8 Interaction of Binary Black Holes with Gas

Interstellar gas might play an important role in the dynamical evolution of a binary SBH. Interactions with gas complement interactions with the stellar environment (Section 4) and with other SBHs (Section 5). Any gas situated close to a binary is disturbed by the SBHs and exerts gravitational force on them, thereby affecting their orbit. Furthermore, if SBH coalescence is accompanied by the presence of gas, an observable electromagnetic afterglow might follow coalescence.

The collisional, dissipative nature of interstellar gas gives rise to a behavior fundamentally different from that of the point-mass dynamics of stellar systems. It is natural to distinguish between two classes of flows in dynamical systems containing gas. In hot flows the gas temperature is comparable to the virial temperature of the system, while in cold flows the gas temperature is significantly below the virial temperature. The virial temperature can be defined as $Tvir = GM12mmp/2ak$ , where $m$ is the mean particle mass in units of the proton mass $mp$ , and $k$ is the Boltzmann constant. The prototype of a hot flow is the spherical, “Bondi” accretion onto a single black hole, in which the accreting gas is supported by pressure against free infall toward the accretor. The prototype of a cold flow is a thin disk, in which the gas is rotationally supported against infall. Even in hot flows rotational support is realized close to the accretor when the gas has nonzero net angular momentum (e.g. [109 ]).

The angular momentum barrier is central to SBH formation theories. Any model for how material is channeled into an accreting black hole must describe the mechanism by which angular momentum is removed from the material. Whatever this mechanism may be, it is expected that it operates universally during the epoch in which SBHs grew to their present masses by rapidly accreting material onto pre-existing black hole “seeds”. This is also the period when galaxy merging peaks [97 , 83 , 222 , 216 ]. While still elusive to astronomical probes due to severe obscuration [190 ], the nuclei of merging galaxies, which are also the sites for the formation of binary SBHs [18 ], are expected to contain the largest concentration of dense gas anywhere in the universe. The inevitable abundance of gas motivates an inquiry into the role of gas dynamics as an alternative to stellar dynamics in the process of SBH coalescence. Some of the mechanisms that remove angular momentum of interstellar gas and thus channel it into the neighborhood of SBHs include the torquing of gas flow by the rapidly-fluctuating potential of merging galaxies [144 ] and by nested stellar bars [199 ], angular momentum transport by hydrodynamical turbulence that might be driven by the onset of self-gravity [198 , 60 , 73 ] or by supernovae embedded within a large-scale toroidal circumnuclear flow [218 ], angular momentum extraction by magnetohydrodynamical turbulence [11 ] or by magnetic braking [22 ], and more speculatively, by Rossby vortex instabilities [118 ].

Astronomical observations offer abundant evidence for both hot and cold gas flows in the immediate vicinity of SBH candidates. The origin and the dynamical impact of the two classes of gas flow are distinct and are discussed here separately.

8.1 Interaction with hot gas
8.2 Interaction with cold gas