Evolution of binary black holes in self gravitating discs: dissecting the torques
by Roedig, Constanze and Sesana, Alberto and Dotti, Massimo and Cuadra, Jorge and Amaro-Seoane, Pau and Haardt, Francesco
14 pages, 12 Figures, submitted to MNRAS
We analyse 3D SPH simulations of the evolution of initially quasi-circular massive black hole binaries (BHBs) residing in the central hollow (cavity) of self-gravitating circumbinary discs. We perform a set of simulations adopting different thermodynamics for the gas within the cavity and for the ‘numerical size’ of the black holes. We study the interplay between gas accretion and gravity torques in changing the binary elements (semi-major axis and eccentricity) and its total angular momentum budget. We pay special attention to the gravity torques, by analysing their physical origin and location. We show that (i) the BHB eccentricity grows due to gravity torques from the inner edge of the disc, independently of the accretion and the adopted thermodynamics; (ii) the semi-major axis decay depends not only on the gravity torques but also on their subtle interplay with the disc-binary angular momentum transfer due to accretion; (iii) the spectral structure of the gravity torques is predominately caused by disc edge overdensities and spiral arms developing in the body of the disc; (iv) the net gravity torque changes sign across the BHB corotation radius: gas inside this radius exerts a net positive torque, while streams located outside this radius (but within the cavity) exert a net negative torque. The relative importance of the two might depend on the thermodynamical properties of the instreaming gas and is crucial in assessing the disc–binary angular momentum transfer; (v) the net torque manifests as a purely kinematic effect as it stems from the low density cavity, where the material flows in and out in highly eccentric orbits. Thus both accretion onto the black holes and the interaction with gas streams inside the cavity must be taken into account to assess the fate of the binary.