Taking the First Picture of a Black Hole

7. Challenges in Obtaining an Image of a Supermassive Black Hole


“Seeing a black hole” has been a long-cherished desire for many astronomers, but now, thanks to the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA) projects, it may no longer be just a dream. To make it possible to image the shadow of the event horizon of Sagittarius A*, many researchers and cutting-edge technologies have been mobilised — because obtaining an image of a black hole is not as easy as snapping a photo with an ordinary camera.

Sagittarius A* has a mass approximately four million times that of the Sun, but it only looks like a tiny dot from Earth, 26 000 light-years away. To capture its image, incredibly high resolution is needed. As explained in the fifth post of this blog series, the key is to use Very-Long-Baseline Interferometry (VLBI), a technique that combines the observing power of and the data from telescopes around the world to create a virtual giant radio telescope.

The resolution of a telescope can be calculated from the radio wavelength the telescope is observing at and the size of the telescope — or in VLBI, the distance between the antennas. However, while actually observing, several kinds of noise and errors interfere with the telescope’s performance and affect the resolution.

In VLBI, each antenna is equipped with an extremely precise atomic clock to record the time at which radio signals from the target object were received. The gathered data are synthesised using the times as a reference, so that the arrival time of the radio waves to each antenna can be accurately adjusted.

But this process isn’t always straightforward because the Earth’s atmosphere blocks a certain range of wavelengths. Several kinds of molecules such as water vapour absorb a fraction of radio waves that pass through the atmosphere, with shorter wavelengths more susceptible to absorption. To minimise the effect of atmospheric absorption, radio telescopes are built on high and dry sites, but even then they are still not completely immune from the effect.

The final ALMA antenna arrives at Chajnantor
ALMA is located at 5000 metres above sea level, on the Chajnantor plateau in the Atacama desert of northern Chile. Credit: A. Marinkovic/X-Cam/ALMA (ESO/NAOJ/NRAO).

The tricky part of this absorption effect is that the direction of a radio wave is slightly changed when it passes through the atmosphere containing water vapour. This means that the radio waves arrive at different times at each antenna, making it difficult to synthesise the data later using the time signal as a reference. And even worse: since VLBI utilises antennas located thousands of kilometres apart, it has to take into account the differences in the amount of water vapour in the sky above each site, as well as the large fluctuations of water vapour content during the observation period. In optical observations, these fluctuations make the light of a star flicker and lower the resolution. Radio observations have similar problems.

“We have only a few ways to reduce this effect in VLBI observations,” explains Satoki Matsushita at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) of Taiwan. “If there is a compact object emitting intense radiation near the target object, we can remove most of the effect of refraction of radio waves by water vapour by using such an intense radiation source as a reference. However, no such intense reference source has been found near Sagittarius A* so far. And even if there is a reference source, there are still necessary conditions that must be satisfied: the telescopes need to have the ability to observe the target object and reference object at the same time; or the telescopes need to have the high-speed drive mechanism to quickly switch the observation between the target object and the reference object. Unfortunately, not all telescopes participating in the EHT/GMVA observations have this capability. One of the methods to remove the effect is to equip each antenna with an instrument to measure the amount of water vapour, but ALMA is the only telescope that has adopted this method at this point.”

Another major challenge in imaging a black hole is obtaining a high-quality image. By combining the data collected by antennas thousands of kilometres apart, VLBI achieves a resolution equivalent to a radio telescope several thousands of kilometres in diameter. However, VLBI also has a lot of large blank areas that are not covered by any of the antennas. These missing parts make it difficult for VLBI to reproduce a high-fidelity image of a target object from the synthesised data. This is a common problem for all radio interferometers, including ALMA, but it can be more serious in VLBI where the antennas are located very far apart.

It might be natural to think that a higher resolution means a higher image quality, as is the case with an ordinary digital camera, but in radio observations the resolution and image quality are quite different things. The resolution of a telescope determines how close two objects can be to each other and yet still be resolved as separate objects, while the image quality defines the fidelity in reproducing the image of the structure of the observed object. For example, imagine a leaf, which has a variety of veins. The resolution is the ability to see thinner vein patterns, while the image quality is the ability to capture the overall spread of the leaf. In normal human experience, it would seem bizarre if you could see the very thin veins of a leaf but couldn’t grasp a complete view of the leaf — but such things happen in VLBI, since some portions of data are inevitably missing.

