Sagittarius A*: the elusive black hole at the center of our Galaxy

The center of our Galaxy, the Milky Way, has always attracted the attention of astronomers, at least since the age of telescopes began a few centuries ago: it is a place full of mystery due to its peculiar darkening that does not allow visible light pass through it. The discovery of other types of light that our eyes cannot see, such as infrared and radio waves, gave hope of unraveling the enigmas that are hidden in this remote region of the Galaxy.

Thanks to the invention of telescopes that record these types of light, we have been able to learn a lot about this part of the universe, such as, for example, that there are stars that move at high speeds around an invisible point; It is just this point that does not emit light that had eluded astronomers and their instruments for decades… until this year 2022.

One of the most interesting predictions of the theory of general relativity, a theory published by Albert Einstein in 1915 to explain gravity, was the possible existence of black holes: huge concentrations of matter in small regions of space that produce a force of gravitational attraction so intense that not even light can escape its influence. The boundary beyond which nothing can escape a black hole’s gravitational pull is known as its event horizon.

Shortly after the general theory of relativity was published, Karl Schwarzschild solved Einstein’s equations for a static, spherical black hole, finding that the size of its event horizon is directly proportional to its mass. With this result, it became clear that if we could measure the size of the event horizon of a black hole, we could know its mass and thus prove the validity of general relativity in extreme gravitational fields.

Around the 1970s, several physicists began to investigate how the event horizon could be revealed, which would be the closest physical structure to the black hole that could be observed. One of the most successful cases was carried out by Jean Pierre Luminet, who, using the Schwarzschild equations, arrived at the prediction of an image of the light surrounding the event horizon that showed a dark shadow towards its central area (Figure 1). Based on these first results, the possibility of visualizing black holes from their shadow was revealed.

Figure 1. Simulated photograph of a spherical black hole with a thin accretion disk. Credit: J.P. Luminet (1979).

Already in the 21st century, more accurate simulations predicted the size of the shadow of black holes suspected to exist at the center of different galaxies. Due to their apparent size in the sky, the shadows of the black hole in the center of our Galaxy called Sagittarius A* (Sgr A) and that of the galaxy M87 (M87) were the most likely to be detected with current technology.

However, such a feat would take the observation techniques developed by radio astronomy to the extreme, since the detection of these objects requires a telescope the size of the Earth! The simulations also showed that the shadow would only be revealed by observations at millimeter wavelengths, for which a special type of telescope was required.

A planetary-sized telescope is practically impossible to build, but radio astronomers since the mid-20th century have developed a technique called aperture synthesis, commonly known as interferometry. This technique mixes signals detected by an array of telescopes, thus allowing a single one to be simulated with a size equal to the maximum separation between the elements of the array.

For the cases of the event horizons in Sgr A* and M87*, the combination of millimeter wavelength and the array of telescopes located along the face of the Earth results in sufficient sharpness to be able to distinguish their shadows.

This arrangement is precisely the Event Horizon Telescope (EHT), in whose development and scientific exploitation more than 300 researchers around the world participated, and whose objective was precisely to take the first image of these two objects.

After two years of processing and analyzing data, in April 2019 the EHT collaboration released the first image of an event horizon in M87. Almost three years later, with an even greater effort in processing the data obtained, in May 2022 the first image of the event horizon in the center of our own Galaxy, Sgr A, was made public. The images obtained were in agreement with the predictions: a ring of light surrounding a dark region with the characteristics of an event horizon (Figure 2).

Figure 2. Comparison of the sizes of the black holes in M87* and Sgr A*, observed by the EHT. Credits: EHT collaboration (2022).

The Alfonso Serrano Large Millimeter Telescope (GTM) in Mexico played a central role in obtaining both images, both due to its geographical location and its size, which makes it one of the two largest elements on the collecting surface within the EHT array. .

The publication of these images marks a milestone in the history of modern astronomy and, with it, after great planning efforts and management of financial and human resources, the name of Mexico and the GTM have been written in history for contributing to which, without a doubt, will be one of the most amazing feats of science in the 21st century.