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. Pinto & N. Lira 000001 00000 000001 00000 000001 00000 00011101000 00011101000 00011101000 00011010001 00011010001 00011010001 Digital conversion Although by definition black holes cannot 0110010 1110 Digital 0110010 1110 Digital 0110010 1110 Digital be seen, we can detect light which passes very close to the event horizon Atomic clocks before escaping, allowing us to see what is around the black hole. Hard drives at So what would a black hole actually look radio telescopes like if we could observe it? David Hilbert began calculating the bending of light around a Schwarzschild (non-rotating) black hole in 1917. Bardeen (1973) subse- quently calculated the geometrical prop- erties of a rotating black hole’s silhouette against a bright background (an orbiting star). Although the likelihood of a black (Synchronised) hole passing in front of a star is very small, black holes never appear “naked” in astrophysical environments since their extreme gravitational fields will pull and Correlator compress matter from their surroundings, eventually forming a disc of luminous plasma. Luminet (1979) performed simu- Data reduction lations of a black hole surrounded by a geometrically thin, optically thick, accre- tion disc. Falcke, Melia and Agol (2000) demonstrated that an accreting black hole embedded in a plasma that is opti- cally thin at millimetre wavelengths (like the plasma expected to surround Sgr A*) Image would produce a bright ring of emission with a dim “shadow” cast by the black hole event horizon in its interior. They suggested that such a shadow might be detectable towards the Galactic Centre in an angular diameter on the sky of ~ 50 Figure 2. A schematic diagram of the VLBI tech- using the technique of very long baseline and ~ 40 μas (as viewed from the Earth) nique. Radio wave signals collected by individual antennas are converted from analogue to digital and interferometry (VLBI) at millimetre for Sgr A* and M87, respectively. Although recorded onto hard disks together with the time­ a wavelengths . very small, this angular size can now be stamps provided by extremely precise atomic clocks resolved by the VLBI technique at milli- at each station. In the 2017 campaign, a total of The shadow and ring are caused by a metre wavelengths using the EHT. about 4000 TB of recorded data was obtained. The data were shipped from each station to a central combination of light bending and photon location where a supercomputer (the correlator) capture at the event horizon. The size combined the signals between all pairs of antennas scale of the emission ring is set by Imaging black holes with the (synchronised using the timing information at each the photon capture radius R . For a non-­ Event Horizon Telescope (EHT) station). The output of the correlator is hundreds of c gigabytes, which is further reduced during data rotating Schwarzschild black hole, ­calibration down to tens to hundreds of megabytes. Rc = √27 rg ~ 2.5 RSch. The factor of ~ 2.5 The VLBI technique at millimetre The end product of the VLBI data processing is an comes from gravitational lensing, which wavelengths astronomical image. increases the radius of the photon ring with respect to the Schwarzschild radius For VLBI to work, a network of radio tele- (see, for example, Figure 1) must observe as seen from the observer, resulting scopes spread across different continents the same source at exactly the same time

The Messenger 177 – Quarter 3 | 2019 27 Astronomical Science Goddi C. et al., First M87 Event Horizon Telescope Results and the Role of ALMA