This infographic illustrates how ALMA contributes to the EHT observation
This infographic illustrates how ALMA contributes to the EHT observations. With its shorter baseline, ALMA is sensitive to larger scales than the EHT and so ALMA can fill in the lower-resolution, larger-scale structures that the EHT misses. Credit: NRAO

Researchers have been studying data processing methods to improve image quality for almost as long as the history of the radio interferometer itself, so there are some established methods that are already widely used, while others are still in an experimental phase. In the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA) projects, which are both aiming to capture the shadow of a black hole’s event horizon for the first time, researchers began to develop effective image analysis methods using simulation data well before the start of the observations.

A simulated image of the supermassive black hole Sagittarius A*
A simulated image of the supermassive black hole at the centre of the M87 galaxy. The dark gap at the centre is the shadow of the black hole. Credit: Jason Dexter (left) and Kazunori Akiyama (right)

The observations with the EHT and the GMVA were completed in April 2017. The data collected by the antennas around the world has been sent to the US and Germany, where data processing will be conducted with dedicated data-processing computers called correlators. The data from the South Pole Telescope, one of the participating telescopes in the EHT, will arrive at the end of 2017, and then data calibration and data synthesis will begin in order to produce an image, if possible. This process might take several months to achieve the goal of obtaining the first image of a black hole, which is eagerly awaited by black hole researchers and the general astronomical community worldwide.

This lengthy time span between observations and results is normal in astronomy, as the reduction and analysis of the data is a careful, time-consuming process. Right now, all we can do is wait patiently for success to come — for a long-held dream of astronomers to be transformed into a reality.

Until then, this is the last post in our blog series about the EHT and GMVA projects. When the results become available in early 2018, we’ll be back with what will hopefully be exciting new information about our turbulent and fascinating galactic centre.




6. What is Sagittarius A*?


The Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA) have caused quite a stir with their recent observations of the supermassive black hole at the centre of the Milky Way, also known as Sagittarius A*. These observations are expected to lead to remarkable scientific results, enhancing our understanding of black holes and our theories of space and time. But just twenty years ago, astronomers didn’t know for sure that Sagittarius A* was the site of a black hole. Let’s step back in time and find out how we arrived at this point.

Back in 1974, British astronomer Sir Martin Rees proposed that supermassive black holes could live at the centres of some galaxies, such as those harbouring active galactic nuclei (AGN). Such galaxies shine incredibly brightly in many different wavelengths — as bright as 30 billion Suns or more — and they also spew out powerful jets of charged particles. Rees realised that black holes could be the cause of this energetic turmoil, a fact that is now confirmed.

In the same year, American radio astronomers Bruce Balick and Robert Brown at the National Radio Astronomy Observatory discovered a compact radio source at the very centre of the Milky Way. At radio wavelengths it’s the brightest feature in the Galactic Centre but it is fairly small.

Balick and Brown thought it looked like a faint quasar, a type of AGN that was more common in the distant past. But this object was in our own cosmic backyard — just 26 000 light-years away. They dubbed it Sagittarius A*, or Sgr A* for short, because it is located in the direction of the constellation of Sagittarius. The asterisks arose because in atomic physics, excited states of atoms are denoted by asterisks — and Sgr A* is an incredibly exciting discovery.

Sagittarius A*
Sagittarius A*, taken by NASA's Chandra X-Ray Observatory. Ellipses indicate light echoes. Credit: NASA/CXC/Caltech/M.Muno et al.

Over the next two decades, astronomers studied this bizarre object at different wavelengths and began to fit together the various pieces of the puzzle. As improving technology provided sharper and sharper views, they observed the commotion going on around the object. Gas and stars were whirling around it at incredible speeds — up to five million kilometres per hour — demonstrating that the object must be very small but very massive, with a staggering gravitational influence. Then, shortly after its launch in 1998, the Chandra X-ray Observatory spotted the first X-ray emission from Sgr A*. When matter is swallowed by a black hole, it emits a final “scream” of X-ray radiation before it crosses the event horizon. These X-rays can penetrate the thick clouds of gas and dust that veil the region, providing tell-tale evidence that a black hole lurks within.

Since the mid-1990s, research teams in Germany and the USA have been meticulously tracking the orbits of stars around Sgr A*. The US team, led by Andrea Ghez at the UCLA Galactic Center Group, has been using the W. M. Keck Observatory to measure the positions of thousands of stars in the vicinity of the Galactic Centre. The German team uses ESO’s Very Large Telescope to precisely measure the orbits of 28 stars frantically speeding around Sgr A*. These dedicated groups have produced the most detailed view ever of the region surrounding Sgr A*, and in 2008 confirmed once and for all that Sgr A* is the site of a supermassive black hole.

