Mysterious dark matter is not visible through telescopes of any range. It manifests itself only as a gravitational effect on ordinary matter. This sad truth seems to have to be reconsidered. To the delight of scientists.
In a distant cluster of galaxies, something is absorbing and re-emitting X-rays of a certain energy. And this something cannot be an ordinary substance. This conclusion is made in a study published by a research group led by Joseph P. Conlon from Oxford University. The work is available on the arXiv.org preprint site.
According to the research press release, this detective story began in 2014. Then a scientific team led by Ezra Bulbul (Esra Bulbul) from the Harvard-Smithsonian Center for Astrophysics in Cambridge discovered a strange phenomenon. The X-ray emission from the cluster of galaxies known as the Perseus Cluster showed a spectral emission line with an energy of 3.5 keV. The result was obtained using the instruments of the XMM-Newton and Chandra telescopes. The same line was found in the radiation of 73 other galaxy clusters recorded by the XMM-Newton telescope.
Just a week after the publication of this result, another group led by Alexey Boyarsky from Leiden University in the Netherlands reported observing the same line in the emission of the galaxy M31 and the outskirts of the Perseus cluster on the same XMM-Newton instrument.
No known astrophysical process leads to the formation of such a line. Therefore, astronomers have suggested that they are dealing with the radiation of mysterious dark matter.
Many astronomers have tried to replicate these observations, but the mysterious line was found and then not. This led skeptics to speculate that the scientists were experiencing an error in the operation of the instrument or in the data processing.
In 2016, the new Japanese telescope Hitomi, specially designed to observe X-ray spectral lines, was unable to detect the 3.5 keV line in the radiation from the Perseus cluster. It seemed that the issue was finally closed. But that was just another plot twist.
Conlon's team noticed that Hitomi's images were much less sharp than Chandra's. Therefore, in the image of the Perseus cluster, the signal from two sources was mixed: radiation from hot gas located around a massive galaxy in the center of the cluster, and light emanating from the vicinity of a supermassive black hole in the center of this galaxy itself.
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Clearer images of Chandra make it possible to discern the contribution of these sources. Taking advantage of this, the authors were able to separately analyze the contribution of the black hole and the radiation of the hot gas.
Having in their hands the early observations of "Chandra" made back in 2009, they discovered an amazing thing: a spectral line of 3.5 keV was observed, but in the "X-rays" emitted by the gas, it was a radiation line, and in the radiation of a black hole - a line absorption! As it turned out, the Hitomi telescope mixed the contribution from two sources, as a result, the lines compensated each other and therefore were not observed. The researchers verified this by performing the appropriate calculations.
But how is it that, looking "directly in the eyes" of a black hole, astronomers detect the absorption of quanta with an energy of 3.5 keV, and observing a gas far enough from it, they catch radiation in the form of these quanta?
This phenomenon has long been known to specialists working with optical telescopes. Imagine a star shielded from us by a cloud of gas. Gas absorbs quanta of a certain energy and immediately re-radiates them. But this radiation occurs in all directions: back to the star, perpendicular to the line "star - observer" (the line of sight, as experts say), and so on. Therefore, looking directly at the star, we find an absorption line, since some of the quanta emitted by the star with this energy will not reach us.
Now we proudly turn away from the star and turn our gaze to that part of the cloud, which is "to the side" of it. These gas atoms also absorb the star's radiation and also re-emit it. But this time we do not see the light of the star itself, it spreads at a large angle to the line of sight. But we see that part of the absorbed light that the gas will emit in our direction (after all, it emits light in all directions evenly). Therefore, when looking at these "side" regions of the gas, we will see a radiation line!
Everything, it would seem, is wonderful. And the vicinity of the supermassive black hole does indeed emit quanta with an energy of 3.5 keV, as well as quanta of many other energies from a wide range. But in order to reproduce the picture just described, we need to assume that in the cloud of hot gas around the galaxy there is something that absorbs quanta of this particular energy, and then re-radiates them. And, as mentioned above, ordinary substance is simply not able to do this!
So, it's still dark matter? Conlon and his colleagues think so. They even developed their own model of this mysterious substance that reproduces this behavior. However, the authors have not yet discounted the error option. Subsequent studies should finally clarify the issue.
