Astronomers have tracked down hidden black holes.
The Milky Way is a tranquil galaxy: it has existed for at least 12 billion years, and no significant collisions with large neighbors are anticipated for several more billion years. As a result, our central supermassive black hole consumes surrounding matter slowly, simply because there isn't much left to consume.
This steady pace sets our galaxy apart from many others that have extremely active cores. For instance, the so-called Seyfert galaxies: they may appear as spiral, lenticular, or irregular galaxies, but their radiation reveals a vast accumulation of interstellar matter around the center. This indicates the vigorous activity of a supermassive black hole.
The paradox is that such energetic black holes can be exceedingly difficult to detect. This largely depends on how the galaxy is oriented in relation to us: if we see the entire disk, the central black hole is noticeable, but if the galaxy is viewed "edge-on," it is often nearly completely obscured by a dense veil, surrounding it in a massive "doughnut" shape, that is, a toroidal structure.
A team of international astrophysicists recently attempted to understand whether it is possible to "unmask" such black holes and estimate how many exist in the cosmos. In an article for the journal The Astrophysical Journal, the scientists shared that they had an important clue — the suspicion that although this "doughnut" is indistinguishable in visible light and even in the faint X-ray range, it must be actively emitting some radiation: infrared. This emission is very weak and long-wavelength, but by tuning into it, one can discover a lot of new information.
The researchers meticulously examined data from the infrared space observatory IRAS, which operated for several months in 1983 at an altitude of 900 kilometers above Earth. The hypothesis proved correct: this invisible radiation revealed hundreds of potential hidden black holes at the centers of galaxies.
To definitively confirm their "identity," the scientists further analyzed data from the NuSTAR X-ray observatory (which has been flying at an altitude of 445 kilometers since 2012). The calculations indicated that the highest-energy X-ray beams should also penetrate the toroidal gas-dust blanket.
After comparing the two light spectra, some candidates had to be eliminated, but for the most part, the "hunt" for hidden black holes was successful. Now astrophysicists suggest that at least 35 percent of all supermassive black holes in the universe are significantly obscured by surrounding matter, and possibly even more.