“Undoubtedly the most spectacular aspect of our long term study is that it has delivered what is now considered to be the best empirical evidence that supermassive black holes do really exist,” commented Reinhard Genzel, leader of the German team from the Max Planck Institute for Extraterrestrial Physics. “The stellar orbits in the Galactic Centre show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt.”

Images collected over 16 years have been assembled into a time-lapse video. The real motion of the stars has been accelerated by a factor 32 million. Credit: ESO/ R.Genzel and S. Gillessen. More info and download options here.

We have learned a great deal about Sgr A* over the years, as constant research has slowly revealed more and more of its secrets. Today, we know it is more than 4 million times as massive as the Sun but is extremely small, only about 40 million kilometres across — approximately equivalent to the distance from Mercury to the Sun. It is also fairly quiet for a black hole — it doesn’t emit an enormous amount of radiation, indicating that it typically doesn’t consume a lot of material. Astronomers have, however, captured blinding flares of X-rays hundreds of times brighter than the usual emissions. These are thought to be caused by the breaking apart of an asteroid falling into the black hole, or the entanglement of magnetic fields lines within the inflowing gas. Astronomers have also tracked a dust gas cloud called G2 that is in orbit around Sgr A* and imaged its closest approach in 2014. These observations, along with many more, add to our knowledge of the behaviour of this black hole and its surrounding turbulent environment.

The dusty cloud G2 passes the supermassive black hole at the centre of the Milky Way (annotated)
This annotated composite image shows the motion of the dusty cloud G2 as it closes in on, and then passes, the supermassive black hole at the centre of the Milky Way. These observations were made with ESO’s VLT and show that the cloud appears to have survived its close encounter with the black hole and remains a compact object. Credit: ESO/A. Eckart

But mysteries still remain about this cosmic beast at the centre of our galaxy — and about black holes in general. Sgr A* has helped cement the idea that every large galaxy hosts a supermassive black hole at its core, but astronomers are still scratching their heads about how such supermassive black holes form, and how they affect their host galaxies.

The GMVA and the EHT teams are using Very-Long-Baseline Interferometry to image Sagittarius A* in greater detail than ever before, linking up telescopes around the world. As well as probing the tumultuous regions around the black hole, the astronomers are also looking for one of the last pieces of the puzzle — observing the event horizon. This is the radius around a singularity beyond which matter and energy cannot escape a black hole’s gravitational pull; it is literally the point of no return.

The EHT and GMVA teams aim to observe the event horizon’s shadow. Not only will that demonstrate its existence, but measuring its shape and size will provide an unprecedented way to verify Einstein’s theory of general relativity. But of course, this is no easy task...

This is the sixth post of a blog series following the EHT and GMVA projects. Next time, we’ll talk about the challenges involved in imaging a supermassive black hole.




5. How to build an Earth-sized radio telescope


A black hole is an extraordinary object with extremely strong gravity, but when observed from Earth it would look like a tiny dot. To capture an image of a black hole, a telescope with extraordinarily high resolution is required. But how can such a revolutionary telescope be created?

As we discovered in the previous post, the larger the diameter of the telescope, the better the resolution. This is true both for optical and radio telescopes, which means that a tremendously large telescope is needed to observe a small object that can barely be seen from Earth — like a black hole.

The Atacama Large Millimeter/submillimeter Array (ALMA), which is operated in Chile by a global partnership, combines multiple antennas spread over distances from 150 metres to 16 kilometres. This allows it to simulate a single giant telescope much larger than any individual dish that could be built, achieving a resolution equivalent to up to a 16-km diameter telescope. ALMA’s resolution reaches 1/100 of 1/3600 of a degree angle — 5000 times better than the human eye!

And yet even with such an exceptionally good “eyesight”, ALMA would need to improve its resolution 100 fold in order to capture a black hole at the centre of the Milky Way galaxy.

To simulate a telescope with 100 times the resolution of ALMA, telescopes must be spread over a much larger area — far beyond the Chilean Andes, beyond even South America and extending to North America and Europe. Using the Very-Long-Baseline Interferometry (VLBI) technique, telescopes of several thousand kilometres in diameter can be simulated. The Event Horizon Telescope (EHT) and Global mm-VLBI Array (GMVA) both form Earth-sized telescopes that combine the observing power and the data collected by a range of telescopes across the world, including ALMA. The participating telescopes are listed below.

