Taking A Photo Of The Impossible

We finally get to see our galaxy’s black hole

By Matt Woods

On the 12th of May, humans finally got to see Sagittarius A*, the supermassive black hole that sits at the core of our galaxy, the Milky Way. The image was created from data that was collected by the Event Horizon Telescope Collaboration. In 2019, this ground-breaking collaboration released the historic first image of a black hole, that of the supermassive black hole at the centre of the Galaxy M87. Up until now, we were fairly sure that our Galaxy also had a supermassive black hole at its centre, but we were missing the proof. While some theories over the past years suggested we did not, this stunning image finally proves that we do.

This image shows us the accretion disk around our black hole, a disk-like flow of ionised gas, dust, and stellar debris that orbits the black hole, not quite falling in. We can see this disk because the immense force of the black hole’s gravity accelerates the spinning particles, and they smash into each other releasing X-rays and gamma rays. We can also see the event horizon, the inner hole of the doughnut which is the threshold where to escape the pull of the black hole, you would need to move faster than the speed of light.

The ring shape in the image comes from the gravity around the black hole bending the light from the accretion disk behind the black hole so that we can see it. The image is actually a monochromatic image, the team uses orange to add colour to the image, so it is easier to see the details in the image. The three bright nodules are clumps of dust and gas. The top right one is brighter due to the doppler effect.

Infrared shot of the centre of our galaxy. Image Credits: NASA, ESA, SSC, CXC, STScI

Now this intergalactic doughnut might seem cool, but what makes it stunning is the story of how they were able to create this image and what this image tells us about our supermassive black hole. To reach that incredible resolution, you need an enormous telescope, or you need to be very clever with physics. The Event Horizon Telescope collaboration did the latter to create the image.

It is possible to combine observations from radio telescopes that are separated on Earth by a certain distance, that way it makes them equivalent to a telescope the size of their separation. This is a technique known as Very Long Base Interferometry, and this is the method that the Square Kilometre Array which is being built in South Africa and Western Australia will use as well. The Event Horizon Telescope has observatories across the world from Greenland to Antarctica, from Europe to Hawai’i and together, they function as a telescope as big as the Earth.

You must be thinking this is an extremely big telescope and it must collect a lot of data, and you would be right, they do not just collect gigabytes or terabytes of data, they collected petabytes. They collect so much data they cannot send it via the Internet, they must fly these hard drives to and from the observatories and the supercomputer that crunches the numbers and creates the simulations based off that data.

Now by this stage, you are probably thinking why the need for such a large telescope, are these astronomers working for Dr Evil? Well, the good news is, that they are not working for Dr Evil. This collaboration is funded by the United States National Science Foundation. The reason we need such a large telescope to see our black hole is that it is extremely hard to see it.

Firstly, black holes are by their very nature black. Nothing, not even light can escape their gravity. We also live in the middle of our Galaxy, and our view of the galactic centre is hidden to us in the visible spectrum of light by the dust in the Sagittarius arm of our Galaxy. It is as if we are looking at it with frosted glasses. We need to use other parts of the light spectrum to unveil the galactical curtain to see what is behind it, and therefore the collaboration used radio waves.

Simulations and years of observations in other light waves including in x-ray and radio have made it possible to partially mitigate the effects of this blurring. As supermassive black holes go, Sagittarius A* is not the most massive, it is quite a common-size black hole. Do not expect it to be going into the cosmic nightclubs and making it rain with money, this is a working-class black hole. Its mass is over four million times the mass of our Sun, and the event horizon is about twelve million kilometres across, that is thirty times the Earth-Moon distance. If we tried to look at it from another galaxy, we could not detective it with today’s current technology. The reason we can detect it at all is that it is only 26,000 light-years away from us. What the Event Horizon Telescope team did was the equivalent of seeing a doughnut on the Moon from Earth.

These observations have greatly improved our understanding of what happens at the very centre of our galaxy. It offers new insights into how these giant black holes interact with their surroundings. The scientists were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity. The size of the shadow of the singularity is not precisely known just yet. This is because the scientists do not know if the black hole itself is spinning. The simulations suggest it spinning, but to what rate, the scientists are not too sure yet, and that spin can have a small effect on the diameter. Sagittarius A* also matches the predictions made by studies that looked at the motions of stars orbiting Sagittarius A* as well.

Because Sagittarius A* is 1,600 times smaller than M87*, the scientists needed to capture their data quickly as the environment around it changes more quickly, which made creating the image much more challenging. The scientists had to be careful of blurry images as the innermost stable orbit for Sagittarius A* is about 4.5 light-minutes

instead of weeks like M87*. The M87* image is the size of the solar system and is gorging on dust and gas which shoots out jets of material from its poles that extend as far as the edge of its galaxy. The Sagittarius A* image is the size of mercury and is not shooting jets as it doesn’t have a lot of dust and gas around it like M87*, only every so often a cloud of dust or gas gets too close, and it gets to feed.

If Sagittarius A* was a human, it would eat a piece of rice every million years, that is one hell of a diet. While some Black holes can be very efficient in converting gravitational energy into light, Sagittarius A* traps nearly all the energy it captures (one part in a one thousand is converted into light) so it is ravenous by inefficient. It is only putting out a few hundred times as much energy as the Sun, but it is four million times as massive.

Other discoveries the scientists found, were that we see Sagittarius A* face on, so the black hole is not aligned to the galaxy, its slightly tilted towards us. Sagittarius A* has a strong magnetic field to the gas around the black hole but it is weaker than the fridge magnets on your fridge, but everything is relative. They also found the accretion disk is trillions of degrees kelvin.

The Event Horizon Telescope team is not just stopping there. They have been conducting observing campaigns throughout the pandemic in 2021 and 2022 with even more telescopes, expanding their observations to other objects. The first issue they needed to deal with is the scattering of radio waves by the Sag arm of the galaxy. The scientists are already starting to push their equipment to look in the 345 gigahertz range. This will cut the scattering in half and produce sharper images.

With support from the Nation Science Foundation and other partners, they are also designing next-gen equipment and adding new telescopes to the Event Horizon Collaboration. This way they can go beyond still images and reveal the first-ever footage of a changing supermassive black hole. So, make sure you’re strapped in as we are only at the beginning of this amazing journey.

ALMA looking up at the Milky Way. Image Credits: D. Kordan/ESO