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Binary black holes and tori in AGN I. Ejection of stars and merging of the binary

Christian Zier·Peter L. Biermann
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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 1 pc, 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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Christian Zier, Peter L. Biermann. 2001-08-21. Binary black holes and tori in AGN I. Ejection of stars and merging of the binary. https://doi.org/10.1051/0004-6361%3A20010862

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