Project Contributing Telescopes
EHT ALMA, APEX (Chile), JCMT, SMA (Hawaii, US), ARO/SMT (Arizona, US), LMT(Mexico), IRAM 30m (Spain), NOEMA (France), SPT (South Pole)
GMVA ALMA, VLBA (eight locations in U.S.), GBT 100m (West Virginia, U.S.), IRAM 30m (Spain), OAN 40m (Spain), Max Planck Institute for Radio Astronomy 100m (Germany)
EHT infographic
Telescopes contributing to the EHT and GMVA observations of Sagittarius A*. The connected telescopes simulate a telescope equivalent to the dimensions of the whole western hemisphere of the Earth. Credit: ESO/O. Furtak

But how does VLBI work? With ALMA, it’s relatively straightforward: each antenna receives signals from a target object and then sends them via optical fibres to a central location on site, where they are processed and combined by a dedicated supercomputer. However, when telescopes are located thousands of kilometres apart — halfway across the world from each other — it’s impossible to connect them via optical fibres to a central location and transmit such enormous volumes of data. VLBI therefore uses a different technique: the data are first recorded at each individual telescope and stored on recording devices on site. These devices are then shipped or flown back to one place and played back all together in a computer for data synthesis.

A schematic diagram of the VLBI mechanism.
A schematic diagram of the VLBI mechanism. Each antenna, spread out over vast distances, has an extremely precise atomic clock. Analogue signals collected by the antenna are converted to digital signals and stored on hard drives together with the time signals provided by the atomic clock. The hard drives are then shipped to a central location to be synchronised. An astronomical observation image is obtained by processing the data gathered from multiple locations. Credit: ALMA (ESO/NAOJ/NRAO), J.Pinto & N.Lira.

A key component of the VLBI technique are the clocks — and these are no ordinary clocks. To synthesise the data gathered simultaneously by contributing telescopes around the world, each telescope requires a clock set to the accurate time with astonishing precision. These clocks measure small differences in the arrival time of the radio waves coming from the target object to each antenna of the array. Every telescope taking part in VLBI is equipped with an extremely precise, specially-developed atomic clock — so accurate that over a period of 100 million years, each clock would be off by less than 1 second!

Hydrogen maser atomic clock installed at the ALMA AOS.
Hydrogen maser atomic clock installed at the ALMA Array Operations Site (AOS), along with the technicians who installed it. Credit: ALMA (ESO/NAOJ/NRAO), C. Padilla.

Another key element in VLBI is the device used to record the data. The first VLBI experiments were carried out in the 1960s and used magnetic tapes to record observations, but since entering the 21st century, more and more VLBI observations have been recorded on hard drives because of their larger storage capacity and lower prices. The hard drives used in the EHT and GMVA observations are based on magnetic-disk technology and incorporate primarily low-cost PC components.

One important aspect of such a recording device is the speed with which it records data. The faster the data recording speed, the more extensive the range of frequency signals that can be recorded, which improves the overall sensitivity of the observations. Some of the hard drives used in the EHT observations can record data at up to a total rate of 16 gigabits per second! Of course, the storage capacity of the hard drives is also important. The capacity of the hard drives ALMA used for the EHT/GMVA observations exceeds 1 petabyte (1 million gigabytes) in total.

The dedicated supercomputer to process the recorded data is called a “correlator”. The EHT correlator was developed by the Massachusetts Institute of Technology in the US, while the GMVA correlator was developed by the Max Planck Institute for Radio Astronomy located in Bonn, Germany. The enormous amount of data obtained is firstly recorded at the telescopes around the world, and then sent to these two locations where it is read from each disk to the correlator at up to 4096 MB per second. The data is then processed by the correlator to form an astronomical image.

The VLBI technique can use these cutting-edge technologies to form an Earth-sized telescope capable of achieving extremely high resolution. This raises the question: Can any type of celestial object be revealed in detail by VLBI observations? Unfortunately, the answer is no. Some objects will be a good target of the VLBI observations, while others will not.

If you’ve ever used a microscope to observe a small object, you might be familiar with the experience of raising the magnification — only to see a dimmer view. The same thing happens with telescopes. Increasing the resolution means seeing an object in a view that is divided into many smaller parts. This inevitably results in decreasing the amount of light received from each part of the target object. Eventually, as the resolution increases, fainter objects becomes invisible.

For this reason, the extraordinarily high resolution of VLBI is mainly suitable for observing bright objects. VLBI is often used to observe objects that emit intense radio waves, such as astrophysical masers (similar to the kind of lasers that we are familiar with, but at microwave wavelengths) occurring around young and old stars, and high-speed jets of gas ejected from supermassive black holes. Since a high-temperature gas disk around a supermassive black hole is thought to emit strong radio waves, the EHT and GMVA observations aim to use the VLBI technique to capture the elusive black hole at the centre of the galaxy.

