Large galaxies like the Milky Way could have halos of four to twenty supermassive black holes in a radius of about a million light-years. This is predicted by numerical simulations dedicated to the study of the formation of these black holes during collisions and fusions between galaxies.
In the late 1960s, some researchers, including Martin Rees but especially his colleague and compatriot, the British astrophysicist Donald Lynden-Bell (recently deceased, February 6, 2018), understood that the majority of large galaxies must host in their center supermassive black holes. Skeptics in the 1970s, colleagues Lynden-Bell and Rees would gradually be convinced. Today, it is a thesis mostly accepted for at least 20 years, even if a definitive proof of the presence of objects having a horizon of events consistent with the predictions of the theory of general relativity is still missing.
We know that these black holes contain at least the equivalent of one million solar masses and sometimes several billion. The reality could be even more amazing if we believe an article filed on arXiv by an international team of astrophysicists, which has been now published. The existence of the supermassive black hole at the heart of the Milky Way is well known to science. Another candidate, called an intermediate black hole was known, since it probably contains about 100,000 solar masses, compared to so-called stellar black holes of some dozens of solar masses at most, is also suspected to exist. But according to the researchers, large galaxies like ours could actually have four to twenty supermassive black holes in a radius of about a million light-years.
Where does this new theory come from? No yet observations, but from a numerical simulation of the world of galaxies, specialized in the formation of supermassive black holes called Romulus. We do not know how these giant are born, but scientists suspect that collisions between galaxies contribute to their growth. Indeed, scientists observe fusions of galaxies, which must lead the giant black holes they contain in their center to dive gravitationally towards the center of the new galaxy formed, to finally merge by emitting a powerful flash of gravitational waves.
But according to the Romulus simulation, mergers between galaxies would not all lead to mergers of black holes. Some of the giant black holes would actually end up in orbits in the galactic halo around the center of the galaxies. There could be two to eight supermassive black holes within a 30,000 light-year radius around the center of the Milky Way. We would not have detected these black holes until now, because they would not be surrounded by enough gas to produce easily detectable radiation by accretion. Nor would they be numerous enough to be easily detectable by the effect of gravitational lenses.
Still, these nomadic supermassive black holes could stay in the halo for a few billion years. But according to astrophysicist Michael Tremmel, a postdoctoral researcher at the Yale Astronomy and Astrophysics Center, who led the team behind this work: “It is extremely unlikely that any wandering supermassive black hole will come close enough to our sun to have any impact on our solar system. We estimate that a close approach of one of these wanderers that is able to affect our solar system should occur every 100 billion years or so, or nearly 10 times the age of the universe”.