Scattata la prima foto di un buco nero (by Media INAF)

L’Event Horizon Telescope, collaborazione internazionale che vede la partecipazione di centri di ricerca in tutto il mondo, svela oggi la foto del secolo. Due ricercatrici dell’Inaf, Elisabetta Liuzzo e Kazi Rygl, sono tra i protagonisti della rivoluzionaria osservazione del gigantesco buco nero nel cuore della galassia Messier 87, come parte del progetto BlackHoleCam. Un altro italiano, Ciriaco Goddi, è segretario del consiglio scientifico del consorzio Eht e responsabile scientifico del progetto BlackHoleCam.

L’Event Horizon Telescope (Eht) è un gruppo di otto radiotelescopi da terra che opera su scala planetaria, nato grazie a una collaborazione internazionale e progettato con lo scopo di catturare le immagini di un buco nero. Oggi, in una serie di conferenze stampa coordinate in contemporanea in tutto il mondo, i ricercatori dell’Eht annunciano il successo del progetto, svelando la prima prova visiva diretta mai ottenuta di un buco nero supermassiccio e della sua ombra.

Questo incredibile risultato viene presentato in una serie di sei articoli pubblicati in un numero speciale di The Astrophysical Journal Letters. L’immagine rivela il buco nero al centro di Messier 87, un’enorme galassia situata nel vicino ammasso della Vergine. Questo buco nero dista da noi 55 milioni di anni luce e ha una massa pari a 6,5 miliardi e mezzo di volte quella del Sole.

La rete di radiotelescopi di Eht. Crediti: Eso/O. Furtak

L’Eht collega gli otto radiotelescopi dislocati in diverse parti del pianeta dando vita a un telescopio virtuale di dimensioni pari a quelle della Terra, uno strumento con una sensibilità e una risoluzione senza precedenti. L’Eht è il risultato di anni di collaborazione internazionale e offre agli scienziati un nuovo modo di studiare gli oggetti più estremi dell’universo previsti dalla teoria della relatività generale di Einstein, proprio nell’anno del centenario dell’esperimento storico che per primo ha confermato questa teoria.

«Quello che stiamo facendo è dare all’umanità la possibilità di vedere per la prima volta un buco nero – una sorta di ‘uscita a senso unico’ dal nostro universo», spiega il direttore del progetto Eht Sheperd Doeleman del Center for Astrophysics della Harvard University. «Questa è una pietra miliare nell’astronomia, un’impresa scientifica senza precedenti compiuta da un team di oltre 200 ricercatori».

I buchi neri sono oggetti estremamente compatti, nei quali una quantità incredibile di massa è compressa all’interno di una piccola regione. La presenza di questi oggetti influenza l’ambiente che li circonda in modo estremo, distorcendo lo spazio-tempo e surriscaldando qualsiasi materiale intorno.

«Se immerso in una regione luminosa, come un disco di gas incandescente, ci aspettiamo che un buco nero crei una regione scura simile a un’ombra, un effetto previsto dalla teoria della relatività generale di Einstein che non abbiamo mai potuto osservare direttamente prima», aggiunge il presidente dell’Eht Science Council Heino Falcke della Radboud University, nei Paesi Bassi. «Quest’ombra, causata dalla curvatura gravitazionale e dal fatto che la luce viene trattenuta dall’orizzonte degli eventi, rivela molto sulla natura di questi affascinanti oggetti e ci ha permesso di misurare l’enorme massa del buco nero di M87».

Vari metodi di calibrazione e di imaging hanno rivelato una struttura ad anello con una regione centrale scura – l’ombra del buco nero – risultato che ritorna nelle molteplici osservazioni indipendenti fatte dall’Eht.

Le osservazioni dell’Eht sono state possibili grazie alla tecnica nota come Very-Long-Baseline Interferometry (Vlbi) che sincronizza le strutture dei telescopi in tutto il mondo e sfrutta la rotazione del nostro pianeta per andare a creare un enorme telescopio di dimensioni pari a quelle della Terra in grado di osservare ad una lunghezza d’onda di 1,3 mm. La tecnica Vlbi permette all’Eht di raggiungere una risoluzione angolare di 20 micro secondi d’arco. Un livello di dettaglio tale da permetterci di leggere una pagina di giornale a New York comodamente da un caffè sul marciapiede di Parigi.