achieve an angular resolution as fine as operating in over 20 countries/regions. M87* 11 April 2017 20 µas, which is sufficient to resolve the The key elements of the road­map shadow of both Sgr A* and M87. There- towards an imaging array were the addi- fore, the VLBI technique effectively mimics tion of new sites to better sample the EHT Collaboration EHT a virtual telescope with the size of the Fourier plane, and the improvement in Earth. sensitivity needed to detect weak signals on short timescales (Doeleman et al., While VLBI is well-established at centi­ 2010). Two technological developments metre wavelengths, its extension to wave- were crucial for the latter: (1) improve- lengths as short as 1.3 mm only began in ment in the observing bandwidth by the 1990s (for example, Padin et al., 1990; increasing recording rates — over the last Krichbaum et al., 1997; ­Doeleman & 10 years, EHT data rates increased from Krichbaum, 1999). Challenges at shorter 4 to 64 Gb per second (Gbps); and (2) wavelengths include the reduced aper- development of phased-array systems to 50 µas ture efficiency and small diameter of radio- combine the collecting area of existing telescopes, increased noise in radio connected-element (sub)millimetre inter- 5 April 6 April10 April receiver electronics, higher atmospheric ferometers, which has led to the inclusion opacity, and above all, stronger distortion in the EHT of ALMA and the Submilli­ effects on the wavefronts from water meter Array (SMA) and the future incor- vapour in the troposphere, which limits the poration of the NOrthern Extended Milli- phase coher­ence to only a few seconds. meter Array (NOEMA). By 2003, several active galactic nuclei 0 123 456 (AGN) had been detected on interconti- Brightness temperature (109 K) nental baselines between Pico Veleta Phasing ALMA: turning ALMA into a (Spain) and the Heinrich Hertz Telescope giant single-dish VLBI station Figure 3. Image of the supermassive black hole M87* as obtained with the EHT (on four different (HHT – Arizona, USA) at wavelengths of days) in April 2017. Top panel: EHT image of M87* both 1 and 2 mm (Krichbaum et al., 2004; ALMA is the most sensitive (sub)millimetre- from observations on 11 April as a representative Doeleman et al., 2005). wavelength telescope ever built. It consists example of the images collected during the 2017 of 54 12-metre and 12 7-metre antennas campaign. The angular resolution of the observation (20 μas) is shown in the lower right corner. The located on the Chajnantor ­plateau in image is shown in units of brightness temperature. Formation of the EHT project the Atacama desert in Chile, the highest, North is up and east is to the left. Bottom panels: ­driest (accessible) desert on the Earth, similar images taken on different­ days showing the Early pathfinder experiments (Krichbaum and it ordinarily operates as a connected- stability of the ring structure across the observing week. et al., 1998) detected Sgr A* with the element interferometer. Although the baseline between Pico Veleta and the implementation of a VLBI mode was and in the same frequency band. Individ- Plateau de Bure Interferometer, but not part of the baseline project, the desir- ual antennas record their signals (plus the resolution was insufficient to probe ability of phasing the entire array for VLBI time stamps from very precise atomic horizon scales. In the mid-2000s, a had been recognised (Wright et al., 2001; clocks) onto computer hard disks which focused effort to boost sensitivity through Shaver, 2003) and some of the architec- are then shipped to a central location, increased bandwidth led to the develop- ture needed to sum signals from all where a supercomputer (called a correla- ment of fully digital VLBI backends, ALMA antennas was built into the ALMA tor) combines (cross-correlates) the sig- with the goal of intercontinental 1.3-mm correlator (Escoffier et al., 2007). nals between all pairs of antennas, syn- VLBI of Sgr A*. These systems were chronising them using the recorded deployed at sites in Arizona, California, Motivated by the prospect of using timing information from each station. Fig- and Hawai’i, and event-horizon-scale ALMA for horizon-scale observations of ure 2 is a diagram illustrating the data structures were detected in both Sgr A* supermassive black holes (Doeleman acquisition and processing path in a VLBI and M87 (Doeleman­ et al., 2008, 2012). et al., 2009a, 2010), the case for phasing experiment. These precursory scientific results moti- ALMA was renewed. An international vated a strategy aimed at building a global team, led by Doeleman at MIT Haystack The achievable angular resolution for an 1.3-mm VLBI array capable of imaging Observatory, proposed an ALMA Phasing interferometer is given by q ~ l/B (in the shadows of the SMBHs in both Sgr A* Project (APP) to the US National Science ­radians), where l is the observed wave- and M87, and spurred on the formation Foundation. The APP was accepted by length and B is the maximum distance of the EHT project, which was proposed the ALMA Board in 2011 and was com- between the telescopes (or baseline). during the US 2010 Decadal Survey pleted in 2018 thanks to an international Hence, higher frequencies (shorter wave- (Doeleman et al., 2009a)b. effort with contributors from the USA, lengths) and longer baselines provide Europe, and Asia. the highest resolving power. At 1.3 mm Since then, the EHT collaboration (or (corresponding to a radio frequency of EHTC) 1 has grown to include over 250 The heart of the APP is a beamformer 230 GHz), Earth-diameter VLBI baselines members representing ~ 60 institutes, system, which electronically combines

