First ever image of black hole in our galaxy
A first image of the black hole in the centre of our own galaxy was released by the EHT consortium, May 12th 2022! The black hole is called Sagittarius A* and it is 27.000 lightyears away from us, and has a mass equal to 4 millions suns. The team started working on this image already in 2017, when the first observations were done by the Event Horizon Telescope (EHT), the worldwide network of telescopes.
Supermassive Black Hole at the Galactic Center of the Milky Way
The image provides direct evidence for the presence of a black hole in the center of our Milky Way, something that was predicted already for many years. It also shows for the very first time how material orbits Sagittarius A*, and reveals the complex processes that are created by the extreme gravity of the black hole.
It is not the first ever image of a black hole, this was already released on April 10th, 2019 and revealed the black hole in the center of the galaxy M87, 1000 time more massive and more distant. The new image of the Milky Way black hole is very similar to the M87 one, despite the big difference in mass, which was quite a surprise to the team. In both images of the black holes we see how gas lights up just before it falls into the black hole, i.e. over the event horizon. But apparently, the difference in mass of the black hole has less effect on how that process takes place.
But the difference in mass between M87 and Sagittarius A* did have a severe impact on the analysis of the 2017 data for the black hole in our own Milky Way, despite it being a 1000 times closer by. The reason for that has to do with the timescale over which changes happen in the gas that we see orbiting the black hole - we call that the dynamical timescale. As Sagittarius A* is much more less massive, the dynamical timescale is much shorter, in the order of minutes compared to weeks in the case M87. This means that in a typical period of observations say, one night, the gas flow of M87 is more or less fixed, while for Sagittarius A* a lot of things may have changed. It is like trying to make a photograph of a dog chasing its own tail.
Pressconference at UNAM, announcing the new image of Sagittarius A*
An extra complication in that is that the camera you are using to take the image of the dog that is running around, does not work all the time or is slow, and the result is an image in which the dog appears in multiple places in one image. This is also the case for the EHT telescope, a network of telescopes around the globe. This network does not have telescopes all around the globe yet, there is a hole in the network in Southern Africa. And as the Earth rotates during one night of observations, there are not enough telescopes in the network to capture all the changes in the flow of gas around the black hole.
With the AfricaMillimetreTelescope (AMT) project we want to improve this situation: by adding an extra telescope in Namibia we fill the hole in the EHT network. Moreover, from the site in Namibia, the majestic Gamsberg mountain, we can observe the centre of our Milky Way for the longest time, as it moves directly overhead.
So, while the new image of the black hole is a major step forward scientifically, like many big discoveries it also come with new questions and shows that for the future of the EHT it is necessary to extend the network with telescopes. While there are many efforts ongoing in both Europe and the US, the AMT is potentially the first of the, hopefully many, new telescopes is the EHT.