About the black hole at the centre of the Milky Way galaxy

16. May 2022.

The Event Horizon Telescope (EHT) Collaboration published the second historical image of the black hole on Thursday, 12 May 2022. Although in 2019, the image showing the M87 black hole garnered a lot of attention, there was no lack of interest in the new image, since, for the first time, we are able to see the black hole directly at the heart of the Milky Way galaxy. Hence our epithet.

Radioastronomers collected the data for both images in April 2017, but the Saggitarius A black hole, which is located at the centre of our galaxy, posed a greater challenge. Its mass is actually only about 4 million times that of the sun, which, for massive black holes, at the centres of galaxies, is rather small. ’Its size is comparable to the size of Mercury’s orbit around the sun’, explains Dr Marko Vojinović of the Group of Gravity, Particles and Fields at the Institute of Physics, Belgrade, adding that due to a small event horizon, the material in the so-called accretion disk (spinning around it) manages to orbit in only a few minutes.

This is too short a time to complete the measurement on Earth since the data collection procedure lasts for about 24 hours. This makes the image of the black hole ‘blurry’, since the exposition time is too long for the gas orbiting it so fast’, states Dr Vojinović and reminds that the members of the collaboration compared this with the situation when a camera set to a long exposition (for example 10 seconds) tries to take a picture of a puppy chasing its tail.

The Saggitarius A black hole, which is 26,000 light-years from Earth, does not pose danger, and it was named after the Saggitarius constellation where it is located in the night sky. Since it is at the centre of our galaxy, this black hole is closer to the sun than any others, but there are multiple gaseous clouds of interstellar material between it and the Solar System, which block it from the telescope observations. A central dark region where the hole is located, surrounded by the light coming from the super-heated accelerated gas, is visible in the image, which was published in the Astrophysical Journal Letters along with the accompanying paper.

The image enables a watcher to spot the position of the plane of rotation of the black hole and the angular magnitude of the silhouette of the event horizon. ‘This last information is particularly important because it is directly linked to the black hole’s mass, which has been measured based on the orbits of the surrounding starts circling it’, explicates Dr Vojinović, and emphasizes that the relation between the mass and the size of the black hole event horizon was predicted by the general theory of relativity.’It is a rather formidable theoretic prediction, without any kind of free parameters. Therefore, it is extremely important to measure the mass and the size of the horizon by independent measurements, and thus test the theory prediction’, claims Dr Vojinović. The new image represents the first such measurement in history, and it is a new qualitative test of the General theory of relativity in the strong-field-regime gravity.

At the same time, the image represents the first direct confirmation that there is a black hole at the centre of our galaxy, whose existence was indirectly assumed by means of measurements of orbits around the place where it was believed to be,  for which the Nobel Prize in Physics was awarded in 2020.

Since this is the second image of a black hole in history, comparisons are inevitable. One could say that ‘our’ black hole is much less conspicuous.’Unlike the black hole in the M87 galaxy, which actively consumes the material of the surrounding stars, therefore bringing about particularly evident effects, the black hole in our galaxy is comparatively inactive – its so-called accretion disk, i.e. the material in the near orbit it consumes is relatively small and shines with much less intensity’, describes Dr Vojinović, adding that the team who worked on the image does not rest here, and that they will attempt to get better quality images and better understand these objects.

Text by: Jovana Nikolić/IPB Communication department

Photo: Event Horizon Telescope