28 The Messenger 177 – Quarter 3 | 2019 the collecting area of ALMA by aligning precise hydrogen maser (required to Figure 4. The supermassive black hole at the centre the signals from individual ALMA anten- properly tune and synchronise the signals of M87. Left: The black hole feeds on a swirling disc of glowing plasma (shown in red), driving a powerful nas in phase to form a coherent sum in the VLBI array). Numerous software relativistic jet across several thousands of light years ­signal. Currently, up to 43 12-metre enhancements were also required, (shown in grey; simulation by Davelaar et al. 2018). antennas are used for the phased array, including the implementation of an ALMA Bottom right: Approaching the black hole, gravity is but the number may be smaller depend- VLBI Observing Mode (VOM) and a so strong that light is severely bent, creating a bright (almost circular) ring. The north–south asymmetry ing on the array configuration and the phase solver to adjust the phase of each in the emission ring is produced by relativistic beam- weather conditions. This effectively turns of the ALMA antennas during observa- ing and Doppler boosting (matter in the bottom ALMA into a giant virtual single dish tions to allow coherent summation of part of the image is moving toward the observer) and (hereinafter called “phased ALMA”), and their signals. The initial scope of the APP is mediated by the black hole spin (which is pointing away from Earth and rotating clockwise). Gravita- is equivalent to adding a ~ 70-metre dish effort and implementation can be found tional lensing magni­fies the apparent size of the to existing mm-VLBI arrays. In order to in Doeleman et al. (2010); the final working black hole’s event horizon into a larger dark shadow. phase ALMA successfully and incorpo- implementation is described in ­Matthews The emission between the photon ring and the rate it as a VLBI station, the APP had to et al. (2018) and Goddi et al. (2019). event horizon is due to emitting plasma either in the accretion flow and/or at the footprint of the jet (this add several new hardware and software emission is generally too dim to be detected by the components. These included an optical A broad science case for the use of the EHT; see Younsi et al., in preparation for detail). Top fibre link system to transport the phased- ALMA Phasing System was assembled right: While the EHT can zoom in very close to the sum signal from the ALMA Array Opera- by the international community in white event horizon, down to scales of only 0.01 light years c (or 3.7 light days), i.e., a region comparable to the tions Site (at an altitude of 5100 m) to papers (Fish et al., 2013; Tilanus et al., size of our Solar System, the relativistic jet (extended the ALMA Operations Support Facility (at 2014). Starting in 2016, VLBI as an across several thousand light years) can be probed 2900 m), where a set of Mark 6 VLBI observing mode was made available to using ALMA intra-baselines, recorded during the recorders was installed. ALMA’s original the astronomy community through the EHT observations (greyscale image; Goddi et al. 2019 and in prep.). rubidium clock was replaced with a more normal ALMA proposal system, with an C. Goddi,C. Z. Younsi, J. Davelaar/M.Kornmesser/ESO

The Messenger 177 – Quarter 3 | 2019 29 Astronomical Science Goddi C. et al., First M87 Event Horizon Telescope Results and the Role of ALMA

M87* K-jet, snapshot responds to SNR > 10 on non-ALMA of imaging its key science targets. The baselines and > 100 on ALMA base- global array included eight telescopes in lines. Therefore, the addition of ALMA six different geographical sites: the South into the EHT array greatly facilitates Pole Telescope (SPT), the Arizona Radio

J. Davelaar, Roelofs F. detections, especially for weak signals Observatory’s Submillimeter Telescope (for example, long baseline length or (SMT), the Large Millimeter Telescope bad weather). Using ALMA as a highly Alfonso Serrano (LMT) in Mexico, the sensitive reference station has enabled IRAM 30-metre telescope in Spain, the critical corrections for ionospheric and SMA and the James Clerk Maxwell Tele- tropospheric distortions at the other EHT scope (JCMT) in Hawai’i, and APEX and sites (see EHTC et al., 2019c for details). ALMA in Chile. These telescopes pro- II. ALMA has a central location in the EHT vided baseline lengths up to 10700 km array (see Figure 1), and is therefore towards M87, resulting in an array with a essential for the baseline coverage and resolution of ~ 20 μas (details are pro- image fidelity. Even though ALMA and vided in EHTC et al., 2019b). the Atacama Pathfinder Experiment (APEX) are extremely close geographi- Besides the EHT, which operates at a cally, ALMA’s superior sensitivity wavelength of 1.3 mm (i.e., a frequency of allows the EHT to detect signals with 230 GHz, ALMA Band 6), complementary 1 23456 the required 10-second integration VLBI observations with ALMA were also (Brightness temperature (109 K))0.5 times between all baselines, i.e., to find conducted at 3.5 mm or 86 GHz (ALMA VLBI fringe solutions, which has a dra- Band 3) in concert with the Global mm- Figure 5. Black hole model for M87* used for the matic impact on the imaging capability VLBI Array (GMVA) 3, which consists of up image reconstructions shown in Figure 6. This ­specific model has a relatively bright jet footprint appearing of the EHT (see Figure 6). to 18 telescopes located in Europe, North in front of the photon ring and a more extended jet III. VLBI observations with ALMA also America, and Asia. Figure 1 displays the emission extending towards west. Note the bright provide connected-element interfero- geographical locations of all the partici- knot to the south-west at the point where the jet metric data, which are archived, as pating telescopes in the EHT and the sheath crosses the photon ring in projection (see Davelaar et al., 2019). with any standard ALMA project, and GMVA in 2017 (plus additional telescopes are available to the user in the ALMA that joined after or plan to join in the near expected maximum time allocation of archive (after the appropriate proprie- future). ~ 5% of the total ALMA observing time. tary period). As outlined in Goddi et al. (2019), the calibration of such interfero- The EHT 2017 science observing cam- metric data allows one to determine paign was scheduled for April when Impact of ALMA in the EHT array the absolute amplitude calibration Sgr A* and M87 are night-time sources of the co-located sites ALMA–APEX in and tropospheric conditions tend to be Owing to the combination of a large physical flux-density (i.e, Jy). This, in the best, averaged over all sites in the effective aperture, its central location in turn, allows us to bootstrap source array. At that time, ALMA was in a more the VLBI array, excellent typical atmos- fluxes and calibrate longer baselines compact configuration as required for pheric conditions and ultra-low noise across the entire array (i.e., network phased array operations. About 40 EHT receivers, the addition of ALMA drasti- calibration). In addition, since VLBI astronomers travelled to four continents cally changed the overall capabilities of observations are always performed in the global EHT array, boosting the full-polarisation mode (in order to Figure 6. Image reconstructions from synthetic achievable signal-to-noise ratio (SNR) of ­supply input to the polarisation conver- data generated using the M87* model displayed in VLBI baselines by more than an order of sion process at the VLBI correlators — Figure 5 as input. Each panel shows a reconstruc- magnitude with respect to the first hori- see Martí-Vidal et al., 2016; Goddi et tion from simulated observations with a different array: 2017 EHT array (left column); planned 2020 zon-scale detections (Doeleman at al., al., 2019), the ALMA full-polarisation array (middle column); and a future EHT array includ- 2008, 2012). More specifically, the inclu- interferometric datasets can be used ing AMT in Namibia, and LLAMA in Argentina (right sion of ALMA in the EHT provides three to derive mm-wavelength emission column). A comparison between the top row (with key advantages: a boost in sensitivity; and polarisation properties of each ALMA) and the middle row (without ALMA) highlights the importance of ALMA, demonstrating that the improved baseline coverage; and an ­target observed by the EHT on arcsec- shadow feature can only be recovered if ALMA accurate measure­ment of the absolute ond scales (Goddi et al., in preparation). is part of the array. A comparison between the mid- flux-density scale and polarisation frac- dle row and the bottom row (where APEX is also tions of EHT targets (using standard excluded) clearly demonstrates that, even with a ­single-dish telescope in the same geographical loca- ALMA interferometric data). We expand The first global VLBI campaigns with tion of ALMA, it is not possible to recover an image on these characteristics below. ALMA with sufficiently high fidelity to discern the shadow. Although with future arrays the quality of the recon- I. The median thermal noise of non- Phased ALMA joined the EHT array for structed image will improve and it will be possible to recover new features (for example, the jet), the addi- ALMA baselines is 7 mJy, and 0.7 mJy the first time in April 2017, performing tion of new stations cannot fully compensate for the in ALMA baselines; for M87 this cor­ VLBI observations with an array capable loss of ALMA (middle and right columns).

