Astronomers suspect that hundreds of medium-sized black holes are roaming loose in the Milky Way galaxy. These blach holes are the orphaned central black holes of the many smaller galaxies that the Milky Way has swallowed over its billions of years of existence. If one of them is discovered, it could provide important clues about the evolution of our galaxy. Astronomers are convinced that each of the hundreds of billions of galaxies in the cosmos formed a massive or supermassive black hole at its center. Researchers also sustain the idea that when big galaxies collide - a relatively common event - their central black holes eventually merge. Some observational evidence supports this idea. Another similar theory is that galaxies often grow by absorbing smaller, satellite galaxies, such as the Large Magellanic Cloud, which orbits the Milky Way. This theory isn't yet supported by tangible evidence.
Two researchers from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts - theoretical astrophysicists Ryan O'Leary and Abraham Loeb - have proposed a way to detect evidence that a large galaxy - in this case, the Milky Way - has collided with a satellite galaxy. Using computer simulations they assumed that sometimes the central black hole of a dwarf galaxy might remain independent after a galactic collision. The gravitational interaction between a supermassive black hole and a smaller galactic cousin can sometimes kick the smaller black hole out of the smaller galaxy's center - much as a black hole's intense gravity can sometimes produce huge jets of matter.
The ejected black hole would not move fast enough to escape the galaxy's gravity entirely, but it would move faster than the background stars - something that makes it detectable, because it would also be dragging along a small cluster of surrounding stars.
The ejected black hole would not move fast enough to escape the galaxy's gravity entirely, but it would move faster than the background stars - something that makes it detectable, because it would also be dragging along a small cluster of surrounding stars.
The ability of two interacting black holes to kick one out into space "is turning out to be an important process in a wide variety of astronomical settings," says the astronomer Christopher Mihos of Case Western Reserve University in Cleveland, Ohio. "It gives the observers something to look for and the theorists something to keep scratching their heads over."
