The Alma radio telescope, in conjunction with other instruments, has revealed two active galactic nuclei at the heart of a galaxy merger. Behind these cores is the binary supermassive black hole with the smallest known distance between these two compact stars, suggesting that gravitational wave hunting with eLisa in the near future will be more successful than hoped.
It was once thought that galaxies essentially grew by merging with each other, with the earliest-born large galaxies swallowing up dwarf galaxies. Observations made with the Hubble telescope also showed many examples of galaxies colliding, all the more so as we observed far into space, and therefore backward in time.
It was also thought that it was during collisions that the quasars were ignited, that is to say the active nuclei of galaxies made particularly luminous by a massive supply of gas at the heart of these galaxies, gas which was accreted by a rotating supermassive Kerr black hole.
However, over the past decade, the paradigm has changed with new observations and their interpretation through sophisticated numerical simulations. If we continue to think that supermassive black holes and galaxies grow together, given that we see a law of proportionality respected for the vast majority of galaxies between the mass in the form of stars they contain and that of their supermassive black holes, collisions are now thought to be not as frequent as once believed and in fact play a secondary, if not minor, role in the growth of galaxies.
Everything indicates indeed that it is the accretion of currents of baryonic matter left by the Big Bang, via filaments of cold dark matter channeling them, that new stars are forming massively and that central black holes are growing.
In any event, we will try to find out more about the growth of galaxies/supermassive black holes by fusion by detecting on the horizon of the 2030s the gravitational waves emitted by the fusion of supermassive black holes at the end of the fusion of galaxies and this using the eLisa gravitational wave detector. Current detectors, such as Ligo and Virgo, do not observe in the appropriate wavelength band to highlight the losses of energy in the form of gravitational waves from supermassive black holes. Losses that decrease the sizes of their orbits in the binary system formed by the merger of two galaxies.
Estimates of the amount of binary black holes that can be expected to be observed in a given volume of spacetime have been given. The more a sphere surrounding the Milky Way can be probed by increasing detector sensitivity, the more binary supermassive black holes are expected to be found. But this conclusion is tempered by the fact that the further these objects are, the less luminous they are for a detector for eLisa.
UGC 4211, a laboratory for understanding binary supermassive black holes?
It seems, however, thanks to recent high-resolution observations in the millimeter wave range with theAtacama Large Millimeter/submillimeter Array (Alma) that there are more nearby binary supermassive black holes than previously thought, which bodes well if this is indeed the case.
This is according to an article published recently in The Astrophysical Journal Letters by Michael Koss and his colleagues who have also mobilized other instruments observing at other wavelengths, such as Muse equipping ESO’s VLT or the telescopes of the WM Keck Observatoryto further study UGC 4211, a pair of merging galaxies just 500 million light-years from the Milky Way in the constellation Cancer.
We then note not only the presence of two or three supermassive blacks separated by only about 750 light-years, which is already the record for the smallest separation for known supermassive black holes in a binary system, but above all, and it is also new, that they are accreting matter to the point of forming not one, but two active galactic nuclei at the heart of a molten galaxy, which had never been seen before.
” The simulations suggested that most of the population of black hole binaries in nearby galaxies would be inactive because they are more common, not two growing black holes as we found. Alma is unique in that it can see through large columns of gas and dust and achieve very high spatial resolution to see things very close together. Our study has identified one of the closest pairs of black holes in a galaxy merger, and since we know that galaxy mergers are much more common in the distant Universe, these black hole binaries may also be much more common than previously thought “, explains moreover Michael Koss in a press release from the National Radio Astronomy Observatory (NRAO), in the United States.
His colleague Ezequiel Treister, astronomer at theCatholic University of Chile and co-author of the research, adds that: “ There could be many pairs of growing supermassive black holes at the center of galaxies that we haven’t been able to identify so far. If so, in the near future we will observe frequent gravitational wave events caused by mergers of these objects across the Universe. »
This video shows an illustration and Alma footage of two black holes dining together and greedily devouring dust, gas and other materials disturbed by the collision. © Alma (ESO/NAOJ/NRAO), M. Koss et al (Eureka Scientific), S. Dagnello (NRAO/AUI/NSF)
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