30 The Messenger 177 – Quarter 3 | 2019 EHT2017 EHT2020 EHT2020 + AMT + LLAMA F. Roelofs,F. M. Janssen, Goddi C.

70 µas 70 µas 70 µas

0.5 1.01.5 2.0 2.5 3.0 3.5 4.0 1234 1 2 34 (Brightness temperature (109 K))0.5 (Brightness temperature (109 K))0.5 (Brightness temperature (109 K))0.5

EHT2017, no ALMA EHT2020, no ALMA EHT2020 + AMT + LLAMA, no ALMA

70 µas 70 µas 70 µas

0.5 1.01.5 2.0 2.5 3.0 3.5 0.5 1.01.5 2.0 2.5 3.0 3.5 0.5 1.01.5 2.0 2.5 3.0 3.5 4.0 (Brightness temperature (109 K))0.5 (Brightness temperature (109 K))0.5 (Brightness temperature (109 K))0.5

EHT2017, no Chile EHT2020, no Chile EHT2020 + AMT + LLAMA, no Chile

70 µas 70 µas 70 µas

0.5 1.01.5 2.0 2.5 0.5 1.01.5 2.0 2.5 3.0 3.5 4.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 (Brightness temperature (109 K))0.5 (Brightness temperature (109 K))0.5 (Brightness temperature (109 K))0.5

The Messenger 177 – Quarter 3 | 2019 31 Astronomical Science Goddi C. et al., First M87 Event Horizon Telescope Results and the Role of ALMA