I telescopi che hanno contribuito a questo risultato sono stati Alma, Apex, il telescopio Iram da 30 metri, il telescopio James Clerk Maxwell, il telescopio Alfonso Serrano, il Submillimeter Array, il Submillimeter Telescope e il South Pole Telescope. L’enorme quantità di dati grezzi – misurabile in petabyte, ovvero milioni di gigabyte – ottenuta dai telescopi è stata poi ricombinata da supercomputer altamente specializzati ospitati dal Max Planck Institute for Radio Astronomy e dal Mit Haystack Observatory.

Ciriaco Goddi, project scientist di BlackHoleCam

La costruzione dell’Eht e le osservazioni annunciate oggi rappresentano il culmine di decenni di lavoro osservativo, tecnico e teorico. Un esempio di lavoro di squadra globale che ha richiesto una stretta collaborazione da parte di ricercatori di tutto il mondo. Tredici istituzioni partner hanno lavorato insieme per creare l’Eht, utilizzando sia le infrastrutture preesistenti che il supporto di diverse agenzie. I principali finanziamenti sono stati forniti dalla US National Science Foundation (Nsf), dal Consiglio europeo della ricerca dell’UE (Erc) e da agenzie di finanziamento in Asia orientale.

«L’Eso ha l’onore di aver contribuito in modo significativo a questo risultato attraverso la sua leadership europea e il suo ruolo chiave in due dei telescopi componenti di Eht, che si trovano in Cile – Alma e Apex», commenta il direttore generale dell’Eso Xavier Barcons. «Alma è la struttura più sensibile dell’Eht e le sue 66 antenne ad alta precisione sono state fondamentali per questo successo», conclude Ciriaco Goddi, segretario del consiglio scientifico del consorzio Eht, che si è occupato della calibrazione Alma per l’Eht.

Elisabetta Liuzzo, ricercatrice all’Inaf Ira di Bologna

L’Inaf può vantare un importante coinvolgimento nella rivoluzionaria osservazione come parte del progetto europeo BlackHoleCam(Bhc), di cui lo stesso Goddi è il project scientistElisabetta Liuzzo e Kazi Rygl dell’Istituto nazionale di astrofisica (all’Ira di Bologna) sono due ricercatrici del nodo italiano dell’Alma Regional Centre, uno dei sette che compongono la rete europea che fornisce supporto tecnico-scientifico agli utenti di Alma, e che è ospitato proprio presso la sede dell’Inaf di Bologna. Nel 2018 entrambe sono entrate a far parte del progetto Bhc finanziato dall’Erccome partner del progetto EHT, e fanno a tutti gli effetti parte dell’Event Horizon Telescope Consortium, in cui sono membri dei gruppi di lavoro che si occupano di calibrazione e imaging.

«La calibrazione dei dati Eht è stata una grande sfida: i segnali astronomici sono deboli nella banda millimetrica, e distorti per effetto dell’atmosfera, che varia molto velocemente a queste frequenze», sottolinea Liuzzo, che insieme a Rygl ha partecipato allo sviluppo di uno dei tre software usati per la calibrazione dei dati Eht.

Kazi Rygl ricercatrice all’Inaf Ira di Bologna

Pur operando come un unico strumento che abbraccia il globo l’Eht, infatti, rimane una miscela di stazioni con design e operazioni diverse. Questo ed altri fattori, insieme alle sfide associate alla Vlbi, hanno dato impulso allo sviluppo di tecniche specializzate di elaborazione e calibrazione. «Tre diversi gruppi di ricerca, ognuno dei quali ha utilizzato un diverso software di calibrazione, hanno convalidato in modo incrociato questi dati e hanno trovato risultati coerenti», specifica Rygl, aggiungendo che «è estremamente gratificante vedere come i dati calibrati possano essere tradotti in fisica dei buchi neri».

