This Thursday May 12 humanity knew the first direct image of the black hole that is located in the center of our galaxythe Milky Way, which was achieved by the global network of radio telescopes EHT in the company of several other scientific institutions.
This is one more step for an achievement already made by the EHT, which three years ago had already shown the first image of a black hole, captured in the galaxy M87, our neighbor. This time, Sagittarius A* is a supermassive black hole that is four million times more massive than the Sun.
What can be seen and analyzed is a ‘monster’ that devours matter at a slow pace, is rotating, and its spin axis points only 30 degrees from us. The result proves that the size of the black hole is proportional to the mass it contains, confirming the theory of general relativity.
The image has been achieved by averaging images obtained from eight facilities of the Event Horizon Telescope (EHT) collaboration: Alma and Apex in Chile, Iram in Spain and LMT in Mexico, Jcmt, SMT and SMA in the United States and SPT in the Pole South.
These four questions will allow you to learn more about this achievement:
Didn’t we already know about the black hole at the center of our galaxy?
Previous studies, including those that in 2020 earned Reinhard Genzel and Andrea Ghez the Nobel Prize in Physics, had already shown that at the center of the Milky Way there is a supermassive object – called Sagittarius A* or Sgr A*–, with a mass four million times greater than that of the Sun. They deduced it from the movement of the stars that revolve around it.
What is presented now for the first time is his image, a direct visual evidence. Although the hole itself is not actually visible, its enormous gravity swallows all light, but its dark shadow surrounded by a ring of hot, glowing gas. The shadow is about 52 microarcseconds across, which is the equivalent of seeing a CD on the Moon from Earth. It is about seeing a hole of 3 light minutes at a distance of 27,000 light years.
Since the size of the shadow is proportional to the mass, it is confirmed to have about four million solar masses, a result that is in perfect agreement with Einstein’s theory of general relativity.
What differences are there between the image of Sagittarius A* and that of M87* presented in 2019?
Apparently the two images are similar, despite the fact that they are two quite different holes. The one in our galaxy is more than a thousand times smaller, but it is closer, and it is also less massive: Sagittarius A* has 4.3 million solar masses compared to 6.6 billion for M87*, which is much further away, 55,000,000 light years away. Their orientations towards us are also different.
However, the fact that the two images look alike confirms a key aspect of general relativity, since predicts that all black holes behave and look the same, regardless of its mass. This implies that the entire universe is full of these luminous ‘doughnuts’.
In addition, the two rotating holes are also ‘fed’ at a different rate. The gas takes between days and weeks to orbit around M87* –the big one–, but in Sgr A* –the little one– it completes an orbit in just a few minutes. This makes observations difficult, as the brightness and pattern of the gas swirling around our galaxy’s hole changes rapidly.
While M87* was an easier and more stable lens, in which almost all of its images looked the same, this was not the case with Sagittarius A*. The image presented is an average of many different ones captured by the international collaboration of the Event Horizon Telescope (EHT).
How was the image obtained?
In 2017 the EHT used a network of eight radio telescopes distributed around the world (Alma and Apex in Chile, Iram in Spain, LMT in Mexico, Jcmt, SMT and SMA in the United States and SPT in the South Pole) that function as a virtual one of the size of the Earth. To create it and combine all the signals, a technique called very-long-baseline interferometry (VLBI, where mathematical operations are used instead of lenses) is used.
But even if it has a planetary scale, this global telescope is made up of a limited number of antennas, and reconstructing a ‘photograph’ with all its data is equivalent to guessing a sentence knowing only some of its letters. To solve it and offer the final average image Algorithms and powerful computers are used.
What challenges lie ahead?
One of the most important is to present not an image of Sagittarius A*, but a ‘film’ of the gas orbiting the black hole. In fact, it is what was announced three years ago when the image of M87* was presented, but at the moment there is not enough information. The recent addition to the EHT network of more radio telescopes (GLT in Greenland and NOEMA in France), as well as the updates of those that already existed and the new observation campaigns –the last one, in March of this year–, will help to achieve this objective.
In addition, the EHT scientific collaboration will try to reconstruct the magnetic field of this and other supermassive black holes, since it is an essential factor in the physics and formation of the relativistic jets associated with this type of object.
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