and drove to mountaintops to carry out are provided in EHTC et al. (2019c,d). the black hole was modelled with general the observations, which included six Multiple independent analyses were also relativistic calculations spanning a wide ­science targets: the primary EHT targets, performed in order to verify the results range of possible accretion states (see Sgr A* and M87, and the AGN targets, (EHTC et al., 2019e,f). After about two next subsection). By tracing the peak of 3C 279, OJ 287, Centaurus A, and years of dedicated work by many dozens the emission in the ring we can determine NGC 1052d. Weather is a crucial factor­ of EHT scientists in multiple working the shape of the image, which is close for VLBI observations at mm wave- groups (from instrument through data to circular with an axial ratio 4:3 (corre- lengths, which is why the EHT uses flexi- processing to theory), the collaboration sponding to a 10% deviation from circu- ble observing schedules with windows was finally ready to communicate our larity). The emission in the ring is asym- that are about twice as long as the breakthrough to the world. metric and is brighter in the south, which approved number of observing nights. An can be explained as relativistic beaming array-wide go/no-go decision was made of plasma rotating (close to the speed a few hours before the start of each The breakthrough: first image of a black of light) in the clockwise direction around observing night, based on the weather hole shadow the black hole as seen by the observer conditions and predictions at each site, (i.e., the bottom part of the emission ring as well as technical readiness at each of The first image of a black hole was is Doppler-boosted towards the Earth). the participating telescopes. Observa- ­published by the EHT Collaboration on Based on our modelling and information tions were triggered over a 10-night win- 10 April 2019 in a series of six scientific on the inclination angle of the relativistic dow 5–14 April 2017 — on 5, 6, 7, 10, publications (EHTC et al., 2019a,b,c,d,e,f). jet (observed on larger scales), we derive and 11 April. During the whole campaign, The announcement and the historic the sense of rotation of the black hole to the weather was good to excellent at image were released worldwide in six be in the clockwise direction, i.e., the spin most stations. In addition to favourable simultaneous press conferences — axis of the black hole points away from us. weather conditions, the VLBI-specific in Washington, Brussels, Santiago, technical setup and operations at all sites ­Shanghai, Taipei and Tokyo — with addi- A number of elements reinforce the were successful, which resulted in fringe tional satellite events in Rome, Madrid, robustness of the result. The data analy- detections across the entire array. Munich, Leiden and Nijmegen, amongst sis used four independent data sets others. The core of M87, as imaged taken on four different days (spanning a By the end of the campaign, 96 disk mod- with the EHT, was renamed M87*, in line one-week observing window) in two ules were used, each containing eight with the name of the black hole candidate ­separate frequency bands (centered at helium-filled hard disks (of either six or at the centre of the Galaxy, Sgr A*. 227 and 229 GHz). The top part of Fig­- eight terabyte [TB] capacity), correspond- ure 3 shows an image of M87* on 11 April, ing to more than 5 petabytes (PB) of The most striking feature of the image while the bottom panels show similar (removable) storage; about 4 PB of data (displayed in Figure 3) is a bright circular images from three different days. The were eventually recorded in total. Given ring with an asymmetric brightness distri- diameter and width of the ring remain this huge volume of data, the observing bution and a dark region at its centre, stable and the image features are broadly campaign VLBI data could not be trans- which identifies the black hole shadow. consistent across all four observing days, ferred over the internet, but were shipped The ring reveals the curvature of space- except the position angle of the bright from each remote station to the two EHT time due to the extreme gravitational part in the asymmetric azimuthal profile, correlator centres for processing (the field around a SMBH, which bends light which varies in the range 150–200 degrees shipping took at least several days, and around it, creating an almost circular measured from north towards the east many months in the case of the South shadow at its centre. In fact, GR predicts between the first two days and the last Pole telescope). the shadow to be circular to within a two days. few percent, whereas alternative theories of gravity predict distorted, non-circular Overall, the size, circularity, asymmetry, Path to the image shapes (Younsi et al., 2016; also see Fig- and brightness contrast of the observed ure 7 of Goddi et al., 2017). image are consistent with the shadow of The EHT data were correlated at the MIT a “Kerr” black hole as predicted by Haystack Observatory in Westford, USA The ring has a measured diameter of GR and provide the strongest evidence to and at the MPIfR in Bonn, Germany. 42 ± 3 μas and the central brightness date of the existence of SMBHs in the Three independent data calibration pipe- depression has a contrast ratio > 10:1. nuclei of external galaxies. lines and three imaging pipelines, each The measured angular size, assuming a using a different software package and distance of 16.8 Mpc (EHTC et al., 2019f), associated methodology, were used for implies a black hole mass of M = (6.5 ± Modelling and physical interpretation of 9 the data processing in order to produce 0.7) × 10 M☉, or 6.5 billion times the the black hole image images independently. This approach mass of the Sun (consistent with one encourages the data analysis and sci- ­earlier mass measurement — Gebhardt The appearance of M87* has been mod- ence teams to minimise their biases in et al., 2011). To convert the measured elled using 3D general-relativistic mag- terms of both methodology and human diameter of the ring into the mass of the netohydrodynamic (GRMHD) simulations, decision making; further details of this black hole, the radiating plasma around which provide the physical conditions of