«Il progetto Black Hole Cam è partito nel 2014 con l’obiettivo di misurare, comprendere e ‘vedere’ i buchi neri e fare test sulle principali previsioni della teoria della relatività generale di Einstein», aggiunge Ciriaco Goddi. «Nel 2016 il progetto è entrato a far parte, insieme ad altri partner internazionali, dell’Event Horizon Telescope Consortium visto il comune obiettivo: ottenere la prima immagine di un buco nero».

«Abbiamo raggiunto un risultato che solo una generazione fa sarebbe stato ritenuto impossibile», conclude Doeleman. «I progressi tecnologici e il completamento dei nuovi radiotelescopi nell’ultimo decennio hanno permesso al nostro team di assemblare questo nuovo strumento, progettato per vedere l’invisibile».

Un risultato incredibile, che prometta di essere un punto non di arrivo ma di partenza nella strada per la comprensione del nostro universo.

Originally published by Media Inaf – Apr 10, 2019

Astronomers Capture First Image of a Black Hole (by Radboud University)

For the first time, astronomers have managed to take a photo of a supermassive black hole and its shadow. They used the Event Horizon Telescope (EHT), a worldwide network of eight radio telescopes, that together form a virtual telescope the size of the earth. The news was presented in six press conferences around the world simultaneously.

Radboud University is represented in the management of the EHT. Astronomers from Nijmegen, the University of Amsterdam, Leiden University, and JIVE are involved in the project, as well as the NOVA-technical submm-group of the University of Groningen.

The results will be published in six scientific articles in a special edition of Astrophysical Journal Letters. The photo shows the black hole at the centre of Messier 87, a massive galaxy in the nearby Virgo galaxy cluster. This black hole is 55 million light-years from earth and is 6.5 billion times the mass of the Sun.

Interlinking the eight telescopes has resulted in unprecedented sensitivity and resolution. Time after time, independent observations with the EHT, using different imaging techniques, have revealed a circular-type structure, with a dark area in the middle, a shadow of the black hole in M87.

“Scientists from all over the world have worked together”, says Prof. Anton Zensus of MPIfR in Bonn, chair of the EHT management. The director of the EHT Project, Sheperd S. Doeleman of the American Harvard/Smithsonian Center for Astrophysics, speaks of “a milestone in astronomy, achieved with a team of over 200 researchers from 18 countries”.

Simulation of the Messier 87 black hole, made by Radboud astrophysicist Jordy Davelaar

In the beginning

Heino Falcke, Professor of Astroparticle Physics and Radio Astronomy at Radboud University, is the chair of the EHT Science Council and was there when the idea to photograph a black hole using a network of telescopes was first proposed. “If the black hole exists in a bright area, such as a disc of glowing gas, we expect that it will create a very dark area, comparable to a shadow. We have also compared the photo with supercomputer simulations of different black-hole models. These simulations match up surprisingly well with the observations and make it possible to determine the characteristics of the black hole.”

The shadow is created by deflection of the light caused by the curvature of space and by the absorption of light in the so-called event horizon of the black hole. The horizon is the edge of the area from which nothing, not even light, can escape from the black hole. Falcke: “Shape and size of the shadow perfectly match our expectations based on Einstein’s general theory of relativity and the existence of an event horizon.”

Exotic objects

Black holes are exotic cosmic objects which have enormous mass, but are small in size. A black hole exerts extreme influence on its environment. It curves spacetime and heats surrounding matter to super-high temperatures. “The size of the shadow is related to the mass of a black hole and we managed to actually measure the enormous mass of the black hole in M87”, says Sera Markoff, Professor of Astrophysics in Amsterdam, who is a member of the EHT Science Council and coordinator of the Multiwavelength Working Group.

“We know that black holes exert an enormous influence over their surroundings, at scales hundreds of millions times bigger than those of its event horizon. Using the EHT, we have been able to observe the origin of this process for the first time”, adds Markoff.

New instrument

With the EHT, scientists have a new instrument to study the most extreme objects in the universe, which were predicted by Einstein. The result comes exactly 100 years after the experiment that first proved Einstein’s theory.

