Black-Hole Mergers Face Final Parsec

When two galaxies merge, dynamical friction slows their central supermassive black holes until they orbit within a few parsecs of each other. By then, they may have flung away the surrounding stars and gas needed to drain more energy, leaving them stalled around one parsec, or 3.26 light-years, apart and too distant for gravitational waves to finish the merger. Yet supermassive black holes appear to merge anyway, creating the cosmic mystery known as the final parsec problem. Astronomers still debate whether fresh stars or gas, self-interacting dark matter, eccentric orbits, or a third black hole pushes them across the gap.

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