16 mins read 12 May 2022

From the South Pole to the Milky Way's Supermassive Black Hole

Humanity’s first image of the central supermassive black hole at the heart of the Milky Way Galaxy has been revealed by the Event Horizon Telescope collaboration. We chat with University of Melbourne PhD student Aman Chokshi who works with the EHT in Antarctica about this historic event.

This is the first image of Sgr A*, the supermassive black hole at the centre of our galaxy. Credit: EHT Collaboration.

Dressed in appropriate gear to fight off the -40°C conditions, Aman set up his camera, positioning it to look back at the two isolated radio telescopes that emerge from the endless ice of the Antarctic terrain. The skies were perfectly blue that day, except for a band of clouds that hovered above the horizon away in the distance, accompanied by the soft, high-pitched whimper of the wind.

Here, in December, the Sun never sets. Rather, it moves horizontally across the sky – unfamiliar to the rhythmic rising and falling our circadian cycles are tuned into. This is exactly what Aman had hoped for, as he fixed the camera with an external power pack, covering it in hand warmers to ensure the bitter cold would not drain the battery.

At regular intervals, and with the correct settings programmed, his camera would snap a picture of the two telescopes at the South Pole Observatory as the Sun moved along its path directly above these majestic instruments, which sit housed atop the Dark Sky Laboratory. This day seemed like every other regular day for Aman, with the light from above, hitting the bright white snow and reflecting to become the light from below, whilst he carried on with his assigned astrophysics duties. But this was no regular day.

Slowly, the Sun’s searing light grew darker, as the Moon gracefully positioned itself in front of the bright white orb – part of a cosmic dance that the inhabitants of Earth have gawked over for as far as time remembers. It began with a small portion, but as the day passed, more of the solar sphere started to disappear. It’s hard to describe these occurrences, but for those who have ever been lucky enough to witness such events, the exhilaration is unforgettable.

At first, it feels like nothing big is happening. The speed at which the Moon is moving across the sky seems negligible to our hurried lives, but as the shadow of our celestial sibling falls upon Terra’s surface, it races across at over 1,600 km/h.

And then, for those lucky enough to lie along the path of the shadow, for a brief moment of time – when time itself stands still, it happens.

Eclipse. Totality. Darkness.

Suspended in this moment, living beings (of all kinds) become dazed and confused, unsure on how to behave during totality. Humans howl to the sky like wolves, birds stop chirping, and bats think the night has come early.

To seize the momentous occasion – to memorialise the gravity of standing at Earth’s southern pole during a solar eclipse, Aman and his colleagues stood between the two radio telescopes, raising their hands above their heads, mimicking the paraboloid shape of dish antennas.

Snap. The camera takes another picture.

And just like that, the moment passes, the Moon slowly moves away from its perfectly positioned sky real estate, and the brightness returns. Life at the south pole carries on.

The December 2021 eclipse over Antarctica, captured by Aman – pictured standing with four other colleagues between the two telescopes at the South Pole Observatory. Credit: Aman Chokshi/Instagram.

Aman Chokshi is a PhD Student, from the University of Melbourne currently stationed at the Amundsen-Scott research station, sitting atop a 2,800-metre-high plateau in Antarctica. His work in radio astronomy involves looking for some of the tantalising hints of when early neutral hydrogen first became ionised, during the period when some of the Universe’s first stars fired up their core-nuclear engines.

Additionally, Aman works as part of a global collaboration of astronomers who come together to image some of the most massive structures our Universe contains. This international collaboration is known as the Event Horizon Telescope (EHT).

And today, for the first time in history, Aman and the EHT have revealed humanity’s first view of the gargantuan supermassive black hole that resides at the heart of our Milky Way Galaxy, a behemoth known as Sgr A* (pronounced “Sagittarius-A-Star”).

“It’s been absolutely amazing being a part of something so gigantic,” said Aman, who is currently working with the EHT team on the recent imaging campaigns the collaboration has undertaken.

“Coordinating with telescopes all around the world, pointing at distant black holes whose size boggles the mind, while realizing that they’re so distant that we require a telescope the size of a planet to observe them! I’ve never been a part of something so huge”

This image was made possible through the combination of an array of global telescopes, all working together in unison - as something of this magnitude and scale requires the might of all of these combined instruments to resolve a practically invisible target.

