Millions of Black Holes Are Hiding in the Milky Way Eating Matter from Interstellar Space

Across the Milky Way, there are millions of undiscovered black holes that are consuming matter from interstellar space, draining our galaxy of dust and gas floating in the voids between stars. Now, a pair of Japanese scientists have announced a plan to hunt down these "lost" black holes.

The idea that there are millions of black holes hiding in the Milky Way is not new. In 2018, NASA-funded research found there could be between 10,000 and 20,000 black holes gathered around the supermassive black hole at the center of the galaxy. Research published February also said there are potentially 100 million "quiet black holes" in the Milky Way.

Tracking these regions in space is difficult, however. The gravitational forces around a black hole are so intense not even light can escape. This means they cannot be detected using traditional methods used to observe other celestial objects.

However, astronomers Daichi Tsuna and Norita Kawanaka, from the University of Tokyo and Kyoto University in Japan, claim to have found a new way of detecting these "isolated black holes" (IBHs).

Their study, which has been posted on the preprint server arxiv.org, suggests observing IBHs through their X-ray emissions. The paper, which is yet to be peer reviewed so has not been assessed by other scientists working in the field, builds on the understanding that IBHs, like all black holes, feed on matter from interstellar space. This produces an accretion disk of matter that surrounds the black hole, which—for the biggest black holes—produces X-rays that can be detected by scientists.

But for IBHs, the accretion flow is "unusually weak," meaning it does not produce strong X-ray emissions. However, because of the weak accretion flow, there is a much stronger outflow of material. As LiveScience first reported, Tsuna and Kawanaka focus on the shock produced when this outflow hits the surrounding material. When this happens, electrons are accelerated, producing radio waves that could be detected.

The scientists say the technique is optimistic—detecting IBHs in this way would require extremely sensitive equipment and would probably could not be used to find the vast majority of these hidden black holes.

Astrophysicist Simon Portegies Zwart, from Leiden University in the Netherlands, who was not involved in the study, told LiveScience that Tsuna and Kawanaka's method of detecting IBHs "would be great" but "the sensitivity may pose a problem."

The authors say the forthcoming Square Kilometer Array (SKA) telescope, which will be based in Australia, may offer capabilities required. "We propose in this paper that IBHs can be one of the promising targets for SKA, and aim to give an estimate on the number of detectable IBHs in the whole Galaxy by the two SKA phases," they say.