This is the fifth post of a blog series following the Event Horizon Telescope and the Global mm-VLBI Array projects. The next topic will focus on the supermassive black hole Sagittarius A* located at the centre of the Milky Way, a high-priority target object of the EHT/GMVA observations.




4. How do Radio Telescopes work?


Can you imagine yourself hearing only the bass of a music recording? Or only seeing objects of a particular colour? Well, in a way, you experience this every day. The human eye can only detect a narrow part of the electromagnetic spectrum: a section we call visible light. But a broad range of electromagnetic waves exist with the same nature — for example radio waves, which have much longer wavelengths than the light we can detect with our eyes. Radio wavelengths range from 1 millimetre to over 10 metres, while visible light wavelengths are only a few hundred nanometres — one nanometre is 1/10 000th the thickness of a piece of paper!

Radio waves are not visible to us directly, but in 1867 their existence was predicted by James Clerk Maxwell. By the end of the 19th century, scientists had developed instruments that could transmit and detect electromagnetic waves at the radio end of the spectrum. A few decades later, it was discovered that these instruments could not only be used for communication, but could also be directed towards space — hidden parts of the Universe were suddenly revealed!

The first detection of radio waves from an astronomical object was in 1932, when Karl Jansky observed radiation coming from the Milky Way. Then came the phenomenal discovery of the cosmic microwave background in 1964, worthy of a Nobel Prize in Physics. Soon afterwards, Jocelyn Bell Burnell observed the first pulsar with an array of radio aerials in 1967, which led to another Nobel Prize. And this was only the beginning — a dazzling array of discoveries have been made since.

ALMA's world at night.
This panoramic view of the Chajnantor plateau shows some of the 66 antennas of the Atacama Large Millimeter/submillimeter Array (ALMA). Credit: ESO/B. Tafreshi (twanight.org)

But how do radio telescopes work?

In order to detect signals from astronomical objects, every radio telescope requires an antenna and at least one receiver. They come in a variety of shapes and sizes, reflecting the need to be able to detect a great breadth of radio waves across many wavelengths.

The antennas of most radio telescopes working at wavelengths shorter than 1 metre are paraboloidal dishes. The curved reflector concentrates incoming radio waves at a focal point. For shorter wavelengths, such as millimetre waves collected by ALMA and VLBI networks like the EHT and GMVA, the perfection of the dish’s surface is critical: any warp, bump, or dent in the parabola will scatter these tiny waves away from the focus, and valuable information is lost.

In addition to the main dish, most radio telescopes have secondary reflectors that send the concentrated waves to receivers. These receivers select, detect and amplify the radio signals of the desired frequencies. The receiver delivers these signals in an analogue format, which is converted into a digital signal and fed into a computer. Astronomers can then stitch these signals together to create a map of the sky measured by radio brightness.

Radio telescopes point at a radio source for hours in order to detect the faintest signals coming from the near and distant Universe. This technique is a similar to keeping the shutter of a camera open for a long exposure at night. After combining these signals with a computer, astronomers can analyse the radiation emitted by many astronomical phenomena — such as stars, galaxies, nebulae and supermassive black holes.

The galactic centre.
The view of the centre of our galaxy with a closer view of the object known as Sagittarius A*, the bright radio source that corresponds to the supermassive black hole. Credit: NRAO/AUI/NSF

Here’s the problem in radio astronomy: because radio wavelengths are so long, it is difficult to achieve a high resolution of the objects being observed. Even the shortest radio wavelengths observed by the largest single telescopes only result in an angular resolution slightly better than that of the unaided eye. The resolution (or degree of detail in the image) of a single telescope can be calculated by dividing the length of the radio wave by the diameter of the antenna. When this ratio is small, the angular resolution is large and therefore finer details can be observed. The larger the diameter of the telescope, the better the resolution, therefore radio telescopes tend to be much larger than telescopes suited for other, shorter wavelengths like visible light.

The longest wavelengths, on scales of metres, pose a particular challenge because it is hard to achieve good resolution from a single dish. The largest moveable dish is the Green Bank Telescope (100 metres across). Dishes that don’t move can be much, much larger. The world's biggest radio dish is the newly-constructed Five-hundred-meter Aperture Spherical Telescope (FAST) in China: a fixed dish supported by a natural basin in the landscape. FAST can observe radio waves up to 4.3 metres in wavelength. There are also other similar dishes, such as the historic 300-metre Arecibo Observatory, which was the largest telescope for five decades until FAST was completed in 2016.

But building antennas any larger than this is not feasible, so here we reach a limit when it comes to observing at longer and longer wavelengths. But what can be improved is the angular resolution, opening the door of investigation into the finest details of the low-energy Universe.

