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Dynamical friction and the evolution of supermassive black hole binaries: the final hundred-parsec problem

Dosopoulou, Fani and Antonini, Fabio 2017. Dynamical friction and the evolution of supermassive black hole binaries: the final hundred-parsec problem. Astrophysical Journal 840 (1) , 31. 10.3847/1538-4357/aa6b58

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Abstract

The supermassive black holes originally in the nuclei of two merging galaxies will form a binary in the remnant core. The early evolution of the massive binary is driven by dynamical friction before the binary becomes "hard" and eventually reaches coalescence through gravitational-wave emission. We consider the dynamical friction evolution of massive binaries consisting of a secondary hole orbiting inside a stellar cusp dominated by a more massive central black hole. In our treatment, we include the frictional force from stars moving faster than the inspiralling object, which is neglected in the standard Chandrasekhar treatment. We show that the binary eccentricity increases if the stellar cusp density profile rises less steeply than . In cusps shallower than , the frictional timescale can become very long due to the deficit of stars moving slower than the massive body. Although including fast stars increases the decay rate, low mass-ratio binaries () in sufficiently massive galaxies have decay timescales longer than one Hubble time. During such minor mergers, the secondary hole stalls on an eccentric orbit at a distance of order one-tenth the influence radius of the primary hole (i.e., for massive ellipticals). We calculate the expected number of stalled satellites as a function of the host galaxy mass and show that the brightest cluster galaxies should have of such satellites orbiting within their cores. Our results could provide an explanation for a number of observations, which include multiple nuclei in core ellipticals, off-center AGNs, and eccentric nuclear disks.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Physics and Astronomy
Publisher: American Astronomical Society
ISSN: 0004-637X
Date of First Compliant Deposit: 19 February 2020
Date of Acceptance: 2 April 2017
Last Modified: 20 Feb 2020 12:15
URI: http://orca.cf.ac.uk/id/eprint/129842

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