Supermassive black hole binaries in gaseous and stellar circumnuclear discs: orbital dynamics and gas accretion

Abstract

The dynamics of two massive black holes in a rotationally supported nuclear disc of mass Mdisc= 108 M⊙ is explored using N‐body/smoothed particle hydrodynamics simulations. Gas and star particles are copresent in the disc. Described by a Mestel profile, the disc has a vertical support provided by turbulence of the gas, and by stellar velocity dispersion. A primary black hole of mass 4 × 106 M⊙ is placed at the centre of the disc, while a secondary black hole is set initially on an eccentric corotating orbit in the disc plane. Its mass is in a 1:1, 1:4, and 1:10 ratio, relative to the primary. With this choice, we mimic the dynamics of black hole pairs released in the nuclear region at the end of a gas‐rich galaxy merger. It is found that, under the action of dynamical friction, the two black holes form a close binary in ∼10 Myr. The inspiral process is insensitive to the mass fraction in stars and gas present in the disc and is accompanied by the circularization of the orbit. We detail the gaseous mass profile bound to each black hole that can lead to the formation of two small Keplerian discs, weighing ≈2 per cent of the black hole mass, and of size ∼0.01 pc. The mass of the tightly (loosely) bound particles increases (decreases) with time as the black holes spiral into closer and closer orbits. Double active galactic nucleus activity is expected to occur on an estimated time‐scale of ≲10 Myr, comparable to the inspiral time‐scale. The double nuclear point‐like sources that may appear during dynamical evolution will have typical separations of ≲10 pc.

Nonlinear Dynamical Friction of a Circular-Orbit Perturber in a Gaseous Medium

“…Nevertheless, the idealized models considered in this paper help to isolate the effect of the perturber size or orbital radius on the nonlinear DF force. The results of this work will be particularly useful to justify large perturber sizes employed in recent hydrodynamic simulations, such as for SMBHs at galactic nuclei (e.g., Escala et al 2004Escala et al , 2005Dotti et al 2006Dotti et al , 2007Mayer et al 2007;Cuadra et al 2009) and companions in common-envelope binaries (e.g., Ruffert 1993;Sandquist et al 1998;Ricker & Taam 2008), etc. These simulations usually treat the perturber using a softened point mass, with its size inevitably limited by numerical resolution.…”

Evolution of binary black holes in self gravitating discs

“…Disc-like structures are actually observed in ULIRGs which are thought to be gas rich post-merger star-forming galaxies (e.g., Sanders & Mirabel 1996;Downes & Solomon 1998;Davies et al 2004a,b;Greve et al 2009). In the models, the two nuclear BHs efficiently spiral to sub-pc scales owing to dynamical friction against the massive circumnuclear disc (Dotti et al 2007), eventually opening a cavity (or hollow) in the gas distribution (Goldreich & Tremaine 1980). The subsequent evolution of the system is determined by the efficiency of energy and angular momentum transfer between the BHB and its outer circumbinary disc.…”

“…Additionally, it is still unclear how Nature bridges the gap between the two theoretically well understood stages of BHB evolution: (i) the dynamical friction driven stage, when the two BHs spiral in toward the centre of the merger remnant down to pc separations and (ii) the final inspiral driven by gravitational waves (GWs), which become efficient when the two BHs are at a separation < ∼ 10 −2 pc. Both dense stellar and gaseous environments have been shown to be effective in extracting the binary energy and angular momentum (see, e.g., Escala et al 2005;Dotti et al 2007;Cuadra et al 2009;Khan et al 2011;Preto et al 2011), likely driving the system to final coalescence (an extensive discussion on the fate of sub-parsec BHBs can be found in Dotti et al 2012). Scenarios involving cold gas are particular appealing not only A&A 545, A127 (2012) because they might produce distinctive observational signatures, but also because cold gas dominates the baryonic content in most galaxies at redshifts higher than one, providing a natural reservoir of energy and angular momentum to drive the BHB towards coalescence.…”

Multiband photometric decomposition of nuclear stellar disks

“…Each of these scenarios is likely to be correct for some, but not all, objects. In both of them, the gas is efficiently directed toward the galaxy center (e.g., Barnes & Hernquist 1996;Athanassoula et al 2005;Dotti et al 2007;Eliche-Moral et al 2009;Hopkins & Quataert 2010), where it first dissipates and settles onto an equilibrium plane and then forms into stars. A striking example of an on going process of dissipational formation is provided by NGC 4486A (Kormendy et al 2005).…”