Clues to the photo of Sagittarius A*

The first historical image of black hole at the center of our galaxy is not very different from that of M87* that we saw three years ago, a coincidence that already predicted the Einstein’s general relativity. Now the challenge for the EHT scientific collaboration that has captured them is to record a ‘movie’ of these dark shadows and their bright ring of gas.

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 observing 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 be about four million solar masses, a result that is in perfect agreement with Einstein’s general theory of relativity.

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 it predicts that all black holes behave and look the same, regardless of their 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 days to 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 the many different ones that have been captured by the international collaboration Event Horizon Telescope (EHT).

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 Earth size. 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 although 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 computers are used.

One of the most important is to present not an image of Sagittarius A*, but a ‘film’ of 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 collaboration will try to reconstruct its magnetic field, since it is an essential factor in the formation of the relativistic jets of these objects, trying to clarify why the spin axis of Sgr A* points almost towards us (only 30 degrees ) instead of coinciding with that of our galaxy.

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Clues to the photo of Sagittarius A*