Project manager of the EHT Project Remo Tilanus (Leiden University and Radboud University) is delighted: “This fantastic result follows years of hard work by teams all over the world to technically realise the EHT and have it ready for the observations by 2017. This has been a golden year: not only did everything work smoothly, but the weather was perfect everywhere too.”

Team work

At Radboud University itself, a team of 10 researchers and students, co-managed by astrophysicists Monika Moscibrodzka and Ciriaco Goddi, have worked hard over the past two years to achieve this result. They took part in the observations with the different telescopes and made a crucial contribution to the data analysis and the development of the theoretical models.

Important contributions were provided by the University of Amsterdam in the area of modelling and interpretation, by the Allegro group of the Leiden Observatory in relation to the calibration of the observations, by JIVE in the field of data-analysis software, and by the NOVA submm group of the University of Groningen in relation to specialised equipment.

The next step

Falcke is looking forward to achieving clearer imaging after upgrades in the network. “It is the beginning of a new era in which the ultimate limit of space and time is no longer an abstract concept, but a measurable reality. To increase the sensitivity, we want to expand the EHT network and build a millimetre telescope in Africa. We are fortunate to already have the first supports in place, from different parties and even businesses.”

More information

Heino Falcke received the Spinoza prize from the Netherlands Organisation for Scientific Research (NWO) in 2012 and a large grant from the European Research Council in 2013, which facilitated the research of the BlackHoleCam group. This Synergy Grant of €14 million was awarded to Falcke and co-principle investigators Luciano Rezzolla (Goethe Universiteit Frankfurt) and Michael Kramer (Max-Planck Instituut Bonn). Partner institutes are JIVE, IRAM, MPE Garching, IRA/INAF Bologna, SKA and ESO. The BlackHoleCam team is part of the EHT.

All astronomers from the Netherlands who are involved with the Event Horizon Telescope are listed here.

Originally published in the Radboud University News page of the Astronomy Department– Apr 10, 2019

Astronomers Capture First Image of a Black Hole

An international collaboration presents paradigm-shifting observations of the gargantuan black hole at the heart of distant galaxy Messier 87.

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.

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 [1], 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 [2].

The EHT links telescopes around the globe to form an Earth-sized virtual telescope with unprecedented sensitivity and resolution [3]. 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 [4].

“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 [5]. “This makes us confident about the interpretation of our observations, including our estimation of the black hole’s mass.”

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 very-long-baseline interferometry (VLBI) which synchronises 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 micro-arcseconds — enough to read a newspaper in New York from a sidewalk café in Paris [6].

The telescopes contributing to this result were ALMA, APEX, the IRAM 30-meter telescope, the James Clerk Maxwell Telescope, the Large Millimeter Telescope Alfonso Serrano, the Submillimeter Array, the Submillimeter Telescope, and the South Pole Telescope [7]. 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.”

Notes

[1] 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.

[2] 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.

[3] 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.

[4] 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.

[5] 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.

[6] Future EHT observations will see substantially increased sensitivity with the participation of the IRAM NOEMA Observatory, the Greenland Telescope and the Kitt Peak Telescope.

[7] 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.

More Information

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 that summarizes the published studies.

Press release images in higher resolution (4000×2330 pixels) can be found here in PNG (16-bit), and JPG (8-bit) format. The highest-quality image (7416×4320 pixels, TIF, 16-bit, 180 Mb) can be obtained from repositories of our partners, NSF and ESO. A summary of latest press and media resources can be found on this page.

The EHT collaboration involves more than 200 researchers from Africa, Asia, Europe, North and South America. The international collaboration is working to capture the most detailed black hole images ever by creating a virtual Earth-sized telescope. Supported by considerable international investment, the EHT links existing telescopes using novel systems — creating a fundamentally new instrument with the highest angular resolving power that has yet been achieved.

The individual telescopes involved are; ALMA, APEX, the IRAM 30-meter Telescope, the IRAM NOEMA Observatory, the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope Alfonso Serrano (LMT), the Submillimeter Array (SMA), the Submillimeter Telescope (SMT), the South Pole Telescope (SPT), the Kitt Peak Telescope, and the Greenland Telescope (GLT).

