Black holes in the Milky Way

Black holes are thought to be common in the universe, with a supermassive gobbler lurking at the core of galaxies such as our own Milky Way. But might they also be found roaming outside the galactic centers? A new study estimates that approximately 300 black holes may lurk throughout the Milky Way, remnants of the building blocks that came together billions of years ago to form the galaxy. What is more, those black holes may retain properties that would allow astronomers to identify them, providing an archaeological record of the galaxy's formation.

As galaxies merge, so do their respective central black holes. Previous studies have theorized that the final stage of black hole mergers in low-mass galaxies could release enough energy in the form of gravitational waves, or ripples in the fabric of space and time, to knock the resulting black hole right out of its parent galaxy. This recoil would not be powerful enough, however, to eject the black hole from the Milky Way's gravitational clutches, leaving these free-floating gobblers embedded within the Milky Way.

Scientists simulated the mergers of smaller galaxies in the Milky Way's history, finding that recoiled black holes, ranging in size from roughly 1,000 to a few hundred thousand times the mass of the sun, could exist in relatively large quantities in the Milky Way. (That is small relative to the supermassive black hole presumed to lurk at the Milky Way's core, which has a mass of some four million suns.) None would be particularly close by, the researchers say—at least a few thousand light-years away. A new study estimates that approximately 300 black holes may lurk throughout the Milky Way, remnants of the building blocks that came together billions of years ago to form the galaxy. What is more, those black holes may retain properties that would allow astronomers to identify them, providing an archaeological record of the galaxy's formation.