The first direct visual evidence of a black hole and its “shadow” has been revealed today by astronomers working on the Event Horizon Telescope (EHT). The image is of the supermassive black hole that lies at the centre of the huge Messier 87 galaxy, in the Virgo galaxy cluster. Located 55 million light-years from Earth, the black hole has been determined to have a mass 6.5 billion times that of the Sun, with an uncertainty of 0.7 billion solar masses. Although black holes are inherently invisible because of their extreme density and gravitational field, the researchers have managed to obtain images near the point where matter and energy can no longer escape – the so-called event horizon.
“We are giving humanity its first view of a black hole — a one-way door out of our universe,” says Sheperd Doeleman of the Haystack Observatory at the Massachusetts Institute of Technology (MIT) who is the EHT’s lead astronomer. “This is a landmark in astronomy, an unprecedented scientific feat accomplished by a team of more than 200 researchers.” Doeleman says that the result would have been “presumed to be impossible just a generation ago”, adding that breakthroughs in technology and the completion of new radio telescopes over the past decade have allowed researchers to now “see the unseeable”.
The results, announced today at multiple press conferences around the world, have been published in six papers in a special issue of Astrophysical Journal Letters, which is published by the Institute of Physics on behalf of the American Astronomical Society.
Discs of glowing gas
Supermassive black holes are thought to lie at the centres of most galaxies in the universe, and astronomers are keen to decipher their key properties – such as how their extreme gravity affects the space–time around them, and how some of them fuel the massive jets of material that spew out from the galaxies that host them. A key feature of a black hole is its event horizon – the boundary at which even light cannot escape its gravitational pull, as the velocity required to do so would be greater than the speed of light, which is forbidden by Einstein’s general theory of relativity. And while that theory has passed many tests, researchers want to see how well it holds up at the “ultimate proving ground” – a black hole’s edge.
Despite their name, black holes are not, however, all dark. The gas and dust trapped around them in an accretion disc is so compact that it is often heated to billions of degrees even before the matter eventually succumbs to the black hole, making them glow brightly. Indeed, general relativity also predicts that a black hole will have a “shadow” around it, measuring around three times larger than the event horizon. The shadow is of great interest as its size and shape depend mainly on the mass and – to a lesser extent – on any possible spin of the black hole, thereby revealing its inherent properties.
“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,” says Heino Falcke from Radboud University in the Netherlands, who chairs the EHT’s science council. “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.”
Portrait of a black hole
An orange on the Moon
To directly observe the black hole at the centre of Messier 87 – dubbed M87* – astronomers require a telescope with an angular resolution comparable to the event horizon, which is on the order of tens of micro-arcseconds across. But to achieve that resolution with an ordinary telescope – which is like spotting an orange on the surface of the Moon – would require a dish the size of our planet, which is clearly impractical.
EHT astronomers instead use the radio-astronomy technique of very-long-baseline interferometry (VLBI). It involves picking up radio signals from an astronomical source by a network of individual radio telescopes and telescopic arrays scattered across the globe. The EHT, which first turned on in 2007, consists of eight radio dishes in six different locations across the globe all operating at a wavelength of 1.3 mm. These telescopes include the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the South Pole Telescope (SPT) in Antarctica, and the IRAM 30-metre telescope in Spain (see image left). The distance between individual EHT telescopes – known as the “baseline” – ranges from 160 m to 10,700 km.
The signals received at each individual telescope dish in the network are precisely tagged with a very accurate time stamp, normally using an atomic clock at each location. Each telescope produces roughly 350 terabytes per day, which is stored on high-performance helium-filled hard drives. The data is then later correlated and used to build up a complete image by supercomputers that are located at the Max Planck Institute for Radio Astronomy in Bonn, Germany, and the MIT Haystack Observatory in the US. This process makes the EHT the highest-resolution instrument on Earth, capable of taking images up to 2000 times better resolution than the Hubble Space Telescope and able to resolve features as small as 20 micro-arcseconds.
An astronomy “landmark”
As a black hole’s size is proportional to its mass, the more massive a black hole, the larger its shadow. Thanks to its enormous mass and relative proximity, M87* was predicted to be one of the largest viewable from Earth — making it a perfect target for the EHT. Astronomers observed M87* on 5, 6, 10 and 11 April 2017, with the telescope taking a series of scans of three to seven minutes in duration each day.
These multiple independent EHT observations have now resulted in the first image of a black hole including its shadow, revealing a ring-like structure with a dark central region. The diameter of the ring is 42 micro-arcseconds with a width less than 20 micro-arcseconds. By comparing the image with theoretical models such as general relativistic magnetohydrodynamic (GRMHD) simulations, the observed image is consistent with expectations for the shadow of a Kerr black hole – one that is uncharged and rotates about a central axis – as predicted by the general relativity.
The researchers were able to deduce the mass of the M87* at 6.5 billion times that of the Sun. Previous estimates — based on models as well as spectroscopic observations of the galaxy by the Hubble Space Telescope — ranged between 3.5 and 7.7 billion solar masses. EHT scientists also deduced the radius of the event horizon as 3.8 micro-arcseconds. They also found that the rotation of the black hole is in a clockwise direction, and that its spin points away from us. The brightness in the lower part of the image is due to the relativistic movement of material in a clockwise direction as seen by us, so that it is moving towards us.
“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 predictions surprisingly well,” says Paul Ho, director of the East Asian Observatory and an EHT board member. “This makes us confident about the interpretation of our observations, including our estimation of the black hole’s mass.”
As well as unveiling the properties of M87*, the EHT has now lifted a veil on the event horizon, showing that it is now possible to experimentally study the region via electromagnetic waves. This, the researchers write, has now transformed the event horizon from a purely “mathematical concept” to a “physical entity”.
“The production of radio images with a resolution comparable to the angular size of a black hole event horizon, for the first time, is a major breakthrough in high-energy astrophysics,” says astrophysicist Rob Fender from the University of Oxford, who is not part of the EHT collaboration. Fender adds that the EHT observations are our best look yet at the region where the jet of the black hole is formed. “The region close to the black hole, just above the event horizon, is the site of much of the most extreme astrophysics in our universe since the Big Bang,” he says. “These jets carry an enormous amount of energy away from the central black hole, via processes which are not well understood.”
This is not the first result of come out of the EHT. In 2012 scientists working on the array managed to observe, for the first time, the base of the jet emanating from the M87 galaxy. The work established that the black hole at the heart of M87 is spinning and that the accretion disc follows the direction of spin. Three years later, researchers on the EHT measured the first direct evidence of magnetic fields near the event horizon of Sagittarius A* — the black hole at the centre of our Milky Way galaxy lying around 26,000 light-years away but with a mass around three orders of magnitude smaller than M87*. By studying the right- and left-handed circular polarization of the incoming radio waves, they were able to infer the direction of linear polarization that traces the magnetic field finding that it even changed on a daily basis and revealing the extreme dynamics at play at the heart of the black hole.
Astronomers now hope to carry out further observations of M87* to deduce the shape and depth of the shadow region more accurately. They are also hopeful to add more telescopes to the array that will allow for higher-resolution images. As well as M87*, the EHT team is attempting to take the first image of Sagittarius A*. But this is more difficult to resolve — despite being nearer — because it is more dynamic than M87*, changing on the scale of minutes rather than days.
The results are published in Astrophysical Journal Letters.