The EHT collaboration consists of 13 stakeholder institutes; the Academia Sinica Institute of Astronomy and Astrophysics, the University of Arizona, the University of Chicago, the East Asian Observatory, Goethe-Universitaet Frankfurt, Institut de Radioastronomie Millimétrique, Large Millimeter Telescope, Max Planck Institute for Radio Astronomy, MIT Haystack Observatory, National Astronomical Observatory of Japan, Perimeter Institute for Theoretical Physics, Radboud University and the Smithsonian Astrophysical Observatory.

Lifting the veil on the black hole at the heart of our Galaxy

Including the powerful ALMA into an array of telescopes for the first time, astronomers have found that the emission from the supermassive black hole Sagittarius A* (Sgr A*) at the center of our Galaxy comes from a smaller region than previously thought. This may indicate that a radio jet from Sgr A* is pointed almost toward us. The paper, led by the Nijmegen PhD student Sara Issaoun, is published in the Astrophysical Journal.

So far, a foggy cloud of hot gas has prevented us from making sharp images of the supermassive black hole Sgr A* and causing doubt on its true nature.  Astronomers have now included for the first time the powerful ALMA telescope in northern Chile into a global network of radio telescopes to peer through this fog, but the source keeps surprising them: its emission region is so small that the source may actually have to point directly at us.

Observing at a frequency of 86 GHz with the technique of Very Long Baseline Interferometry (VLBI), which combines many telescopes to form a virtual telescope the size of the Earth, the team succeeded in mapping out the exact properties of the light scattering blocking our view of Sgr A*. The removal of most of the scattering effects has produced a first image of the surroundings of the black hole.

Top left: simulation of Sgr A* at 86 GHz. Top right: simulation with added effects of scattering. Bottom right: scattered image from the observations, this is how we see Sgr A* on the sky. Bottom left: the unscattered image, after removing the effects of scattering in our line of sight, this is how Sgr A* really looks.  Credit: S. Issaoun, M. Mościbrodzka, Radboud University/ M. D. Johnson, CfA

The high quality of the unscattered image has allowed the team to constrain theoretical models for the gas around Sgr A*. The bulk of the radio emission is coming from a mere 300 millionth of a degree, and the source has a symmetrical morphology. “This may indicate that the radio emission is produced in a disk of infalling gas rather than by a radio jet,” explains Issaoun, who has tested several computer models against the data. “However, that would make Sgr A* an exception compared to other radio emitting black holes. The alternative could be that the radio jet is pointing almost at us.”

Issaoun’s supervisor Heino Falcke, Professor of Radio Astronomy at Radboud University, calls this statement very unusual, but he also no longer rules it out. Last year, Falcke would have considered this a contrived model, but recently the GRAVITY team came to a similar conclusion using ESO’s Very Large Telescope Interferometer of optical telescopes and an independent technique. “Maybe this is true after all”, concludes Falcke, “and we are looking at this beast from a very special vantage point.”

Supermassive black holes are common in the centers of galaxies and may generate the most energetic phenomena in the known universe. It is believed that, around these black holes, matter falls in a rotating disk and part of this matter is expelled in opposite directions along two narrow beams, called jets, at speeds close to the speed of light, which typically produces a lot of radio light. Whether the radio emission we see from Sgr A* comes from the infalling gas or the outflowing jet is a matter of intense debate.

Sgr A* is the nearest supermassive black hole and ‘weighs’ about 4 million solar masses. Its apparentsize on the sky is less than a 100 millionth degree, which corresponds to the size of a tennis ball on the moon as seen from the Earth. To measure this, the technique of VLBI is required. The resolution achieved with VLBI is further increased by the observation frequency. The highest frequency to date to use VLBI is 230 GHz. “The first observations of Sgr A* at 86 GHz date from 26 years ago, with only a handful of telescopes. Over the years, the quality of the data has improved steadily as more telescopes join,” says J. Anton Zensus, director of the Max Planck Institute for Radio Astronomy.