A Nobel Prize winning technique called interferometry opened this door: if the signals from many antennas spread over a large area are combined, then the antennas can operate together like a gigantic telescope — an array. Modern arrays usually bring the signals together at a central location in digital form using optical fibres, and then process them in a special-purpose supercomputer called a correlator.

ALMA array from the air.
An aerial view of the Chajnantor plateau, located at an altitude of 5000 meters in the Chilean Andes, where the array of ALMA antennas is located. Credit: Clem & Adri Bacri-Normier (wingsforscience.com)/ESO

One such array is the Atacama Large Millimeter/submillimeter Array on the Chajnantor plateau in the Atacama Desert. ALMA comprises 66 high-precision antennas up to 16 kilometres apart, working together as an interferometer. The resolution of an interferometer depends not on the diameter of individual antennas, but on the maximum separation between them. Moving the antennas further apart increases the resolution.

The signals from the antennas are brought together and processed by the ALMA correlator. The antennas work together in unison, giving ALMA a maximum resolution which is even better than that achieved at visible wavelengths by the NASA/ESA Hubble Space Telescope. This is because the maximum distance between the antennas can be very large, increasing the resolving power of the interferometer and allowing it to detect smaller details.

The ability to link antennas over baselines of many kilometres is crucial to obtain extremely good resolution and a high degree of detail in the images. This gives astronomers the possibility to go even further than arrays like ALMA; by combining the signals from radio telescopes all across the world, the distances between the antennas can be Earth-sized — and even larger, in the case of space-based antennas like Spektr-R.

The telescopes do not have to be physically connected; rather, the signals recorded at each telescope are later “played back” in the correlator. This technique, called very-long-baseline interferometry (VLBI), provides exquisite angular resolution and paves the way for phenomenal new discoveries — including the detailed observation of the supermassive black hole at the centre of our galaxy.

This is the fourth post of a blog series following the Event Horizon Telescope and the Global mm-VLBI Array projects. Next time, we’ll talk about how to build an Earth-sized radio telescope.




3. What’s so interesting about the event horizon?


We know that black holes are fascinating objects, capable of bending not only our minds but also reality itself. They squeeze matter into an extraordinarily miniscule space, resulting in an object with an immense gravitational pull. Around this object is a boundary beyond which nothing can escape, not even light: the event horizon. Besides attracting enormous quantities of matter, the event horizon is attracting a lot of attention from astronomers around the world. But why?

As we discovered in the previous post, black holes are impossible to observe directly. Photons aren’t emitted, and therefore nothing reaches the astronomer’s telescope (except, of course, small amounts of Hawking radiation). But scientists can learn a lot from the bright material surrounding black holes.

When matter comes under the gravitational spell of a black hole, material will either be sucked directly into it or will be pulled into a doomed orbit like water circling a drain. The gravitational pull near the event horizon is so strong that the matter around it reaches relativistic speeds (i.e. speeds comparable to the speed of light). The friction between the material heats it to incredibly high temperatures, turning it into glowing plasma. Close to the event horizon photons are pulled into nearly circular orbits, and this form a bright photon ring which outlines the black “shadow” of inside the event horizon itself.

Simulated image of an accreting black hole.
Simulated image of an accreting black hole. The event horizon is in the middle of the image, and the shadow can be seen with a rotating accretion disk surrounding it. Credit: Bronzwaer/Davelaar/Moscibrodzka/Falcke/Radboud University

Einstein’s theory of general relativity predicts the existence of event horizons around black holes. But until now, the resolution of our telescopes has not been high enough to “see” a black hole. Despite the fact that the event horizon can be millions of kilometres in diameter, black holes are elusive. They are very far away and often hidden behind significant amounts of interstellar gas and dust. At 26 000 light-years from Earth, our galaxy’s supermassive black hole — called Sagittarius A* — is just a tiny pinprick on the sky.

By linking up different telescopes across the globe, the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA) can achieve the resolution necessary to perceive the pinprick of Sagittarius A*. Without doubt, these observations are incredibly exciting. They will allow for the study of black holes in more detail — as well as acting as a test for Einstein’s theory of General Relativity.

“Einstein's wonderful general relativity has been around for about a hundred years now and is very unintuitive, but despite that, it has managed to overcome all tests so far,” explains Ciriaco Goddi, astronomer from the EHT. “However, these tests have not been done in such strong gravitational fields.”