Relative to the other individual objects within our Galaxy, Sgr A* is a monster. Its mass has been measured at approximately 4.3 million solar masses (one solar mass = the mass of our Sun, roughly 1.98 x 1030 kg), all squeezed into a space about 25 million kilometres wide. That might seem like a large number, but it’s only about 17% of the distance between the Earth and the Sun.

Black holes (including the supermassive variety) are invisible to all forms of electromagnetic radiation because nothing escapes beyond their grasp – not even light. So, to infer the mass of Sgr A*, astronomers have been observing and documenting the movements of a handful of stars from the surrounding region for over 20+ years, as they fling around during their orbits around this invisible object. And by timing the speed at which these stars are moving, they can determine the ‘pull’ that the object has, which is related to its mass.

From our perspective, Sgr A* resides 26,000 light-years away behind walls of dust and gas that form the inner arms and structures of the Milky Way’s disc. This causes any optical light to become ‘extinct’ along its path towards us – so we can’t visually see it. However, radio telescopes operate at frequencies that can see through this material and reveal details that regular optical telescopes cannot, and that is what the EHT is – a collection of global radio telescopes working in unison. Which is where Aman steps in.

Aman and Allen Foster, standing in front of the tiled dish antenna of the South Pole Telescope. Credit: Aman Chokshi (supplied).

“My winter-over partner, Allen Foster, and I operate the South Pole Telescope [SPT] for a year down at the South Pole,” said Aman, about his role at Earth’s south pole, and with the EHT.

“A week before EHT observing begins, we halt usual SPT observations to begin preparing for EHT. This includes installing alternate optics on the SPT dish which redirects the light from the primary SPT dish away from our usual receiver, into the EHT detector.”

“Once the new optics are installed, and all the EHT electronics have been turned on and tested, we begin making a pointing model. This is a 24-hour process where we observe bright sources all across the sky to improve the accuracy of the SPT pointing from arcminutes to arcseconds, which enables us to be sure that we’re actually observing the distant EHT targets,” he said.  

“When EHT observing begins, we coordinate with all the other telescopes in the collaboration to ensure that the SPT is on target and recording data on time – the margin of error on these observations is less than two seconds. EHT observations occur for 16 hours a day for five days. It was a very intense couple of weeks, but so rewarding.”

Aurora and the Milky Way rising over the South Pole Observatory, which features the South Pole Telescope (left) and BICEP telescope (right). Credit: Aman Chokshi (supplied).

The SPT itself has been in operation since 2007 and is often used to study the faint, diffuse emissions from the dull glowing relic of the Big Bang, known as the cosmic microwave background. Its sensitivity, location and frequency range also make it one of the world’s most ideal telescopes to help study (in accordance with other telescopes around the world) small, compact objects – like Sgr A*.

“The SPT primary dish has a 10m diameter and operates in three frequency bands – 95, 150, 220 GHz,” said Aman, describing the telescope.

“Our primary objective involves mapping the Cosmic Microwave Background (CMB) over 1,500 square degrees of the southern sky. The CMB is the first observable relic radiation from after the Big Bang and contains a host of information regarding the early Universe.”

Sydney’s Connection to the Galaxy Centre

One of the first radio telescopes in the world to observe the radio signals from Sgr A* was located in Sydney’s Dover Heights, operated by astronomers from Australia’s national science agency, CSIRO. Credit: CSIRO archives / J. Bolten et al.

It’s now almost 100 years since Karl Jansky noticed a ‘strange hissing sound’ that came from the Galactic Centre in 1933 from his now-famous antenna, which was designed to monitor lightning strikes generated by storms. Jansky noted that it recurred every 24 or so hours and seemed to come from the direction of the centre of the Milky Way plane, whenever it passed overhead. Unfortunately, Jansky’s findings were not supported any further, so not much more came from it at the time.

