic particles – those that the EHT does not see any optical light but move at almost the in a mag- rather radio radiation with wavelengths of The black hole in the center of the netic field – also release synchrotron radia- just over one millimeter. is much closer than that in Messier 87. Why tion. So around a black hole, it “shines”, was the EHT still successful with M87? while the hole itself, as the name suggests, How is your Institute involved in the Event Our Milky Way is a bit hesitant to disclose appears black. It is this blackness that we’ve Horizon Telescope? its deepest secrets (laughs). But joking been observing. The EHT includes the 12-meter APEX tele- aside, there are, of course, sound reasons. scope, which is operated by our Max On one hand, the heart of our Milky Way is What did you discern from the shadow? Planck Institute for Radio Astronomy to- hidden in a dense fog of charged particles. To be honest, we were amazed at how well gether with the European Southern Obser- This leads to a flickering of the radio radi- the dark spot we observed matches the vatory and the Swedish Onsala Space Ob- ation and thus to blurred images of the structure predicted by our computer sim- servatory. It is located close to the ALMA center of the Milky Way. But I am confident ulations. From the shadow itself, the mass, site. The is also in- that we can still solve this problem. On the the rotation, and the of the volved with the IRAM 30-meter antenna other hand, galaxy M87 is about 2000 black hole can be derived. For this purpose, on the Pico Veleta in the Spanish Sierra Ne- times further away. However, the black 60,000 different simulations of black holes vada and will be involved with the NOEMA hole in its center is also 1000 times more were carried out on the computer and telescope near Grenoble in the future. A to- massive than the one in our Milky Way. The compared with the EHT results. tal of thirteen partner organizations from greater mass makes up for the greater dis- all over the world work for EHT. Our Insti- tance. The shadow of the black hole in M87 How will this successful observation tute operates a supercomputer that cali- therefore appears to us to be about half advance astronomy? brates and evaluates the data. In fact, the size of the one from the gravity trap in Black holes are an ideal laboratory for mea- enormous amounts of data are generated. our Milky Way. surements under strong gravity. We are at Each of the EHT’s individual telescopes de- the beginning of a phase in which many liver around 350 terabytes per day. Just what is the shadow of a black hole? new insights await us. We will soon be able A black hole deflects the light even more to confidently exclude alternative explana- You knew what you were looking for: the than our . The theory of relativity pre- tions for black holes – such as boson shadow of a black hole. I suppose theoretical dicts that a radiation ring should be ob- or . We will better understand considerations played an important role? served around a dark spot where the black the galactic centers and obtain a complete Yes, Einstein’s general theory of relativity hole is located. Right where the black hole picture of the formation and evolution of of 1915 provides the theoretical back - is. Some casually refer to this dark spot as active galaxies. We will also be able to ob- ground. Also about a hundred years ago, the shadow of the black hole. serve in the vicinity of the black astronomers observed jets for the first hole in our Milky Way and thus thoroughly time. These are gas flows that shoot out of But where does the light come from? check the general theory of relativity. the center of active galaxies and must be Black holes are black, aren’t they? generated at enormously high energies. According to the general theory of relativi- The interview was conducted by Helmut Hornung Since the 1970s, we have suspected that ty, black holes have an “event horizon”. It de- there are super-massive black holes behind scribes the region within which nothing it. The theory of relativity predicts that a can escape from the black hole. The event massive object can deflect light. The En- horizon – but also the area within – should

