The first image of the black hole at the center of our galaxy – The Post

Event Horizon Telescope (EHT), an international consortium of multiple radio telescopes, has been able to observe for the first time the “shadow” of the black hole Sagittarius A *, located at the center of the Milky Way, our galaxy. The image is reminiscent of the one that always EHT had made in 2019, when he released the first ever image of a black hole. At the time, the observation concerned a black hole in the center of the Messier galaxy (M87), about 55 million light years from Earth. The new observation instead concerns a colleague of his, but closer to us, at least in astronomical terms: 26 thousand light years away and with a mass estimated to be around 4 million times that of the Sun.

The presence of a black hole in the center of the Milky Way had been hypothesized for some time and had found various confirmations, thanks to analyzes of how celestial bodies move in the galaxy, but it had never been possible to observe its presence.

To discover at a distance what happens near a black hole, radio telescopes are used, large antennas that, unlike classic optical telescopes, use their parabolas to detect the radio waves emitted by things (radio sources) that are in Space. Their use allows us to observe what happened a long time ago at enormous distances, such as to require journeys of tens, hundreds and sometimes thousands of years to light.

A photomontage of the parables of the radio telescopes used by EHT around the world (ESO)

In general, the larger the parables, the more precise the observations can be. For this reason a few years ago researchers wondered if it was possible to transform the entire planet into a sort of large antenna, to have a much more powerful instrument. From this idea was born theEvent Horizon Telescope (EHT)which collects telescopes from Chile to Antarctica to Hawaii, synchronized with atomic clocks in order to collect data on the galaxies towards which they are pointed.

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The radio telescope network used by EHT (ESO)

Black holes
A black hole, however, cannot be seen or photographed directly, because these objects have such an intense gravitational field that nothing inside it can escape it: not even light. We can imagine it as a sphere inside which there is the actual massive object. The spherical surface marks the boundary within which the conditions occur whereby nothing can escape, or go back if it has ended up in the black hole: this sphere is called the “event horizon”.

The massive object that is in the center of the sphere, and that creates a deformation in spacetime (the four-dimensional structure of the Universe, if you are confused here it is explained more fully), is instead defined “singularity”: it is called this way because we do not know its characteristics, but we know that they are different from those that regulate the behavior of matter as we know it. It is also hypothesized that the density of the singularity is such as to tend towards infinity.

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Schematic representation of a black hole, click on the image to enlarge it (ESO)

We do not know what happens within the confines of the event horizon, on the other hand from the outside we can observe what happens to matter (gas and dust) when it ends up at the border and, based on its reactions, understand something more about the black hole. . Thanks to EHT it was possible to do this with M87 * and now with Sagittarius A *.

Sagittarius A *
The black hole in the center of the Milky Way is much closer to Earth, but it is smaller and less active than the one in the center of M87, to the point of being more difficult to observe. The material near the event horizon changes very quickly and consequently the shadow of the black hole itself changes rapidly, making observation with synchronized radio telescopes more complicated. The research team also had to deal with interstellar dust in our galaxy, which interfered with observations.

And it was precisely the work of “cleaning” the data that required a lot of time for the research groups. The observation of Sagittarius A * was made in 2017 around the same time that the black hole at the center of M87 was observed. For the latter it took almost two years of analysis, while for the other it took almost five. However, the research groups had started working earlier on M87, as they were deemed less complicated.

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Sagittarius A * as seen by EHT (ESO)

As Katie Bouman, one of the astrophysicists who participated in the initiative, explained: “Taking a picture with EHT is like listening to a song played on a piano with many keys missing.” The data collected has numerous gaps, which must be filled with complex calculations and excluding the interference generated by other celestial bodies. For a long time the research groups have therefore worked without being sure of reaching a sufficiently acceptable and accurate result.

EHT collected 3.5 million gigabytes, an enormous amount of data that could not have been transferred over the Internet. The files were saved on hundreds of hard drives and then transported by radio telescopes to research and analysis centers in the United States and Germany. The data were then analyzed and gradually selected, to the point of being able to draw relevant information: the image of Sagittarius A *.

From the image and other data, researchers from EHT have found that the Milky Way’s black hole is not particularly voracious when compared to other black holes around the Universe. The amount of matter that is in its vicinity and that ends up inside it is rather limited, as explained by another astrophysicist, Michael Johnson, in the press conference presenting the results: “If Sagittarius A * were a person, it would consume a grain of rice every million years “. Of course, everything must be related to the enormous mass of the black hole, much higher than that of a person. Overall Sagittarius A * was found to be quite common and quiet.

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M87 * and Sagittarius A * compared (ESO)

In addition to directly affecting our galaxy, the new result is important because it confirms the reliability of the systems developed by EHT to study black holes. In the coming years, the project managers are confident that they will be able to add new radio telescopes, creating a more precise system that could make it possible to observe a black hole over time, and with better resolution, to understand its evolution.

The first image of a black hole

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The first image of the black hole at the center of our galaxy – The Post

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