It would be a few years later, in 1951, when Jack Piddington and Harry Minnett used a radio telescope located in Potts Hill in Sydney, in which a determination was made that there was indeed a bright, yet discrete radio source in the same region that Jansky had noted it. A few years later, Richard McGee and John Bolton, two other Australian astronomers, used another Sydney radio telescope (this time in Dover Heights), identifying the source as the likely Galactic Centre.

The nucleus, the core of our Galaxy, had finally been found. Its coordinates noted in the logs of the International Astronomical Union thereafter.

Since then, radio telescope technology, engineering, software and high-performance computing have exponentially increased in efficiency and productivity, leading to capabilities, such as using a global array of telescopes to capture such remarkable structures in our Universe – like distant supermassive black holes.

“It blew my mind to learn about the technology involved in the SPT telescopes,” said Aman. “The detectors used in the SPT are Transition Edge Sensor (TES) Bolometers, which function on the edge of the superconducting transition, and are thus required to be kept at incredibly low temperature.”

“The 10 hexagonal detector arrays are maintained at 300mK, i.e., 0.3℃ above absolute zero, which required ingenious nested refrigeration. Incoming CMB radiation hits these TES bolometers, marginally raising their temperature and pushing them away from superconducting, changing their resistance. It’s all just so cool!”

Science Check – What is a Supermassive Black Hole

Visualisation of the components of a supermassive black hole. Credit: NASA Goddard Space Flight Centre/J. Schnittman.

Black holes trigger all sorts of science fiction fascinations amongst all of us – they are extreme, exotic objects which we are yet to fully understand. Even the laws of physics break down when considering how black holes can simply be.

Essentially, a black hole is a region of space-time in which gravity is so strong, that nothing can escape from it – no light, no information, no matter. Consideration for these objects began around the time of Einstein’s General Relativity, which outlines how matter causes space-time to curve and warp. All matter in the Universe does this – including your human body, but at these scales, it is such a small value that it could never (practically) be measured.

But as we scale up the size of the mass, say to something as massive as the Earth or our Sun, then these effects become much more noticeable. Going even to bigger masses, such as some of the very massive stars that range above 10 solar masses, then the curvature effect really starts to take off.

Crushing matter down into smaller spaces also increases density, which is the other ingredient for forming a black hole. Let’s say we were to crush the entire Earth’s mass down to the size of a ping pong ball, it would then have sufficient density to become a black hole. Do the same to the Sun, except reduce its mass content down to about 3-kilometres, and you’ve got yourself another black hole.

However, the forces required to do this crushing are literally astronomical (and we don’t need to ever worry about the Sun, its fate is much more pleasant). But when massive stars die, especially if the star's original mass is above 25 solar masses, the process in which the core collapses during a supernova provides enough power to keep crushing this material inwards to a point known as a singularity. Here, the space-time curvature is infinite, as is the density. The singularity itself is then surrounded by an arbitrary spherical region of space-time out to a distance known as the event horizon. This is the unknown where anything within the event horizon is never transmitted to the outside Universe. Once in, it is lost forever.

Size comparison of the two black holes imaged by the Event Horizon Telescope (EHT) Collaboration: M87*, at the heart of the galaxy Messier 87, and Sagittarius A* (Sgr A*), at the centre of the Milky Way. Credit: EHT collaboration (acknowledgment: Lia Medeiros, xkcd).

Black holes also come in a variety of sizes. The collapse of a massive star forms a stellar-mass black hole, which can range in mass from about 5 – 100 solar masses. These are peppered around the Galaxy, and we have indirectly observed them through their gravitational wave signatures during merger events, as well as a number of x-ray binary objects.

There are also the intermediate-mass black holes, of which we don’t have a large population sample size. These objects are much bigger, with masses that can get as high as 1,000  - 10,000 solar masses.

And finally, there are supermassive black holes – with masses that can reach as high as millions or even billions of solar masses. Plenty of evidence has been accounted for with these objects, as nearly all galaxies that we observe have a supermassive black hole at their centres.

Historically, we have visually inferred that the Milky Way has a supermassive black hole at its centre by watching the motion of those nearby stars – but this new image of Sgr A* is the first direct evidence of the supermassive black hole itself.

