New calculations by Ryan O'Leary and Avi Loeb at the Harvard-Smithsonian Center for Astrophysics reveal hundreds of massive black holes in the Milky Way, left over from the early days of the galaxy.
Rogue Black Holes in the Milky Way Galaxy
Theory states that rogue black holes started out as the cores of tiny, low-mass galaxies early in the history of the Universe. These dwarf galaxies eventually collided over a series of events that created the full-sized galaxies seen today. When two of the dwarf galaxies merged, their central black holes also merged, forming one relic black hole.
"These black holes are relics of the Milky Way's past," says Loeb. "You could say that we are archaeologists studying those relics to learn about our galaxy's history and the formation history of black holes in the early universe."
Location of the Milky Way's Relic Black Holes
When the black holes merged, directional emission of gravitational radiation caused the black hole to be shot out of the center of its dwarf galaxy. The black hole would still remain with its new galaxy, along the outer edges of the galactic halo. In the Milky Way, this would put the relic, or rogue, black holes thousands of light-years safely away from Earth, along the galaxy's periphery. The exact number of rogue black holes depends on the number of dwarf galaxies that merged to form the present-day Milky Way.
How to Find Rogue Black Holes Currently in Our Galaxy
The relic black holes, with masses some 1,000 to 100,000 times our sun, are only visible through the surrounding material that they are pulling in. When the rogue black holes swiftly departed their original dwarf galaxies, they pulled along with them clusters of stars that were located closest to the black hole. Therefore, scientists will look for very compact clusters in the Milky Way's edges. The clusters will be so small and compact as to appear as a single star; however, their spectrums would reveal that multiple stars are present. Another telltale sign will be that the stars are moving rapidly as they are flung around the central black hole.
"The surrounding star cluster acts much like a lighthouse that pinpoints a dangerous reef," explains O'Leary. "Without the shining stars to guide our way, the black holes would be all but impossible to find."
"Until now, astronomers were not searching for such a population of highly compact star clusters in the Milky Way's halo," says Loeb. "Now that we know what to expect, we can examine existing sky surveys for this new class of objects."
Scientists will look for these lighthouses at the edge of our galaxy to learn more about rogue black holes and the evolution of our Milky Way.
 |
