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First M87 Event Horizon Telescope Results and the Role of ALMA Astronomical Science DOI: 10.18727/0722-6691/5150 First M87 Event Horizon Telescope Results and the Role of ALMA Ciriaco Goddi1, 2 5 Onsala Space Observatory, Chalmers lengths that for the first time included Geoff Crew 3 University of Technology, Sweden the Atacama Large Millimeter/ Violette Impellizzeri 4 6 Department of Astronomy and Astro- submillimeter Array (ALMA). The addi- Iván Martí-Vidal 5, 6 physics/Astronomical Observatory, tion of ALMA as an anchor station Lynn D. Matthews 3 University of Valencia, Spain has enabled a giant leap forward by Hugo Messias 4 7 Max-Planck-Institut für Radioastronomie increasing the sensitivity limits of the Helge Rottmann 7 (MPIfR), Bonn, Germany EHT by an order of magnitude, effec- Walter Alef 7 8 Center for Astrophysics | Harvard & tively turning it into an imaging array. Lindy Blackburn 8 Smithsonian, Cambridge, USA The published image demonstrates that Thomas Bronzwaer 1 9 Steward Observatory and Department it is now possible to directly study Chi-Kwan Chan 9 of Astronomy, University of Arizona the event horizon shadows of SMBHs Jordy Davelaar 1 Tucson, USA via electromagnetic radiation, thereby Roger Deane10 10 Centre for Radio Astronomy Tech- transforming this elusive frontier from Jason Dexter 11 niques and Technologies, Department a mathematical concept into an astro- Shep Doeleman 8 of Physics and Electronics, Rhodes physical reality. The expansion of Heino Falcke1 University, Grahamstown, South Africa the array over the next few years will Vincent L. Fish 3 11 Max-Planck-Institut für Extraterres- include new stations on different conti- Raquel Fraga-Encinas 1 trische Physik, Garching, Germany nents — and eventually satellites in Christian M. Fromm12 12 Institut für Theoretische Physik, Goethe space. This will provide progressively Ruben Herrero-Illana18 Universität, Frankfurt am Main, sharper and higher-fidelity images of Sara Issaoun1 Germany SMBH candidates, and potentially even David James 8 13 Joint Institute for VLBI ERIC (JIVE), movies of the hot plasma orbiting Michael Janssen 1 Dwingeloo, the Netherlands around SMBHs. These improvements Michael Kramer 7 14 Anton Pannekoek Institute for Astron- will shed light on the processes of black Thomas P. Krichbaum 7 omy, University of Amsterdam, the hole accretion and jet formation on Mariafelicia De Laurentis 19, 20 Netherlands event-horizon scales, thereby enabling Elisabetta Liuzzo 21 15 Instituto de Radioastronomía Milimétrica, more precise tests of general relativity Yosuke Mizuno12 IRAM, Granada, Spain in the truly strong field regime. Monika Moscibrodzka1 16 Mullard Space Science Laboratory, Iniyan Natarajan10 University College London, Dorking, Oliver Porth14 UK Supermassive black holes and their Luciano Rezzolla12 17 Kavli Institute for Astronomy and Astro- shadows: a fundamental prediction of Kazi Rygl 21 physics, Peking University, Beijing, general relativity Freek Roelofs1 China Eduardo Ros 7 18 ESO Black holes are perhaps the most Alan L. Roy 7 19 Dipartimento di Fisica “E. Pancini,” fundamental and striking prediction of Lijing Shao17, 7 Universitá di Napoli “Federico II”, Einstein’s General Theory of Relativity Huib Jan van Langevelde13, 2 Naples, Italy (GR), and are at the heart of fundamental Ilse van Bemmel13 20 INFN Sez. di Napoli, Compl. Univ. di questions attempting to unify GR and Remo Tilanus1, 2 Monte S. Angelo, Naples, Italy quantum mechanics. Despite their impor- Pablo Torne15, 7 21 INAF–Istituto di Radioastronomia, tance, they remain one of the least tested Maciek Wielgus 8 Bologna, Italy concepts in GR. Since the 1970s, astron- Ziri Younsi 16, 12 omers have been accumulating indirect J. Anton Zensus 7 evidence for the existence of black holes on behalf of the Event Horizon In April 2019, the Event Horizon Tele- by studying the effects of their gravita- Telescope collaboration scope (EHT) collaboration revealed the tional interaction with their surrounding first image of the candidate super- environment. The first such evidence massive black hole (SMBH) at the cen- came from the prototypical high-mass 1 Department of Astrophysics, Institute tre of the giant elliptical galaxy Messier X-ray binary Cygnus X-1, where a star for Mathematics, Astrophysics and 87 (M87). This event-horizon-scale orbits an unseen compact object of ~ 15 Particle Physics (IMAPP), Radboud image shows a ring of glowing plasma solar masses, apparently feeding on University, Nijmegen, the Netherlands with a dark patch at the centre, which is material from its stellar companion at only 2 Leiden Observatory—Allegro, Leiden interpreted as the shadow of the black 0.2 au. More evidence has come from University, Leiden, the Netherlands hole. This breakthrough result, which studies of the Galactic Centre, where 3 Massachusetts Institute