32 The Messenger 177 – Quarter 3 | 2019 the plasma and magnetic field surrounding Implications of the black hole shadow the baseline coverage and sensitivity, the black hole. GR ray-tracing radiative- on tests of GR and complementarity thereby affecting the resulting images, is transfer (GRRT) codes then take this with LIGO by performing simulated observations. GRMHD simulation data as input and cal- Instrument simulators were specifically culate the black hole’s appearance from Simulated images can be used to test built for the EHT to tie theoretical models the emitted radiation field. Approximately basic properties of black holes as pre- to instrument measurements. In parti­cular, 60 000 simulated images were produced dicted in GR (for example, Psaltis et al., they can generate realistic synthetic data, in the process (see EHTC et al., 2019e). 2015), or in alternative theories of gravity taking as input GRMHD model images, Figure 4 showcases the main components (Younsi et al., 2016; Mizuno et al., 2018). and performing synthetic observations of the M87 SMBH and their characteristic They can also be used to test alternatives using a specific EHT array and observing scales by comparing observed and simu- to black holes (Olivares et al., 2019). We schedule (see EHTC et al., 2019d,e). As a lated images (for one specific set of mod- estimate a deviation from circularity of demonstration, in Figure 5 we show a els). In particular, in the simulation to the < 10%, so we can set an initial limit on GRMHD simulation of the jet-launching left (which combines emission at wave- relative deviations from GR. Although it is region of M87 from Davelaar et al. (2019). lengths of 7, 3, and 1 mm; see Davelaar et difficult to rule out alternatives to black This specific model has arelatively ­ bright al., 2018 for details), one can see that the holes in GR — a shadow can be pro- jet footprint appearing in front of the pho- SMBH is embedded in an accretion flow duced by any compact object with unsta- ton ring and a more extended jet emission and powers a bipolar relativistic jet. ble circular photon orbits (Mizuno et al., extending towards west. We can then test 2018) — we can readily exclude exotic how well the input image (the ground truth) Zooming in closer to the centre (the alternatives to black holes, such as naked can be recovered with the current EHT ­simulation to the right in Figure 4 shows singularities or wormholes, which predict array and analyse­ the effect of adding new emission at 1 mm; Younsi et al., in prepa- much smaller shadows than we have stations and/or excluding existing stations. ration), hot magnetised plasma orbiting measured, whereas others like boson and accreting onto the black hole creates stars and gravastars need to be analysed Figure 6 shows some examples of recon- the familiar emission ring structure around with more care (Olivares et al., 2019); also structed images of M87, using the model the event horizon. As stated earlier, the see EHTC et al. (2019e) for further details. shown in Figure 5 as the input model, size of the ring is set by the photon cap- produced with one specific EHT synthetic ture radius: photons approaching the It is worth pointing out that the EHT result data generation pipeline (details are black hole with an impact parameter b < Rc provides a new way to study black hole reported in Roelofs, Janssen and EHTC, are captured and disappear into the black spacetimes and is complementary to the submitted). For instance, using the EHT hole; photons with b > Rc escape to infin- detection experiments of gravitational array and schedule that observed on ity; photons with b = Rc are captured waves from merging stellar-mass black 11 April 2017, the resulting simulated on an unstable circular orbit and produce holes with LIGO/Virgo (Abbott et al., 2016). image (shown in the left panel, top row of the so-called lensed photon ring. There are at least two main complemen- Figure 6) is similar to the one actually tary aspects between gravitational-wave observed by the EHT. The middle and While the EHT can resolve down to and electromagnetic observations of bottom rows show simulated images scales of 0.01 light years (or 3.7 light- black holes: without ALMA, which best showcase its days), i.e., a region comparable to the 1. Since EHT targets SMBHs and LIGO importance by clearly demonstrating size of our Solar System, the jet extends mainly targets stellar-mass black holes, that the familiar ring structure cannot be to much larger scales across several combining measurements from both reconstructed when ALMA is not part of thousand light years and can be probed methods we can test whether one of the array. Although APEX shares the using shorter baselines and/or lower the most fundamental properties of same geographical location as ALMA and frequencies. In Figure 4 (top right), we black holes in GR, that their size scales therefore should provide similar baseline show an image of the M87 jet, which linearly with mass, actually holds over coverage, the quality of the reconstructed extends across about 20 arcseconds, eight orders of magnitude. image is not sufficient to discern the ring corresponding to 5000 light years, 2. Gravitational wave experiments cannot when APEX is in the array and ALMA is obtained at 1.3 mm using ALMA interfer- rely on the possibility of multiple and excluded. These simulations clearly sub- ometric data (with maximum baseline repeated measurements of the same stantiate the need for ALMA’s sensitivity, lengths of only a few hundred metres) source, whereas the EHT can be used which allows for numerous and strong acquired simultaneously with the EHT to measure the shadow shape of M87 detections of weak signale. observations (Goddi et al., 2019 and in with ever increasing precision, leading to preparation). The science section image progressively better constraints on black By adding new stations, the quality of the (p. 24) showcases a montage of images hole parameters and their spacetime. reconstructed image improves and new of the M87 relativistic jet observed at sev- features (for example, the jet) can be eral radio wavelengths with multiple inter- recovered (see middle and right columns ferometers at progressively higher angu- Importance of ALMA in imaging M87 in the top row of Figure 6), but these lar resolution overlaid on the HST optical new stations cannot fully compensate for image: the VLA at 21 cm, VLBA at 7 mm, The most straightforward way to visualise the loss of ALMA (see middle and right GMVA at 3 mm, and EHT at 1.3 mm. how the loss of specific stations changes columns in the middle and bottom rows).

