Astronomers have unveiled the first image of the supermassive black hole at the centre of our own Milky Way galaxy. This result adds overwhelming evidence – over length scales comparable with the event horizon – that the object was indeed a black hole and yielded valuable clues about the workings of such giants, which are thought to reside at the centre of most galaxies. The image, in unprecedented resolution, was produced by Event Horizon Telescope (EHT), a global scientific collaboration, where scientists funded by the European Research Council, through BlackHoleCam project, have played a key role.

The breakthrough follows the EHT collaboration’s 2019 release of the first image of a black hole, called M87*, at the centre of the more distant Messier 87 galaxy. The new image is a long-anticipated look at the massive object that sits at the very centre of our own galaxy. Scientists had previously seen stars orbiting around something invisible, ultra-compact, and very massive at the centre of the Milky Way. This shows beyond reasonable doubt that this object known as Sagittarius A* is a black hole, and today’s image provides the first direct visual evidence of it.

“The idea that our own Milky Way may harbour a supermassive black hole always intrigued and motivated me. Now we can finally see that it is really there. This is a lifelong dream come true,” said Heino Falcke Professor of Astroparticle Physics and Radio Astronomy at Radboud University in Nijmegen, Netherlands.

Professor Falcke together with Professors Michael Kramer from the Max Planck Institute for Radioastronomy, and Luciano Rezzolla from Goethe University Frankfurt, and their teams of postdocs and PhD students and research staff, led BlackHoleCam. The project, proposed in 2013 and supported with an ERC Synergy Grant from 2014 to 2020, aimed not only to produce the first-ever image of a black hole, but also “turn our Galactic Centre into a fundamental-physics laboratory to measure the fabric of space and time with unprecedented precision.” Now these objectives have been fully accomplished. 

Although we cannot see the event horizon of the black hole because it is completely dark, hot and glowing gas around it reveals a telltale signature: a dark central region, called a “shadow”, surrounded by a bright ring-like structure. The new view captures light bent by the powerful gravity of the black hole, which is four million times more massive than our Sun. The EHT team’s results are being published today in a special issue of The Astrophysical Journal Letters. 

“For me the image of Sgr A* is even more exciting than our first black hole image of M87,” said Professor Kramer. “For Sgr A*, we already knew the mass and distance of the black hole from other measurements. This allows the prediction of the size of the shadow, assuming the general relativity is correct, which we can now compare with the observations. In other words, we can do an exciting test of Einstein’s theory! He wins again!”

Because the black hole is about 27,000 light-years away from Earth, it appears to us to have about the same size in the sky as a donut placed on the Moon. To image it, the team created the powerful EHT, which linked together eight existing radio observatories across the planet to form a single “Earth-sized” virtual telescope. The EHT observed Sgr A* on multiple nights, collecting data for many hours in a row, similar to using a long exposure time on a camera.

The two black holes that so far have been imaged – M87* and Sgr A* – look remarkably similar, even though our galaxy’s black hole is more than a thousand times smaller and less massive than the colossus pictured in 2019 and the physical conditions very different.

Piecing together the image of Sgr A* – even though it is much closer to us – was considerably more difficult because of the variations in the data. In comparison, M87* was an easier, steadier target, with nearly all images looking the same, that was not the case for Sgr A*. The image of the Sgr A* black hole is an average of the different images the team extracted, finally revealing the giant lurking at the centre of our galaxy for the first time.  

“Despite all the experience with M87*, imaging Sgr A* has not been a stroll in the park. The rapid variability in the emission and a much more lively environment at the centre of the galaxy has forced us to develop new techniques for the analysis of the data and of the numerical simulations, said Professor Rezzolla. The end result is that we now have much more confidence on how black holes behave and can discard a number of theoretical models that do not fit observations.”

The effort was made possible through the ingenuity of more than 300 researchers from 80 institutes around the world that together make up the EHT Collaboration. Along with the US’s National Science Foundation, the European Research Council (ERC) has provided crucial support to the EHT though the €14 million BlackHoleCam project. Over the past two decades the EU also supported the development and upgrading of the large telescope infrastructure essential to the success of the EHT project, investing more than €30 million in RadioNet.