of Technology represents a powerful confirmation of ~ 30 stars have been tracked in tight, fast Haystack Observatory, Westford, USA Einstein’s theory of gravity, or general orbits (up to 10 000 km s–1) around a 4 Joint ALMA Observatory, Vitacura, relativity, was made possible by assem- radio point source named Sagittarius A* Santiago de Chile, Chile bling a global network of radio tele- or Sgr A* (Gillessen et al., 2009), practi- scopes operating at millimetre wave- cally ruling out all mechanisms The Messenger 177 – Quarter 3 | 2019 25 Astronomical Science Goddi C. et al., First M87 Event Horizon Telescope Results and the Role of ALMA GLT OSO MRO ESO/L. Benassi/O.Furtak MPIFR OAN NOEMA VLBA GBT ARO/SMT KPNO IRAM LMT JCMT SMA AMT ALMA LLAMAAMA APEX GMVA 2017 2020 SPT > 2020 responsible for their motions, except for a its Schwarzschild radius: Figure 1. Locations of the participating telescopes black hole with a mass of about four mil- R = 2 GM /c2 = 2 r , of the Event Horizon Telescope (EHT; shown in blue) Sch BH g and the Global mm-VLBI Array (GMVA; shown in lion solar masses. where rg is the gravitational radius, MBH is yellow) during the 2017 global VLBI campaign. Addi- the black hole mass, G is the gravitational tional telescopes that will observe in 2020 are Perhaps the most compelling evidence constant, and c is the speed of light. The shown in light blue; the GLT also joined in the cam- came in 2015, with the detection by angular size, subtended by a non-rotating paign conducted in 2018. Planned telescopes that may join the EHT in the future are shown in green. the advanced Laser Interferometer BH with diameter 2 RSch is: 6 Gravitational-Wave Observatory (LIGO) of qSch = 2 RSch/D ≈ 40 (MBH/10 M☉)(kpc/D) gravitational waves: ripples in space-time in microarcseconds (µas), where the candidate SMBH in the Universe. With produced by the merger of two stellar- black-hole mass is expressed in units of a mass of 4.15 million solar masses and mass black holes (Abbot et al., 2016). one million solar masses and the black at a distance of 26 400 light years or Despite this breakthrough discovery, hole’s distance (D) is in kiloparsecs. For 8.1 kpc (Gravity collaboration et al., 2019), there was until very recently no direct evi- stellar-mass black holes (with masses this SMBH is a factor of a million times dence for the existence of an event of a few to tens of solar masses), qSch larger than any stellar mass black hole in horizon, the defining feature of a black lies well below the resolving power of any the Galaxy and at least a thousand times hole and a one-way causal boundary current telescope. SMBHs, which are closer than any other SMBH in other in spacetime from which nothing (includ- thought to reside at the centre of most galaxies. The second-best candidate is ing photons) can escape. On 10 April galaxies, are millions to billions of times found in the nucleus of the giant elliptical 2019, the EHT provided the very first the mass of the Sun, but as they are galaxy M87, the largest and most mas- resolved images of a black hole, demon- located at much greater distances, their sive galaxy within the local supercluster strating that they are now observable apparent angular sizes are also generally of galaxies in the constellation of Virgo. astrophysical objects and opening a too small to be resolved using conven- Located 55 million light years from the new and previously near-unimaginable tional observing techniques. Fortunately, Earth (or 16.8 Mpc), it hosts a black hole window onto black hole studies. there are two notable exceptions: Sgr A* of 6.5 billion solar masses. Therefore, and the nucleus of M87. even though M87 is ~ 2000 times as dis- In order to conduct tests of GR using tant, it is ~ 1500 times as massive as astrophysical black holes, it is crucial to Sgr A*, yielding a (slightly) smaller but observationally resolve the gravitational Sgr A* and the nucleus of M87: the comparable angular size of the black hole sphere of influence of the black hole, “largest” black hole shadows in our sky shadow on the sky. Owing to the combi- down to scales comparable to its event nation of their masses and proximity, horizon. For a non-rotating black hole, Sgr A*, at the centre of our own Galaxy, both Sgr A* and the nucleus of M87 sub- the radius of the event horizon is equal to hosts the closest and best constrained tend the largest angular size on the sky 26 The Messenger 177 – Quarter 3 | 2019 among all known SMBHs (qSch ≈ 20 and 15 μas, respectively). This makes Sgr A* Location Location Location and M87 the two most suitable sources A B C for studying the accretion process and jet Radio signal Radio signal Radio signal formation in SMBHs, even enabling tests θθθ of GR at horizon-scale resolution. Radio telescopes The “shadow” of a black hole Analog Analog Analog ALMA (ESO/NAOJ/NRAO), J.
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