A pressing issue in physics is that the theories of general relativity and quantum mechanics seem to be fundamentally incompatible. To get to the bottom of this issue, physicists need to study the places where these theories overlap or break down. However, the conventional view is that this will not be observed at the event horizon of a supermassive black hole: quantum effects are expected to be important only near the horizon of lighter (about 10 microgram) black holes — of which we currently have no evidence for their existence. Yet some theorists argue that there will be deviations from classical general relativity close to the event horizon even for supermassive black holes, and these are potentially observable with the EHT.

“If there is any deviation from Einstein’s predictions near the black hole, where gravitation is at its strongest, we would need a new theory of gravity,” Goddi says, “and that means that we would need to describe space and time in different terms.”

General relativity predicts that the “shadow” of a black hole is circular, but other theories predict the shadow could be “squashed” along either the vertical axis (prolate) or the horizontal axis (oblate). Studying the shadow can therefore test general relativity as well as alternate theories of gravity. Plus, since the diameter of the black hole’s shadow is proportional to its mass, observing a black hole’s shadow may allow astronomers to directly estimate its mass.

Testing general relativity with a supermassive black hole.
This infographic shows a simulation of the outflow (bright red) from a black hole and the accretion disk around it, with simulated images of the three potential shapes of the event horizon’s shadow. Credit: ESO/N. Bartmann/A. Broderick/C.K. Chan/D. Psaltis/F. Ozel

ALMA astronomer Violette Impellizzeri adds: We think that there is a supermassive black hole at the centre of every galaxy. But the inner workings of these black holes remain a mystery. However, we need to ask ourselves the question of why there is a supermassive black hole at the centre of every galaxy. And it’s become more and more clear that black holes play a fundamental role in the formation of galaxies, and how they evolved. So, the links between the black holes, the galaxies, and the Universe are vital to understand.”

The VLBI observations with the EHT and GMVA will make phenomenal new discoveries, addressing the current and pressing problems of gravitational theory.

This is the third post of a blog series following the EHT and GMVA projects. Next time, we’ll explore how radio telescopes work.




2. What is a black hole?


Right now, astronomers are attempting to take the first image of the event horizon of the supermassive black hole at the centre of the Milky Way — but what exactly are black holes?

Black holes are some of the most bizarre and fascinating objects in the Universe. Essentially, they’re reality-bending concentrations of matter squeezed into a very tiny space, creating an object with an immense gravitational pull. Around a black hole is a boundary called an event horizon — the surface beyond which nothing can escape the black hole’s clutches, not even light.

Take a tour of the anatomy of a black hole with our handy infographic:

Infographic with a diagram of different parts of a black hole
Credit: ESO, ESA/Hubble, M. Kornmesser/N. Bartmann

Since no light can escape from a black hole, we can’t see them directly. But their huge gravitational influence gives away their presence. Black holes are often orbited by stars, gas and other material in tight paths that become more crowded and frantic as they’re dragged closer to the event horizon. This creates a superheated accretion disc around the black hole, which emits vast amounts of radiation of different wavelengths.

By observing this radiation from the activity around black holes, astronomers have determined that there are two main types: stellar mass and supermassive.

A stellar mass black hole is the corpse of a star more than about 30 times as massive as our Sun. At the end of its life, such stars violently collapse and don’t stopped collapsing until all of their constituent matter has condensed down into an unimaginably tiny space. It’s easiest to discover stellar mass black holes that are part of an X-ray binary system, where the black hole is guzzling down material from its companion star.

Artist’s impression of the formation of a stellar black hole in a binary system.
Artist’s impression of the formation of a stellar black hole in a binary system. Credit: ESO/L. Calçada/M.Kornmesser

The second type is called a supermassive black hole. These gargantuan black holes are up to billions of times more massive than an average star, and how they formed is much less clear and is a matter of ongoing study. One theory proposes they formed from enormous clouds of matter that collapsed when galaxies first formed; another theory suggests that colliding stellar mass black holes can merge into one enormous object.

Today, these supermassive monsters reside at the centres of almost every galaxy — including our own Milky Way. They exert tremendous influence on their home galaxies, especially when they gorge on gas and stars.

Simulation of gas cloud after close approach to the black hole at the centre of the Milky Way.
Artist’s impression of a gas cloud after a close approach to the black hole at the centre of the Milky Way. The star orbiting the black hole are shown, along with blue lines that mark their fast, tight orbits. Credit: ESO/MPE/Marc Schartmann

26 000 light-years away from Earth, Sagittarius A* (Sgr A* for short) is the supermassive black hole in the hot, violent centre of the Milky Way. It’s over 4 million times more massive than our Sun, approximately 40 million kilometres across, and is spinning at a large fraction of the speed of light. It’s shrouded from optical telescopes by dense clouds of dust and gas, so observatories that can observe different wavelengths — either longer (such as ALMA) or shorter (X-ray telescopes) — are essential to study its properties.

