Galaxy mergers can occur when two (or more) galaxies collide. They are the most violent type of galaxy interaction. The gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies involved, but the exact effects of such mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active area of research. Galaxy mergers are important because the merger rate is a fundamental measurement of galaxy evolution and also provides astronomers with clues about how galaxies grew into their current forms over long stretches of time.
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Galactic merger in the context of Binary black hole
A binary black hole (BBH), or black hole binary, is an astronomical object consisting of two black holes in close orbit around each other. Like black holes themselves, binary black hole systems are classified as either stellar-mass—involving remnants of high-mass binary star systems or formed by dynamic processes and mutual capture—or supermassive, black hole systems believed to arise from galactic mergers.
The existence of stellar-mass binary black holes was directly confirmed by gravitational wave observation in September 2015. Supermassive binary black hole candidates have been proposed based on indirect evidence, but await observational confirmation.