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The Question

(Submitted February 07, 2011)

I've heard that Andromeda and the Milky Way galaxy will collide in 3 to 4 billion years, and that Andromeda and the Milky Way have black holes in their centers. What would happen to the black holes when the two galaxies merge?

The Answer

Thank you for your question. For some period of time the merged Andromeda/Milky Way galaxy will have two massive black holes in the central region, as is sometimes observed in merger remnant galaxies. Dynamical friction caused by interactions with stars cause them to sink to the center of the new galaxy, where they will form a binary system that will get tighter due to further encounters with stars. Eventually as the separation gets smaller than a light-year or so, gravitational radiation takes over and leads to an inevitable merger into a single supermassive black hole. If there is enough gas around to be accreted, the black hole may exhibit quasar-like behavior.

Michael Loewenstein & Amy Fredericks
for ``Ask an Astrophysicist''

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