Soon, through the combined power of ALMA and other millimetre-wavelength telescopes across the globe, we may become much better acquainted with the monstrous heart of our galaxy. The Global mm-VLBI Array is currently investigating the process of how gas, dust and other material accrete onto supermassive black holes, as well as the formation of the extremely fast gas jets that flow from them. The Event Horizon Telescope, on the other hand, is working towards a different goal: imaging the shadow of the event horizon, the point of no return.

This is the second post of a blog series following the EHT and GMVA projects. Stay tuned to find out more about why the event horizon of a black hole is so interesting!




1. What are the Event Horizon Telescope and the Global mm-VLBI Array?


At the centre of our galaxy lurks a cosmic monster: a supermassive black hole called Sagittarius A* with a mass about four million times that of the Sun. Its gravity is so intense that not even light can escape its pull, but if it wasn’t for its strong gravitational influence on the stars and gas around it, we would have no idea that it was there! Now, an ambitious new endeavour is underway to take a never-seen-before image, of the event horizon of the black hole itself.

Two international collaborations of radio telescopes have linked up to create Earth-sized virtual telescopes: the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA), working at different wavelengths. The impressive line-up of telescopes, which stretch across the globe from the South Pole to Hawaii to Europe, will work together to target the supermassive black hole at the heart of the Milky Way.

To do this, astronomers will exploit a technique known as Very-long-baseline Interferometry (VLBI), where telescopes thousands of kilometres apart can link together and act as one. This cooperative technique can achieve a far higher resolution than any single facility could obtain on its own — a resolution 2000 times that of the NASA/ESA Hubble Space Telescope! This super-high resolution is crucial for detecting the black hole, which — despite being about 20 times bigger than the Sun — lies a long way away, over 26 000 light-years from Earth.

Infographic of the EHT on the Earth
This infographic details the locations of the participating telescopes of the Event Horizon Telescope and the Global mm-VLBI Array. Credit: ESO/O. Furtak

The plan to image a black hole has been in the works for years, but it’s only recently that technology has brought the ambitious endeavour within reach. Plus, a radio telescope heavyweight has just joined the team: the Atacama Large Millimeter/submillimeter Array (ALMA).

Located high up on the Chajnantor plateau in Chile’s Atacama Desert, ALMA’s 66 antennas and exquisite receivers make it the largest and most sensitive component of the EHT/GMVA collaboration, increasing the overall sensitivity by a factor of 10. Despite being a state-of-the-art facility, ALMA has undergone several upgrades to take part in the collaboration. Specialist equipment has been installed, including new hard drives that are necessary to store the sheer amount of data produced by the observations, as well as an extremely accurate atomic clock, which is critical to link ALMA to the entire VLBI network.

ALMA’s solitude
ALMA’s solitude: This panoramic view of the Chajnantor Plateau shows the site of the Atacama Large Millimeter/submillimeter Array (ALMA), a place of solitude 5000 metres above sea level in the Chilean Andes. Credit: ESO/B. Tafreshi (twanight.org)

The first groundbreaking observations will be made in April 2017: observations at 3 millimetre wavelengths will be made with the GMVA from 1–4 April 2017, and with the EHT at 1.3 millimetre wavelengths from 5–14 April 2017. The GMVA will investigate the properties of the accretion and outflow around the Galactic Centre, while the EHT will attempt to image, for the very first time, the shadow of the black hole’s event horizon.

There is a long, hard road ahead to process the massive amounts of data that will be acquired during the observation periods, and results are expected to become available towards the end of 2017.

The outcome of these observations is eagerly awaited by the astronomy community worldwide, as their scientific potential is incredibly exciting and the collaboration are pursuing some awesome goals. These could include testing Einstein’s theory of general relativity, which predicts a roughly circular “shadow” around the black hole. Other goals include learning about how material accretes around black holes, as well as the formation of extremely fast jets of gas that blast out from them.

Testing general relativity using the black hole shadow.
Simulated images of the shadow of a black hole: General relativity predicts that the shadow should be circular (middle), but a black hole could potentially also have a prolate (left) or oblate (right) shadow. Future EHT images will test this prediction. Credit: D. Psaltis and A. Broderick.

This is the first post of a blog series that will take you along for the astronomical ride, giving insight into how cutting-edge research is done and what risks are involved.

In the following posts, we’ll explore questions such as: What makes black holes so interesting? How do radio telescopes see the Universe? And what do we really know about the supermassive monster lurking at the centre of the Milky Way?