Tracing Black Hole Mergers Through Radio Lobe Morphology
  Binary supermassive black holes are produced by galactic mergers as the black holes from the two galaxies fall to the center of the merged system and form a bound pair. The two black holes will eventually coalesce in an enormous burst of gravitational radiation. Here we show that the orientation of a black hole's spin axis would change dramatically even in a minor merger, leading to a sudden flip in the direction of any associated jet. We identify the winged or X-type radio sources with galaxies in which this has occurred. The implied coalescence rate is similar to the overall galaxy merger rate, suggesting that the prospects are good for observing gravitational waves from coalescing supermassive black holes. 
David Merritt (Rutgers University) and R.D. Ekers (ATNF)