The Pairing of Accreting Massive Black Holes in Multiphase Circumnuclear Disks: the Interplay between Radiative Cooling, Star Formation, and Feedback Processes

TL;DR

This study uses hydrodynamical simulations to explore how radiative cooling, star formation, and feedback processes influence the orbital decay of massive black hole pairs in circumnuclear disks, revealing complex dynamics and significant delays.

Contribution

It introduces detailed simulations of black hole pair decay considering multiple physical processes, highlighting the impact of feedback and fragmentation on decay timescales.

Findings

Fragmentation causes stochastic torques and potential ejection of the secondary BH.

Feedback can generate hot bubbles that delay decay by shutting off dynamical friction.

Decay timescales vary widely from 10 to 300 million years depending on galaxy conditions.

Abstract

We study the orbital decay of a pair of massive black holes (BHs) with masses 5 * 10^5 and 10^7 M_sun, 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 feedback. In the absence of such processes, the orbit of the secondary BH decays over timescales of ~10 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 and with densities in the range 10^4 - 10^7 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 change significantly 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, a phenomenon we dub "wake evacuation", leading to delays in the decay of possibly ~0.3 Gyr. We discuss the non-trivial implications on the discovery space of the eLISA telescope. Our results suggest 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 ~10 to ~300 Myr.