56 MaxPlanckResearch 2 | 19 Optical: DSS; illustration: NASA/CXC/M. Weiss of the darkthebodiesof mystery The T ral philosopher. versity; hewasalsoadedicated natu- Hebrew andGreek at CambridgeUni- chell hadnot only studiedtheology, highly regardedscholar. Indeed,Mi memorial inthechurch, wasalsoa chell, who,asisclearfromhistablet eval church liveditspriest,JohnMi TEXT than more 230 predicted first such were exotic objects Telescope now allowed has shadow time. the us to ablack of see It hole for first the is that startling space space bend around an and them. irresistible They time and exert Event The attraction. Horizon have only they indirectly on due gravitational to the couldn’tthat observed, they directly be effect Even have though they alarge mass, black are not holes made matter. of Until recently, that meant the 18 HELMUT HORNUNG HELMUT th century nearthetown’s century medi- shire. For26yearsduring English countyofYork- town ofThornhillinthe gins inthequiet little of black heholes be- story - - ment that a very massive star’sment that a very gravity it, hereasonedinathought experi ture onthegravityofstarsin 1783.In august institutionthathegave alec- of theRoyalSocietyinLondon. John Michellbeingelectedamember scientific community, anditresultedin causedquiteastirinthe This theory of differenttypesbelowtheground. earthquakes werepropagatedbywaves earthquake in 1755,hesuggested that an essaypublishedaftertheLisbon His waschieflyinterestedingeology. In It was before the members of this

years ago. years - Simon deLaplacesuggestedinhis1796 cian, physicist,andastronomer Pierre- up thesubject.TheFrenchmathemati - chell, anothernaturalphilosopher took us,” heconcluded. bodies ...theirlightcouldnotarriveat should reallyexistinnatureany[such] be abletoleaveitssurface.“Ifthere might besostrongthatnolightwould emits X-rays in the process. the in X-rays emits and star massive blue neighboring a from material up swallows X- Cynus the swirl: Cosmic More thanadecadeafterJohn Mi- 2 | MaxPlanckResearch 19MaxPlanckResearch _Black Holes History 1 blackhole 57

58 Schwarzschild radiusofsuch anob the basisofgeneralrelativity. The trically-charged, staticblack holeon and behaviorofanon-rotating, elec- Schwarzschild hadcalculated thesize in 1916,however, theastronomer Karl matics article inthejournalA our world. radiation wouldsimplydisappearfrom point singularitiesinwhichmatterand did, indeed,predicttheexistenceof on hisequationspublishedin1915,he ence –albeitnotintentionally. Based ofthese“darkbodies”intosci- entry relativity, who paved the way for the of stein, withhisgeneraltheory scurity. Intheend,itwasAlbertEin ly overlookedandquicklyfellintoob- and Pierre-SimondeLaplacewerelarge on thephysicalworldofJohnMichell obscur, a“darkbody.” Laplace termedsuchanobjectacorps ed ofcorpuscles,itssmallest particles. ofIsaacNewton,consist cepted theory this light,accordingtothegenerallyac light wouldnotbeabletoescapethem; that theircouldbemassivestarswhose work sibility ofsuchblackholes.Previously MaxPlanckResearch 2 MaxPlanckResearch But in1939Einsteinpublishedan However, thethoughtexperiments Exposition duSystèmeMonde attemptingtodisprovethepos- | 19 nnals

