Astronomers have discovered a pair of supermassive black holes in the galaxy NGC 7674, only one light-year apart and ready to merge into an even larger object, according to an article published in the journal Nature Astronomy.
“This pair of black holes is about 400 million light-years from Earth, about twice as close as black holes in galaxy 4C +37.11 or galaxy OJ 287. This, however, will not make it more convenient for observation behind gravitational waves, because, due to a combination of factors, neither pulsar detectors like LIGO nor even eLISA will see them,”write Preeti Kharb of the University of Pune (India) and her colleagues.
The launch of the updated LIGO detector and the detection of several bursts of gravitational waves generated by merging black holes, once again showed that such events occur quite often in the Universe. Nevertheless, to date, scientists are aware of only two galaxies where black holes are very close to each other - OJ 287 and 4C +37.11.
The small number of such objects and the large distance to them prevent scientists from studying the properties of such black holes, including the role that their mergers can play in the formation of galaxies and in the "strangulation" of star formation in them.
Harb and her colleagues added another interesting object to their number by studying the "tail" of a black hole in the center of the spiral galaxy NGC 7674, located about 400 million light-years away from us and located in the constellation Pegasus. Its unusually large length - about two thousand light years - and its structure have long led scientists to believe that not one, but two black holes can live in the center of this spiral galaxy.
Observing it with the eVLA radio interferometer, a "union" of several radio telescopes combined into one virtual radio dish, scientists discovered that at its center there are not one, but two sources of radio emission. Their high brightness and speed of movement indicate only one possible variant of their nature - they are both supermassive black holes.
The total mass of these black holes, according to scientists, is about 36 million times the mass of the Sun, and they make one revolution around a common center of mass in about 100 thousand years. The short duration of the "year" on these black holes surprised Harb and her team, as it means that these objects are very close to each other - they are separated by only 420 light days, or 1.1 light years.
Such a close proximity of black holes and the very fact of their rotation around each other suggests that they are generating strong enough gravitational waves right now. These waves, despite the relatively small distance between NGC 7674 and the Milky Way, cannot be seen either with LIGO or with the space-based gravitational detector eLISA, whose construction was recently approved by ESA.
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The reasons for this, as scientists explain, are different - LIGO is simply not adapted to detect low-frequency gravitational waves that are produced by supermassive black holes. In turn, the "triangle" of eLISA probes, designed to observe mergers of heavy black holes, will not see them for the reason that the mass of objects in the center of NGC 7674 is too small to be able to notice them.
On the other hand, the discovery of the first real traces of two black holes in the center of a spiral galaxy suggests that their collisions and mergers with other large "families of stars" do not always lead to the birth of elliptical galaxies, in which star formation processes are rapidly extinguished due to heating of the gas the emissions of these black holes. Apparently, there are different mechanisms for the merger of galaxies and black holes, which scientists have not yet discovered, Harb and her colleagues conclude.