Black holes are talked about so often in astronomy that it may come as a surprise to discover that we know only a few dozens of them inhabiting the Milky Way. Apart from the central supermassive black hole, Sagittarius A*, all the others are at most 15 times the mass of the Sun, but statistically, there should be many more. Maybe tens of millions more.
Finding all these black holes is currently next to impossible thanks to a very distinctive property: They are black. All the known ones have companions or have been caught gobbling up material that led to emitting light, which can be observed. As reported in the Monthly Notices of the Royal Astronomical Society, astronomers do have an idea on how to detect and study these objects.
Astronomers Daichi Tsuna and Norita Kawanaka, from the University of Tokyo and Kyoto University respectively, think that it is possible to detect the emission from black holes accreting matter from the interstellar medium, the material (gas, dust, and cosmic rays) that fills the space between stars. The amount of material ejected, however, is very small and thus the emission produced is very weak and difficult to measure.
But the researchers believe that this weak emission could be detected by the upcoming Square Kilometer Array, which over the next decade will become the world’s largest radio telescope. The first phase of the telescope is expected to detect about 30 of these isolated black holes. Once it is fully operational up to 700 of these objects should become visible.
According to the astronomers, there could be 100 million black holes hiding in the Milky Way. This is based on the estimate of our galaxy having had roughly 20,000 stellar explosions in the past million years, and having been around for about 13 billion years. While the rate of stars going supernova might have changed, this is a good ballpark figure for how common these objects are.
Assuming that our galaxy is a flat disk (it isn’t, it's warped and bulges in the middle), is 100,000 light-years across, and with an average thickness of 1,000 light-years, there should be isolated black holes every few light-years. Obviously, in reality, there are going to be many more towards the core (something which is already suspected) than in the outskirts where the Solar System is.
But this approach might discover some black holes in the solar neighborhood. Currently, the closest known stellar black hole to Earth is V616 Monocerotis. It is 3,000 light-years away and between nine and 13 times the mass of the Sun. In the next decade, we should hopefully meet more of our neighbors.