If galaxies with massive black holes at their centers collide and merge, the massive black holes will also interact. If there is enough dynamical friction, due to interactions with gas and stars, the massive black hole binaries will eventually coalesce and generate large amounts of gravitational radiation. The expected event rate could be several per year, with large uncertainty.
The figure shows a ground based telescope view (left) of the collision between the galaxies NGC4038 and NGC4039 which reveals long arcing insect-like 'antennae' of luminous matter flung from the scene of the accident. Investigations using the Hubble Space Telescope have revealed over a thousand bright young clusters of stars - the result of a burst of star formation triggered by the collision. The green outline shows the area covered by the higher resolution Hubble image (right). At the distance of the Antennae galaxies (about 63 million light years), a pixel in this image corresponds to about 15 light years. Dust clouds around the two galactic nuclei give them a dimmed and reddened appearance while the massive, hot young stars of the newly formed clusters are blue.
Image by B. Whitmore (STSci), F. Schweizer (DTM), NASA