In addition to developing complex tools to overcome the challenges of imaging Sgr A*, the EHT team worked rigorously for five years, using supercomputers (some of which were fully funded by the ERC) to combine and analyse their data, all while compiling an unprecedented library of simulated black holes to compare with the observations. Scientists are particularly excited to finally have images of two black holes of very different sizes, which offers the opportunity to understand how they compare and contrast. They have also begun to use the new data to test theories and models of how gas behaves around supermassive black holes, and whether alternatives to black holes are compatible with the data. This process is not yet fully understood, but is thought to play a key role in shaping the formation and evolution of galaxies.

Related pages: 

https://erc.europa.eu/news-events/magazine/Behind-the-scene-with-blackhole-scientists

https://erc.europa.eu/projects-figures/stories/astronomers-reveal-first-ever-image-black-hole

https://erc.europa.eu/news/eu-funded-scientists-unveil-first-ever-image-black-hole

https://www.youtube.com/watch?v=Dr20f19czeE

EHT press release  

BlackHoleCam

Other ERC-funded research on black holes: 

• Launching, propagation and emission of relativistic jets from binary mergers and across mass scales (JETSET) https://cordis.europa.eu/project/id/884631
• Mapping Magnetic Fields with INterferometry Down to Event hoRizon Scales (M2FINDERS) https://cordis.europa.eu/project/id/101018682 

The Event Horizon Telescope (EHT) collaboration, who produced the first ever image of a black hole, has today revealed a new view of the massive object at the centre of the Messier 87 (M87) galaxy: how it looks in polarised light. This is the first time astronomers have been able to measure polarisation, a signature of magnetic fields, this close to the edge of a black hole. The observations are key to explaining how the M87 galaxy, located 55 million light-years away, is able to launch energetic jets from its core.

“We are now seeing the next crucial piece of evidence to understand how magnetic fields behave around black holes, and how activity in this very compact region of space can drive powerful jets that extend far beyond the galaxy,” says Monika Mościbrodzka, Coordinator of the EHT Polarimetry Working Group and Assistant Professor at Radboud University in the Netherlands.

On 10 April 2019, scientists released the first ever image of a black hole, revealing a bright ring-like structure with a dark central region — the black hole’s shadow. Since then, the EHT collaboration has delved deeper into the data on the supermassive object at the heart of the M87 galaxy collected in 2017. They have discovered that a significant fraction of the light around the M87 black hole is polarised.

“This work is a major milestone: the polarisation of light carries information that allows us to better understand the physics behind the image we saw in April 2019, which was not possible before,” explains Iván Martí-Vidal, also Coordinator of the EHT Polarimetry Working Group and GenT Distinguished Researcher at the University of Valencia, Spain. He adds that “unveiling this new polarised-light image required years of work due to the complex techniques involved in obtaining and analysing the data.”

Light becomes polarised when it goes through certain filters, like the lenses of polarised sunglasses, or when it is emitted in hot regions of space where magnetic fields are present. In the same way that polarised sunglasses help us see better by reducing reflections and glare from bright surfaces, astronomers can sharpen their view of the region around the black hole by looking at how the light originating from it is polarised. Specifically, polarisation allows astronomers to map the magnetic field lines present at the inner edge of the black hole. 

“The newly published polarised images are key to understanding how the magnetic field allows the black hole to ‘eat’ matter and launch powerful jets,” says EHT collaboration member Andrew Chael, a NASA Hubble Fellow at the Princeton Center for Theoretical Science and the Princeton Gravity Initiative in the US.

The bright jets of energy and matter that emerge from M87’s core and extend at least 5000 light-years from its centre are one of the galaxy’s most mysterious and energetic features. Most matter lying close to the edge of a black hole falls in. However, some of the surrounding particles escape moments before capture and are blown far out into space in the form of jets. 

Astronomers have relied on different models of how matter behaves near the black hole to better understand this process. But they still don’t know exactly how jets larger than the galaxy are launched from its central region, which is comparable in size to the Solar System, nor how exactly matter falls into the black hole. With the new EHT image of the black hole and its shadow in polarised light, astronomers managed for the first time to look into the region just outside the black hole where this interplay between matter flowing in and being ejected out is happening. 