The Event Horizon Telescope

Whilst supermassive black holes contain lots of mass, they are also remarkably small in size dimensions (remember, all that matter needs to be crushed into a small space). This makes them extraordinarily difficult to resolve, because of their tiny size, and location at great distances.

To get around this, astronomers have come up with a very clever technique, known as Very Long Baseline Interferometry (VLBI). In general, the bigger the radio telescope, the better it will be able to resolve smaller objects at greater distances. But we can’t just keep building bigger radio telescopes, because they cost a lot of money, they are hard to maintain and would be impossible to operate.

But here is where VLBI steps in. If there are two radio telescopes at a certain distance apart, then combining the data from these telescopes would simulate a virtual telescope as big as the distance between them.

For example, the CSIRO Parkes radio telescope has a diameter of 64-metres, and the Canberra Deep Space Communication Centre’s DSS 43 antenna has a diameter of 70-metres. Individually, these are grand radio antennas and produce excellent science. They are also located about 340-kilometres apart. So, if we were to use both these antennas in unison, we would simulate a virtual telescope with an aperture of 340-kilometres – a much bigger eye to scan and resolve the sky.

Using this technique, the EHT has antennas that reside in Greenland, Hawaii, North America, Europe, South America and at the south pole, where Aman is involved. By using VLBI across all of these instruments, the EHT simulates a telescope as big as the face of the Earth itself.

“While the Supermassive Black Holes that EHT observes are truly massive and gigantic, they are located at astronomical distances,” said Aman.

“Sgr A* has a diameter 14 times larger than the Sun but is 2 billion times further from the Earth than the Sun is. This is equivalent to trying to observe a softball on the surface of the Moon,” he added.

The very first image of a black hole ever taken of the supermassive black hole at the centre of the massive galaxy, M87, which resides in Virgo. This was one of the first targets of the EHT Collaboration. Credit: EHTelescope.

Historically, the EHT has delivered some remarkable results. We’ve seen the very first image of a supermassive black hole (ever), which resides at the heart of the massive M87 elliptical galaxy, some 55 million light-years away. We’ve seen the twisting relativistic jets produced by the nearby (12 million light-years) Centaurus A galaxy. And we’ve even seen the inner workings of a quasar that exists five billion light-years away.

And now, the team has brought us this beautiful new image of the supermassive black hole at the centre of our Galaxy.

Riding out the Antarctic Winter

The view, looking out over the Antarctic ice sheet towards the Sun, accompanied by a pair of Sun dogs from the South Pole Telescope. The BICEP antenna can be seen in the foreground. Credit: Aman Chokshi (supplied).

Aman’s journey to the south pole and working on projects began many years earlier, whilst he studied as a student across Australian institutions.

“I’ve worked with the Raman Research Institute in India, where I worked on an Epoch of Reionization experiment called SARAS, after which I worked with the Indian Institute of Astrophysics, where I studied the solar prominences with an optical Spectrometer.”

“My PhD research at the University of Melbourne and CSIRO has involved studying the Epoch of Reionization with the Murchison Widefield Array in remote Western Australia,” he said.

And whilst he has worked as part of the global EHT collaboration in obtaining images of these distant monsters, such as this new image of Sgr A*, Aman also works on a number of other projects whilst stationed in Antarctica.

“Most of my work in Antarctica involves working with the South Pole Telescope, where Allen and I maintain and repair the SPT as needed. We ensure that CMB observations are performed on time and that the data quality looks good.”

“I’m also really passionate about astrophotography, and in my free time I try and get outside as much as possible to see and shoot the Auroras and the Milky Way.”

There’s still some time left before Aman heads back to the mainland of Australia but given the opportunity he has to work on such exciting projects, at the bottom of Earth, he is going to take it all in first.

“I should finish my year at the South Pole by December 2022. I’m hoping to spend as much time as possible outside, and soak in as much of this unique place as possible. It’s probably a once in a lifetime experience for me and I’m so lucky to be down here.”

Aman Chokshi playing with light, shadow and the setting Antarctic Sun over the ice sheet. Credit: Aman Chokshi (supplied).

Video credit: ehtelescope/YouTube.

Read the science papers about the EHT images in The Astrophysical Journal