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

Abstract

We study the orbital decay of a pair of massive black holes (BHs) with masses 5×10 5 and 10 7 M ⊙ , 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.

The birth of a supermassive black hole binary

“…The clumps' mean gas density distribution peaks at ∼ 10 2 particles cm −3 , which is in very good agreement with the typical densities of giant molecular clouds (McKee 1999), with only a small tail at higher densities (the mass fraction in gas with density > 10 4 particles cm −3 is < 10%). The gas density is always below the "effective density of the SMBH", defined as that the SMBH mass would have if spread over a sphere with radius the softening length, therefore we are not affected by spurious motions (del Valle et al 2015;Souza Lima et al 2017). Our simulation is also unaffected by an over-estimate of stochastic gravitational interactions with over-dense gas clumps (del Valle et al 2015;Souza Lima et al 2017).…”

“…This would lead, if those clumps are massive enough, to SMBH-SMBH-Clump+background interactions instead of simple SMBH-SMBH+background interactions. Also, the trajectory and the orbit of SMBHs could be strongly affected (Fi- acconi et al 2013; Lupi et al 2015;Souza Lima et al 2017;Tamburello et al 2017). This is discussed in Section 4.2.…”

Dynamical Evolution and Merger Timescales of LISA Massive Black Hole Binaries in Disk Galaxy Mergers

“…In the last decade there has been considerable effort in modelling the orbital decay phases of massive BH pairs in galaxy mergers, using predominantly either numerical simulations that follow the BH binary to very small separations but capture only the gravitational dynamics of the stellar and dark matter components (Milosavljević & Merritt 2001;Berczik et al 2006;Khan et al 2011), or simulations the include the interaction with the gaseous interstellar medium (ISM) but normally cannot follow the decay process beyond pc scales (Escala et al 2005;Dotti et al 2006Dotti et al , 2007Mayer et al 2007;Callegari et al 2009;Chapon et al 2013;Souza Lima et al 2017). Achieving high enough resolution to model the hard-binary phase in simulations of galaxy mergers that include also hydrodynamics in the galaxy merger phase has been first attempted by Khan et al (2012b) but in a limited form.…”

Growth problems of stellar black holes in early galaxies

“…Lupi et al (2016) state that the large reservoir of dense cold gas in a clumpy medium allows super-Eddington accretion via slim-disk evolution. Fiacconi et al (2013), Tamburello et al (2017), Souza Lima et al (2017) studied the orbital decay of massive BH pairs in a clumpy circumnuclear disk, important to predict BH dynamics in galaxy mergers remnants. Indeed, our study neglects that the density field of galaxies can be affected by galaxy mergers; for a comprehensive review of the evolution of galaxy structure from first galaxies to the local universe see Conselice (2014).…”