Evolution of supermassive black hole binaries and tidal disruption rates in non-spherical galactic nuclei

Abstract

Binary supermassive black holes (SMBH) are expected to form naturally during galaxy mergers. After the dynamical friction phase, when the two SMBHs become gravitationally bound to each other, and a brief stage of initial rapid hardening, the orbit gradually continues to shrink due to three-body interactions with stars that enter the loss cone of the binary. Using the stellar-dynamical Monte Carlo code RAGA, we explore the co-evolution of the binary SMBH and the nuclear star cluster in this slow stage, for various combinations of parameters (geometry of the star cluster, primary/secondary SMBH mass, initial eccentricity, inclusion of stellar captures/tidal disruptions). We compare the rates of stellar captures in galactic nuclei containing a binary to those of galaxies with a single SMBH. At early times, the rates are substantially higher in the case of a binary SMBH, but subsequently they drop to lower levels. Only in triaxial systems both the binary hardening rates and the capture rates remain sufficiently high during the entire evolution. We find that the hardening rate is not influenced by star captures, nor does it depend on eccentricity; however, it is higher when the difference between the black hole masses is greater. We confirm that the eccentricity of the binary tends to grow, which may significantly shorten the coalescence time due to earlier onset of gravitational-wave emission. We also explore the properties of the orbits entering the loss cone, and demonstrate that it remains partially full throughout the evolution in the triaxial case, but significantly depleted in the axisymmetric case. Finally, we study the distribution of ejected hypervelocity stars and the corresponding mass deficits in the central parts of the galaxies hosting a binary, and argue that the missing mass is difficult to quantify observationally.

Galaxy Core Formation by Supermassive Black Hole Binaries: the Importance of Realistic Initial Conditions and Galaxy Morphology

“…It is clear from this analysis that mass deficits in triaxial galaxies are smaller than in the spherical geometry, since most stars ejected by the binary are from orbits with very large characteristic radii. The fact that for triaxial systems the hardening of a massive binary is almost entirely driven by draining of centrophilic orbits that arrive from outside the SMBH binary sphere of influence was already noted before (e.g., Vasiliev 2015;Lezhnin & Vasiliev 2019). Here, we have shown that the morphology of a galaxy has a direct impact on the evolution of its nuclear density profile.…”

Tidal disruption event rates in galaxy merger remnants

“…More sophisticated techniques, beyond the scope of this paper, can be used for binary BHs (e.g. Lezhnin & Vasiliev 2019), which often result in an increased rate, at least for a short time (e.g. Chen et al 2009Chen et al , 2011Li et al 2017).…”

Inward Bound: The incredible journey of massive black holes as they pair and merge; I. The effect of mass ratio in flattened rotating galactic nuclei

“…Once the binary forms and hardens, further shrinking the SMBH binary's orbit requires few body interactions that transfer energy and angular momentum to the scattered stars. This few-body scattering phase has been rigorously modeled using a broad range of techniques and underlying assumptions; regardless of the technique, the SMBH binary is either invoked in equilibrium galaxy models (Makino & Funato 2004;Berczik et al 2005Berczik et al , 2006Merritt 2006;Khan et al 2013;Vasiliev et al 2015;Bortolas et al 2016;Gualandris et al 2017;Lezhnin & Vasiliev 2019) or allowed to form self-consistently through galaxy merger simulations (Milosavljević & Merritt 2001;Khan et al 2011;Preto et al 2011;Gualandris & Merritt 2012;Khan et al 2012aKhan et al ,b, 2016Rantala et al 2017;Khan et al 2018a,b). After two decades of work, a converging scenario has emerged: SMBH binaries in the few-body scattering phase continue to shrink at an almost constant rate to a separation where coalescence via gravitational wave emission is inevitable.…”