Why do black holes stick around in galaxies despite their violent dynamical history?
A brilliant young postdoc has an answer!
Dr Bogdanovic has a nifty press release on why black holes do tend to stick around in galaxies
Recent results in numerical relativity have shown that the gravitational radiation reaction is strong and asymmetric.
What this means is that when black holes coalesce through gravitational radiation emission, the final stage of emission is asymmetric, and if the black holes have different (but not too different masses) or different spins, there is an impulsive recoil to the final merged black hole, which can be hundreds of kilometers per second, or even a few thousand km/sec for well tuned mass ratios and spins.
But, we see black holes in almost all galaxies where we can look closely enough, and we simultaneously think that most all black holes have undergone mergers, possibly multiple mergers, with other black holes.
So what is going on?
Tamara has shown that if the black holes have undergone significant gas accretion, which we think they mostly do, then the spins probably align which strongly suppresses the magnitude of the recoil.
So most galaxies keep their black holes.
But some may not, which is also interesting.
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