# The pairing of accreting massive black holes in multiphase circumnuclear disks: the interplay between radiative cooling, star formation, and feedback processes

Lima, Rafael Souza; Mayer, Lucio; Capelo, Pedro R; Bellovary, Jillian M (2017). The pairing of accreting massive black holes in multiphase circumnuclear disks: the interplay between radiative cooling, star formation, and feedback processes. The Astrophysical Journal, 838(1):13.

## Abstract

We study the orbital decay of a pair of massive black holes (BHs) with masses $5\times {10}^{5}$ and 107 ${M}_{\odot }$, using hydrodynamical simulations of circumnuclear disks (CNDs) with the alternating presence of sub-grid physics, such as radiative cooling, star formation, supernova feedback, BH accretion, and BH feedback. In the absence of such processes, the orbit of the secondary BH decays over timescales of $\sim 10\,\mathrm{Myr}$ to the center of the CND, where the primary BH resides. When strong dissipation operates in CNDs, fragmentation into massive objects the size of giant molecular clouds with densities in the range 104–107 amu cm−3 occurs, causing stochastic torques and hits that can eject the secondary BH from the midplane. Outside the plane, the low-density medium provides only weak drag, and the BH return is governed by inefficient dynamical friction. In rare cases, clump–BH interactions can lead to a faster decay. Feedback processes lead to outflows, but do not significantly change the overall density of the CND midplane. However, with a spherically distributed BH feedback, a hot bubble is generated behind the secondary, which almost shuts off dynamical friction. We dub this phenomenon "wake evacuation." It leads to delays in the decay, possibly of $\sim 0.3\,\mathrm{Gyr}$. We discuss the non-trivial implications on the discovery space of the eLISA telescope. Our results suggest that the largest uncertainty in predicting 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 $\sim 10$ to $\sim 300\,\mathrm{Myr}$.

## Abstract

We study the orbital decay of a pair of massive black holes (BHs) with masses $5\times {10}^{5}$ and 107 ${M}_{\odot }$, using hydrodynamical simulations of circumnuclear disks (CNDs) with the alternating presence of sub-grid physics, such as radiative cooling, star formation, supernova feedback, BH accretion, and BH feedback. In the absence of such processes, the orbit of the secondary BH decays over timescales of $\sim 10\,\mathrm{Myr}$ to the center of the CND, where the primary BH resides. When strong dissipation operates in CNDs, fragmentation into massive objects the size of giant molecular clouds with densities in the range 104–107 amu cm−3 occurs, causing stochastic torques and hits that can eject the secondary BH from the midplane. Outside the plane, the low-density medium provides only weak drag, and the BH return is governed by inefficient dynamical friction. In rare cases, clump–BH interactions can lead to a faster decay. Feedback processes lead to outflows, but do not significantly change the overall density of the CND midplane. However, with a spherically distributed BH feedback, a hot bubble is generated behind the secondary, which almost shuts off dynamical friction. We dub this phenomenon "wake evacuation." It leads to delays in the decay, possibly of $\sim 0.3\,\mathrm{Gyr}$. We discuss the non-trivial implications on the discovery space of the eLISA telescope. Our results suggest that the largest uncertainty in predicting 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 $\sim 10$ to $\sim 300\,\mathrm{Myr}$.

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