its winds existed, you just had to know how to look for them

It is well known that, while attracting matter into their interior, supermassive black holes release jets of energy and matter outwards in a violent way. The result is a kind of wind, which has been observed in all black holes of this type. There is only one in which, no matter how many observations have been made, those winds did not seem to exist. That of our own galaxy.

With known physics in hand, Sagittarius A*, the black hole of the Milky Way, should generate strong winds, but no matter how hard they have searched for more than 50 years, they have not been found. Now, a team of scientists from Northwestern University has acted as a mother would: after a “what should I go and find it?” They have made a series of observations and, indeed, they have found it.

Five years and two observatories. The main problem why the Sagittarius A* winds were not being detected is that the black hole itself emits radio waves that acted as interference. Therefore, these scientists have used the radio telescopes of the ALMA observatory with a series of calibrations that allow these interferences from the background glow of the black hole to be eliminated. Besides, They have taken data for 5 years. The more data, the more the noise is reduced.

In this way, they found what appeared to be evidence of the existence of winds. However, they wanted to confirm the data with a second observatory, this time the Chandra X-ray Observatory. Thanks to it, they detected emissions in this part of the spectrum that fit perfectly with what ALMA detected. Sure enough, they had found the lost winds of the black hole.

A cone without cold gas. Thanks to this new calibration method, scientists at Northwestern University were able to observe the gas from very close to the black hole to many light years away. This allowed him to see how it has behaved over the past thousands of years. Thanks to that, a cone-shaped region was detected that lacked cold gas. The most obvious thing would be that very hot and strong winds had carried the gas in that region. It could be the proof they were looking for.

Not all winds are the same. Black holes are objects so massive that almost nothing can escape their attraction. Not even the light. That matter and energy that is directed towards the center of the black hole is concentrated in a region known as the accretion disk, where it rotates at high speed as it gets closer and closer. In this process, the matter is compressed and heated very much, releasing a large amount of energy, which favors the violent expulsion of part of that material. This is how the winds of the black hole are generated. On the other hand, it is known that winds are also generated on the surfaces of stars. Our Sun, for example, causes very intense winds.

To find out what type of winds would be causing the cone, these scientists calculated the energy that would result from the winds of all the surrounding stars. Not even all of them together could give rise to a wind so intense that it could carry all that amount of cold gas. Only the black hole option fit. Furthermore, the cone pointed precisely to Sagittarius A*.

Chandra enters the scene. To verify that their suspicions were not the result of a specific anomaly, they checked their results with the Chandra X-ray Observatory. Thanks to it, they detected X-ray emissions in exactly the same region as the cone. These would correspond to the emissions from the black hole’s winds. Yes, they were facing the winds that no one had been able to locate.

20,000 years without being seen. By analyzing emissions across the galaxy, they saw that this black hole has been active for at least 20,000 years. It is true that it has been relatively silent, since, while the winds of a black hole are more intense than those of a star, it was more similar to a breeze than a storm. But the wind was there, immersed in interference. You just had to look for it conscientiously. This time, physics adds up. What seemed like an exception, really wasn’t.

Image | X-ray: NASA/CXC/Northwestern University/M. Gorski; Radio: ESO/NAOJ/NRAO/ALMA; processing: NASA/CXC/SAO/K. Arcand and P. Edmonds

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