The observations provide new information about the structure of the magnetic fields just outside the black hole. The team found that only theoretical models featuring strongly magnetised gas can explain what they are seeing at the event horizon. 

“The observations suggest that the magnetic fields at the black hole’s edge are strong enough to push back on the hot gas and help it resist gravity’s pull. Only the gas that slips through the field can spiral inwards to the event horizon,” explains Jason Dexter, Assistant Professor at the University of Colorado Boulder, US, and Coordinator of the EHT Theory Working Group. 

To observe the heart of the M87 galaxy, the collaboration linked eight telescopes around the world — including the northern Chile-based Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder EXperiment (APEX), in which the European Southern Observatory (ESO) is a partner — to create a virtual Earth-sized telescope, the EHT. The impressive resolution obtained with the EHT is equivalent to that needed to measure the length of a credit card on the surface of the Moon.

“With ALMA and APEX, which through their southern location enhance the image quality by adding geographical spread to the EHT network, European scientists were able to play a central role in the research,” says Ciska Kemper, European ALMA Programme Scientist at ESO. “With its 66 antennas, ALMA dominates the overall signal collection in polarised light, while APEX has been essential for the calibration of the image.”

“ALMA data were also crucial to calibrate, image and interpret the EHT observations, providing tight constraints on the theoretical models that explain how matter behaves near the black hole event horizon,” adds Ciriaco Goddi, a scientist at Radboud University and Leiden Observatory, the Netherlands, who led an accompanying study that relied only on ALMA observations.

The EHT setup allowed the team to directly observe the black hole shadow and the ring of light around it, with the new polarised-light image clearly showing that the ring is magnetised. The results are published today in two separate papers in The Astrophysical Journal Letters by the EHT collaboration. The research involved over 300 researchers from multiple organisations and universities worldwide. 

“The EHT is making rapid advancements, with technological upgrades being done to the network and new observatories being added. We expect future EHT observations to reveal more accurately the magnetic field structure around the black hole and to tell us more about the physics of the hot gas in this region,” concludes EHT collaboration member Jongho Park, an East Asian Core Observatories Association Fellow at the Academia Sinica Institute of Astronomy and Astrophysics in Taipei. 

More information

This research was presented in two papers by the EHT collaboration published today in The Astrophysical Journal Letters:

“First M87 Event Horizon Telescope Results VII: Polarization of the Ring” (doi: 10.3847/2041-8213/abe71d) 

“First M87 Event Horizon Telescope Results VIII: Magnetic Field Structure Near The Event Horizon” (doi: 10.3847/2041-8213/abe4de).

Accompanying research is presented in the paper

“Polarimetric properties of Event Horizon Telescope targets from ALMA” (doi: 10.3847/2041-8213/abee6a) by Goddi, Martí-Vidal, Messias, and the EHT collaboration, which has been accepted for publication in The Astrophysical Journal Letters.

The EHT collaboration involves more than 300 researchers from Africa, Asia, Europe, North and South America. The international collaboration is working to capture the most detailed black hole images ever obtained 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 Institut de Radioastronomie Millimetrique (IRAM) 30-meter Telescope, the IRAM NOEMA Observatory, the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope (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 consortium 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.  

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of ESO, the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI). ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA. 

The BlackHoleCam research group was awarded the European Research Council €14 million Synergy Grant in 2013. The Principal Investigators are Heino Falcke, Luciano Rezzolla and Michael Kramer and the partner institutes are JIVE, IRAM, MPE Garching, IRA/INAF Bologna, SKA and ESO. BlackHoleCam is part of the Event Horizon Telescope collaboration.

Links

• Research papers
  • Paper VII
  • Paper VIII
  • Goddi et al.
• Additional videos related to the research on the EHT YouTube channel
  • What is polarization
  • How magnetic fields affect black hole images
  • The M87 image as seen with a polarizer
• EHT website
• Images of ALMA
• Images of APEX 
• ESO EHT webpage
• ESO blog post on the EHT project
• For scientists: got a story? Pitch your research

Originally published by ESO March 24, 2021