The research of Issaoun and international colleagues describes the first observations at 86 GHz in which ALMA also participated, by far the most sensitive telescope at this frequency. ALMA became part of the Global Millimeter VLBI Array (GMVA) in April 2017. The participation of ALMA, made possible by the ALMA Phasing Project effort, has been decisive for the success of this project.

The Global Millimeter VLBI Array, joined by ALMA. Credit: S. Issaoun, Radboud University/ D. Pesce, CfA

“Sgr A* is located in the southern sky, thus the participation of ALMA is important not only because of its sensitivity but also because of its location in the southern hemisphere,” says Ciriaco Goddi, from the European ALMA Regional Center node in the Netherlands (ALLEGRO, Leiden Observatory). In addition to ALMA, twelve telescopes in North America and Europe also participated in the network. The resolution achieved was twice as large as in previous observations at this frequency and produced the first image of Sgr A* that iscompletely free of interstellar scattering (an effect caused by density irregularities in the ionized material along the line of sight between Sgr A* and the Earth).

To remove the scattering and obtain the image, the team used a technique developed by Michael Johnson of the Harvard-Smithsonian Center for Astrophysics (CfA). “Even though scattering blurs and distorts the image of Sgr A*, the incredible resolution of these observations allowed us to pin down the exact properties of the scattering,”says Johnson.“We could then remove most of the effects from scattering and begin to see what things look like near the black hole. The great news is that these observations show that scattering will not prevent the Event Horizon Telescope from seeing a black hole shadow at 230 GHz, if there’s one to be seen.”

Future studies at different wavelengths will provide complementary information and further observational constraints for this source, which holds the key to a better understanding of black holes, the most exotic objects in the known universe.

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More information? Contact:

Article:

The Size, Shape, and Scattering of Sagittarius A * at 86 GHz: First VLBI with ALMA: Issaoun, M. D. Johnson, L. Blackburn, C. D. Brinkerink, M. Mościbrodzka, A. Chael, C. Goddi, I. Martí-Vidal, J. Wagner, S. S. Doeleman, H. Falcke, T. P. Krichbaum, K. Akiyama, U. Bach, K. L. Bouman, G. C. Bower, A. Broderick, I. Cho, G. Crew, J. Dexter, V. Fish, R. Gold, J. L. Gómez, K. Hada, A. Hernández-Gómez, M. Janßen M. Kino, M. Kramer, L. Loinard, R.-S. Lu, S. Markoff, D. P. Marrone, L. D. Matthews, J. M. Moran, C. Müller, F. Roelofs, E. Ros, H. Rottmann, S. Sanchez, R. P. J. Tilanus, P. de Vicente, M. Wielgus, J. A. Zensus and G.-Y. Zhao.

Article reference: Issaoun, S., et al. 2019, ApJ, 871, 30 (https://doi.org/10.3847/1538-4357/aaf732).

Arxiv link: https://arxiv.org/abs/1901.06226

Astronomers from the BlackHoleCam team led and worked on the research.

The research team is also part of the EHT consortium, an international partnership of thirteen institutes from ten countries: Germany, the Netherlands, France & Spain (via IRAM), USA, Mexico, Japan, Taiwan, Canada and China (via EAO). http://www.eventhorizontelescope.org/

The participation of ALMA through the ALMA Phasing Project has been decisive for the success of this project. https://www.almaobservatory.org/

The data were correlated at the Max Planck Institute for Radioastronomy (MPIfR). Data analysis software was developed at the MIT Haystack Observatory and the Smithsonian Astrophysical Observatory.

This work is supported by the ERC Synergy Grant “BlackHoleCam: Imaging the Event Horizon of Black Holes”, Grant 610058. The National Science Foundation (AST-1126433, AST-1614868, AST-1716536) and the Gordon and Betty Moore Foundation (GBMF-5278) have provided financial support for this work. This work was supported in part by the Black Hole Initiative at Harvard University, which is supported by a grant from the John Templeton Foundation. The GMVA is partially supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 730562 (RadioNet).

This press release was originally posted at https://www.ru.nl/astrophysics/news-agenda/news/news-ru/lifting-veil-black-hole-heart-our-galaxy/