The Messenger 177 – Quarter 3 | 2019 33 Astronomical Science Goddi C. et al., First M87 Event Horizon Telescope Results and the Role of ALMA

Future directions the EHT’s capabilities over the coming measure parameters like black hole spin. years will bring more exciting scientific While current terrestrial VLBI at 1.3 mm The first EHT image of M87 has provided results. In 2018, the Greenland Telescope can resolve the black hole shadow only very strong evidence for the existence (GLT) joined the EHT (and GMVA) and in Sgr A* and M87, adding satellites of an event horizon and supports the VLBI observations were conducted as in space would significantly expand the notion of SMBHs being located at the part of ALMA Cycle 5 (the analysis of range of sources that can be resolved centre of galaxies. SMBHs present a new these observations is still ongoing). In on horizon scales. Combining ground- tool to explore gravity at its most extreme 2019, EHT observations were abandoned based VLBI at 0.87 mm with space- limit and on a mass scale that was because of operational difficulties at a based VLBI at longer wavelengths would ­hitherto inaccessible. Ongoing analysis of small number of key EHT sites. For 2020, provide better matching beam sizes, existing data and future EHT observa- observations are planned during ALMA which are important for spectral index tions will further help us understand Cycle 7 and will include new telescopes: and rotation measure studies. Studying the nature of black holes and will provide Kitt Peak National Observatory (KPNO) in nearby low-luminosity AGN could fill even more stringent tests of GR. Arizona, and the NOEMA interferometer the gaps in black hole mass, accretion/jet in France. These new stations will provide power, and host galaxy type between Future observations and detailed analysis intermediate baselines (<~ 1000 km) in Sgr A* and M87. Therefore all these devel- of M87 data will explore the shape and Europe (NOEMA–IRAM-30m) and impor- opments will open up very exciting and time variability of the shadow more accu- tant short baselines in the USA (KPNO– new scientific possibilities in the coming rately. The EHT is in the process of study- SMT) (<~ 100 km), thus further extending decades. ing the magnetised plasma around the baseline coverage for both M87 M87 in polarised light, which will allow us and Sgr A*. The possible future addition Finally, if we were to discover a radio pul- to investigate the mechanism by which of the Africa Millimeter Telescope (AMT) sar on a tight orbit (period < 1 year) black holes launch and power their rela- in Namibia (Backes et al., 2016) and around Sgr A*, this would allow us to tivistic jets. the Large Latin American Millimeter Array measure the black hole properties (mass, (LLAMA) in Argentina will add further distance and spin) more accurately As for Sgr A*, the mass-to-distance ratio baseline coverage, including the long than currently possible with orbiting stars is accurately measured from stellar baselines oriented east-west (AMT–ALMA) targeted by the AO-assisted, two-object, orbits in the near-infrared (Gravity Collab- and the intermediate baseline LLAMA– multiple beam-combiner interferometric oration et al., 2019), so measuring the ALMA (180 km). In particular, the addition VLTI instrument, GRAVITY, leading to a shadow shape and diameter provides a of short/intermediate baselines of the clean test of the no-hair theorem (Psaltis, null hypothesis test of GR (Psaltis et al., order of a few hundred km, which are Wex & Kramer, 2016). The detection of 2015). Since its mass is three orders sensitive to extended emission from the a magnetar at a projected distance of of magnitude smaller than that of M87*, jet on scales > 100 µas, may enable us to 0.1 pc from Sgr A* (Eatough et al., 2013) the dynamical timescales are minutes trace the jet down to the SMBH and suggests that finding a pulsar in a close instead of days; therefore observing the directly image the jet launching. See the orbit around Sgr A* should be possible, shadow of Sgr A* will require accounting middle and right columns of Figure 6 to and the recent detection of the Vela pul- for this variability as well as the mitigation evaluate the impact of these new stations sar with phased ALMA (Liu et al., 2019) of scattering effects caused by the inter- in recovering the jet structure; the location opens up the possibility of pulsar stellar medium (Johnson, 2016). Time-­ of the new stations is also displayed in searches with ALMA at high frequencies dependent non-imaging analysis can Figure 1. (where the effect of interstellar scattering potentially be used to track orbits of hot is lower). The combination of the far- spots near the black hole (Broderick & Higher-resolution images can be achieved field measurements (100s–1000sr g) Loeb, 2006; Doeleman et al., 2009b; by going to a shorter wavelength (0.87 mm based on pulsars and stars, with the Roelofs et al., 2017), as reported recently or 345 GHz, i.e., ALMA Band 7). A future near-field tests from imaging of black hole on the basis of interferometric observa- array that combines observations at both shadows (10s rg), has the power to tions in the near-infrared (Gravity Collabo- 1.3 and 0.87 mm will improve the imaging reveal deviations from the Kerr metric and ration et al., 2018). Real-time movies may dynamic range, while multi-frequency provide a fundamental test of GR (Goddi also become possible via interferometric VLBI would also open up spectral index et al., 2017), potentially leading to a dynamical imaging (Johnson et al., 2017). and rotation measure studies. breakthrough in our understanding of Time-domain studies and movies of black physics in the strong gravity regime. holes can then be used to study black In the more distant future, extending hole accretion and to map the black hole VLBI into space would provide the spacetime, leading directly to measure- increased angular resolution necessary Acknowledgements ments of black hole spin and tests of the to image finer structures and dynamics The authors would like to acknowledge all the “no hair” theorem (Broderick et al., 2014). near the black hole shadow (Fish et al., ­scientists, institutes, observatories and funding 2019; Palumbo et al., 2019; Roelofs et al., agencies who are part of and collectively support Although the focus of this article is the 2019). An order of magnitude increase the EHT project. The APP was supported by a Major first EHT result from the 2017 campaign, in angular resolution would allow us Research Instrumentation award from the National Science Foundation (NSF; award 1126433), an ALMA it is noteworthy that enhancement of to perform precision tests of GR and to

