what do we know about the newly photographed black hole

The 2020 Nobel Prize in Physics was awarded on the one hand to Roger Penrose for his theoretical work on black holes and on the other hand to Reinhard Genzel and Andrea Ghez for discovering that at the center of our galaxy, the Milky Way, there is a black holecalled Sgr A* (read 'Sagittarius A star'), whose mass is four million times the mass of the Sun.

The sound of a black hole, audible thanks to a computer process

Know more

Such a conclusion is based on detailed observation of the orbits of stars very close to Sgr A*, carried out over decades. To this end, ESO telescopes such as the Very Large Telescope (VLT) and the VLTI interferometer have been used, among others, supplemented with instrumentation specifically designed to achieve the necessary precision in the position and speed of these stars neighboring the black hole. Armed with these tools, the research teams have used the black hole Sgr A* as a laboratory to verify various predictions of the theory of general relativity, which has successfully passed all tests.

Solidly established the existence of this black hole, we had to "see" it. The EHT consortium (Event Horizon Telescope) already published in April 2019 the first image of a black hole, this truly gigantic (6.5 billion solar masses), in the center of an also gigantic galaxy called M87. For this, this international consortium relied on eight radio telescopes around the world. Among them are the largest millimeter and submillimeter wave observatory ALMA, the APEX telescope (both in the Atacama desert and of which ESO is a partner and carries out its operations, together with other partners), the IRAM 30-meter antenna at Pico Veleta and the NOEMA interferometer in the French Alps.

A telescope as big as Earth

The EHT consortium equipped these and other radio telescopes around the world with the appropriate devices so that their simultaneous observations could be combined by performing long-baseline interferometry. This achieves resolution equivalent to that of a telescope the size of the entire earth.

What do we see in the image of the black hole at the center of M87? We see light emitted by material orbiting the black hole as a result of its gigantic gravitational pull. In the black hole at the center of M87, matter takes weeks to complete an orbit, so the image taken over a few hours or days is a "still picture".

Obtain the image of SgrA* that the EHT consortium has presented today at the ESO headquarters in Garching (Germany) has additional challenges. It is a much less active black hole since it has hardly any material around it that can attract gravitationally and therefore less energy. We see it bright because it is "close" in the center of our own galaxy.

Less active than the one we met in 2019

But what has made the observations of Sgr A* particularly difficult to interpret is that the material orbiting it takes only a few minutes to complete one revolution. So there is no fixed photo, since to obtain the image of Sgr A* you have to spend hours and even days collecting data and during that time everything changes. After many years of work, the EHT consortium has finally been able to show us today that average image of the black hole in the center of our galaxy, with the great difficulty involved in photographing something that moves at breakneck speed.

And what have we learned with this image of Sgr A*? Many things. First, the size of the black hole is the one that corresponds to its mass of 4 million suns, according to the theory of general relativity. This mass, meticulously measured by Reinhard Genzel's team, is exactly what is inferred from the size of the Sgr A* black hole in the image.

On the other hand, the EHT team concludes that this black hole is spinning and that the spin axis is pointing approximately towards us (only 30 degrees away). This result, quite remarkable, is going to make people talk without a doubt.

Difficult data to interpret

There is much more in store. The data, which is not easy to interpret, contains a large amount of information that must be “decoded”. The EHT consortium is at it, as it prepares to expand the number of radio telescopes it uses as well as study other black holes.

I want to remind here that the THAT, European Organization for astronomical research in the southern hemisphere, decided 60 years ago to install its telescopes in the southern hemisphere and one of the reasons was precisely to be able to study in detail the center of our galaxy, the Milky Way. It is reassuring to see that the two great results obtained in relation to the black hole Sgr A* in the center of our galaxy (the orbits of the stars near the black hole and the image revealed today) have taken good advantage of that decision.

Source link