Two galactic surveys have determined that recently discovered mid-sized black holes may be relatively common and exist in up to 20% of galaxies, Andy Ptak and Ed Colbert of Carnegie Mellon University reported at the same meeting.
Supermassive black holes that pack the mass of a billion suns into a space just a few miles across reside at the core of most galaxies. Small black holes form when a star collapses. But mid-sized black holes--between 10 and 1 million times the mass of our sun--are a mystery. They may form when black holes merge and could be an intermediate step toward the formation of supermassive black holes.