Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
C. Zier and P.L. Biermann:
Binary black holes and tori in AGN I.
Ejection of stars and merging of the binary
Astronomy and Astrophysics 377, 23-43 (2001)
Abstract.
Most galaxies have central massive black holes (BH), and merge with
others during their evolution, as strongly suggested by recent
observations.
Consequently a black hole binary (BHB) emerges. Considering the
evolution
of the binary when it has become hard, at a separation of the BHs of
about 1pc, we propose to solve two basic questions in our understanding
of the central activity of galaxies: (1) Do the BHs actually merge? (2)
And does the
effect of the torque of the BHB on the surrounding stellar distribution
help to explain the presence of the ubiquitous torus of molecular
material
surrounding apparently all active galactic nuclei? While the second
question is the topic of a subsequent paper we try to solve the first
in this article. Simulating the evolution of a stellar cluster in the
potential of such a binary by solving the equations of the restricted
three body problem, we obtained the following results: If the cluster
is about as massive as the BHB,
the two BHs coalesce after about 10^7 yr due to ejection of stars with
low angular momentum and finally via emission of gravitational
radiation. In a sequence of models where the mass of the secondary BH
increases while M_1 is fixed, a bigger fraction of stars is ejected.
For a more massive cluster the merger then proceeds on smaller time
scales. The cluster is depleted in the central region and the final
distribution of stars assumes a torus-like structure, peaking at three
times the initial distance of the two black holes.
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