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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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