34 The Messenger 177 – Quarter 3 | 2019 North American Development Augmentation award, Doeleman, S. et al. 2009a, astro2010, 68 Roelofs, F., Janssen, M. and EHTC, submitted to ALMA North America (NA) Cycle 3 and Cycle 4 Doeleman, S. et al. 2009b, ApJ, 695, 59 A&A Study awards, and an ALMA NA Cycle 5 Develop- Doeleman, S. 2010, EVN Symposium Proceedings, Shaver, P. A. 2003, Proc. of the workshop “The ment award. The EHT project has been supported 53 Mass of Galaxies at Low and High Redshift”, by multiple grants from many independent funding Doeleman, S. et al. 2012, Science, 338, 355 ESO Astrophysics Symposia, ed. Bender, R. & agencies, including the ERC Synergy Grant “Black- Eatough, R. et al. 2013, Nature, 501, 391 Renzini, A., 357 HoleCam: Imaging the Event Horizon of Black Holes” EHT Collaboration et al. 2019a, ApJ, 875, L1 Tilanus, R. et al. 2014, arXiv:1406.4650 (Grant 610058) and several USA NSF grants (includ- EHT Collaboration et al. 2019b, ApJ, 875, L2 Wright, M. et al. 2001, ALMA Memo, 382 ing AST-1310896, AST-1440254, and OISE-1743747). EHT Collaboration et al. 2019c, ApJ, 875, L3 Younsi, Z. et al. 2016, Phys. Rev. D, 94, 084025 For the complete list of funding grants and acknowl- EHT Collaboration et al. 2019d, ApJ, 875, L4 edgements please see EHT Collaboration et al. EHT Collaboration et al. 2019e, ApJ, 875, L5 (2019a); they have not been reproduced here for EHT Collaboration et al. 2019f, ApJ, 875, L6 Links ­reasons of space and readability. We gratefully Escoffier, R. P. et al. 2007, A&A, 462, 801 acknowledge the support provided by the staff of Falcke, H., Melia, F. & Agol, E. 2000, ApJ, 528, 13 1 The Event Horizon Telescope webpage: the ALMA observatory. Falcke, H. et al. 2012, The Messenger, 149, 50 http://eventhorizontelescope.org/ Falcke, H. 2017, J. Phys., Conference Series 942, 2 The Global mm-VLBI Array webpage: http://www3. This paper makes use of the following ALMA data: 012001 mpifr-bonn.mpg.de/div/vlbi/globalmm/ ADS/JAO.ALMA#2016.1.01154.V. Fish, V. et al. 2013, arXiv:1309.3519 3 Astronet 2007: https://www.eso.org/public/ Fish, V. et al. 2019, arXiv:1903.09539 archives/oldpdfs/Astronet_ScienceVision_lowres. 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The groundbreaking ALMA array is composed of 66 giant antennas situated on the Chajnantor ­Plateau in the Chilean Andes.

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