http://arxiv.org/abs/1610.01600
We study the orbital decay of a pair of massive black holes (BHs), in the mass range 5 * 10^5 – 10^7 Msun, using a large set of hydrodynamical simulations of circumnuclear disks (CNDs) with varying prescriptions for the sub-grid physics of the interstellar medium, from star formation and supernova feedback to BH accretion and its feedback. In the absence of any of such processes, the orbit of the secondary BH in an adiabatic flow decays over timescales of a few Myr to the center of the CND, where the primary BH resides. As soon as strong dissipation operates in CNDs, fragmentation into massive objects the size of giant molecular clouds occurs, causing stochastic torques as well as direct hits that eject the secondary BH out of the disk plane. Once outside the plane, the low-density medium provides only weak drag, and the return to the CND plane is governed by inefficient dynamical friction in a stellar bulge. Ejections are seen to occur in nearly all of runs with cooling, irrespective of which other processes are modeled. In rare cases, clump-BH interactions can lead to a speed-up of the decay. Feedback processes lead to outflows but do not change significantly the overall density of the CND midplane, thus having minor direct impact on the orbital decay. However, with BH feedback a hot bubble is generated behind the secondary already on the first orbit, which almost shuts off dynamical friction, a phenomenon that we dub “wake evacuation”. In the latter case we find that delays in the decay of up to a fraction of Gyr can occur. We discuss the non-trivial implications on the discovery space of eLISA. Our results suggest that the largest uncertainty in predicting massive BH merger rates lies in the potentially wide variety of galaxy host systems, with different degrees of gas dissipation and heating, yielding decay timescales from as small as 10 Myr to as large as a fraction of Gyr.
R. Lima, L. Mayer, P. Capelo, et. al.
Fri, 7 Oct 16
22/75
Comments: Submitted to ApJ. Comments are welcome
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