of

M athe ------deed, aftertheinitialhype, interest in initially disregardedbyscience. In toimmune skindisease. tions. He diedon 11 May1916 ofanau exact solutionsofEinstein’s fieldequa- ern FrontinRussia–heformulatedthe lieutenantontheEast- was anartillery middle of World War I – Schwarzschild in1909.Afewyearslater,ry inthe - rector oftheAstrophysicalObservato gen toPotsdam,wherehebecameDi- him via Munich,Vienna, and Goettin- astronomical careerwentontotake stars.His orbits ofplanetsandbinary in arenownedjournalcalculatingthe the age of 16 he published two papers talent wasevidentatanearlyage.At a German-Jewishfamily, Schwarzschild’s Frankfurt astheoldestofsixchildren rapid scientificsuccess.Bornin1873 In hisshortlife,Schwarzschildenjoyed EINSTEIN’S FIELDEQUATIONS EXACT SOLUTIONSOF size ofacherry. proximately onecentimeter, orthe For theEarth,thisradiuswouldbeap- beyond which nothing can return.ary pendent onitsmassandisthebound- ject, named after the astronomer, is de But the subject of black holes was - - - escape them. escape to able be not would light whose stars massive be could their that suggested Laplace de Pierre-Simon astronomer and physicist, mathematician, French the portant means of describing objects short wavelengthX-rayspectrum.Uhu- theskyinextremely cally tosurvey al astronomy. Itwaslaunchedspecifi - ru Right relativity. general of basis the on hole black static electrically-charged, non-rotating, a of behavior and size the calculated Schwarzschild Karl astronomer Left mately 30timesthemassof the Sun.It with abluesupergiantstar approxi that theX-raysource wasassociated X-1.Astronomersdiscovered tion ofCygnusstoodout.Itwas named one particularobjectinthe constella mostly neutron stars. Among these, ru who diedin2018. English physicistStephenHawking, studied thembecamestars–likethe taking centerstage.Scientistswho ory. Andthisledtoblack holesalso using newconceptsderivedfromthe- pected propertiescouldbedescribed present inextremestates.Theirunex- or neutronstarsinwhichmatteris theoreticians, forexample,whitedwarfs that, initially, of werethesolepreserve eral relativity”occurred. Clifford Will, “therenaissance ofgen- After this,inthewordsofphysicist tween themid-1920sandmid-1950s. steadily. Thiswasintheperiodbe Einstein’s system of thought waned - usheredinaneweraofobservation discoveredhundredsofsources, In theearly1970s,satelliteUhu- General relativitybecameanim Advanced mathematics. In In mathematics. Advanced Thought experiments. In 1796 1916 , , the the

- - - -

Photos: akg-images (2) History_Black Holes

Astronomers concluded that approximately 4.5 million solar must be » concentrated in a region the size of our planetary system.

has an orbital companion of approxi- concentrated in a region of space the they were able to take a picture of its mately 15 solar masses, and this must size of our planetary system. A massive- “shadow,” an observation which is con- be a black hole. ly compact mass of that kind can only sidered the first direct evidence of a The X-ray emission can be explained mean one thing: a gigantic black hole. gravity well on a galactic scale. by matter falling in toward the black Our Milky Way is also not an excep- But black holes had already been hole due to its gravity. This forms an ac- tion. Scientists believe that such mas- making headlines just a few years ear- cretion disk around the companion due sive monsters lurk in the centers of lier. In September 2015, scientists con- to its extreme gravity, swirling around most galaxies – some even more pow- firmed Einstein’s prediction of gravita- it at unimaginably high speed and erful than *. The supergi- tional waves, with the source calculated heating up to several million degrees ant galaxy Messier 87 has a black hole as the merging of two black holes of 36 due to friction. of about 6.5 billion solar masses! Like and 29 solar masses. The 230-year-old Cygnus X-1 is by no means the Sagittarius A*, this star system, at a dis- history of black holes is far from over. only black hole that astronomers have tance of approximately 55 million light Instead, these observations mark the indirectly detected. They now know of years, was one of the targets of the beginning of a new era in astronomy, many, with a mass ranging from four Event Horizon Telescope. The team, in- which will shed light on the dark uni- to sixteen solar masses. One, however, deed, have succeeded in their goal. It verse – and on the dark monsters that is a great deal more massive. It was dis- was announced on 10 April 2019 that lurk in it. covered at the end of the 1990s and re- sides at the heart of the Milky Way, some 26,000 light years away. In 2002, a group led by from the Max Planck Institute for Extrater- restrial Physics made another sensa- tional discovery. At the of the European Southern Observatory (ESO), the scientists made observations of a star that was approach- ing the center of our galaxy to a dis- tance of only 17 light hours (around 18 billion kilometers). In the following months and years, they kept track of the orbital motion of this star, as it was termed. It orbits the galaxy’s central region, Sagittarius A*, at an average speed of 5,000 kilome- ters per second and with a period of 15.2 years. The motion of S2 and other stars led astronomers to conclude that some 4.5 million solar masses must be

Ripples in space-time. In 2015, astronomers detected gravitational waves for the first time – the image is based on a “numerical relativity simulation.” It depicts two black holes merging deep in space. Photo: S. Ossokine, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme project, W. Ben ger (Airborne Hydro Mapping GmbH)

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