Dr Nader Khonji

Binaries of supermassive black holes [massive black hole binaries (MBHBs)] represent the primary sources of the gravitational wave background (GWB) detectable by pulsar timing arrays (PTAs). The eccentricity with which binaries form in galactic mergers is the key parameter determining their evolutionary timescale from pairing to coalescence. Ho we ver, accurately determining the binary eccentricity at formation is difficult in simulations due to stochastic effects. We present a numerical study of the formation and evolution of MBHBs that are potential PTA sources. We simulate mergers of equal-mass galaxies on different initial orbits and follow the dynamics of the MBHBs through the hardening phase. We find that low-resolution simulations are affected by stochasticity due to torques from the stellar distribution acting at pericentre passages. The dispersion in binary eccentricity decreases with increasing central resolution, as expected for a Poisson process. We provide a fitting formula for the resolution requirement of an N-body simulation of MBHB formation and evolution as a function of the initial eccentricity of the merger, e 0 , and the required accuracy in the binary eccentricity, e b. We find that binaries experience a torque at first pericentre that is approximately independent of initial eccentricity, producing a general trend in which the binary eccentricity decreases abo v e sufficiently large initial orbital eccentricities. While this behaviour is generic, the precise cross-o v er eccentricity (e 0 ∼ 0. 97 in our models) and the sharpness of the drop-off depend on the galaxy initial conditions. We provide a fitting formula for e b (e 0) that can be used in semi-analytical models to determine the merger timescales of MBHBs as well as the amplitude and slope of the GWB.

It has long been thought that nuclear star clusters (NSCs) cannot coexist with the most massive supermassive black holes (SMBHs), since SMBH mergers—unavoidable for the most massive systems—would scatter away NSC stars. However, central concentrations of light have now been reported in up to one-third of all massive ellipticals. We present a new mechanism for forming NSCs in giant elliptical galaxies, arising naturally from SMBH mergers, which could explain these observations. We call this “black hole dragging.” After a major merger of two galaxies and their SMBHs, the newly merged SMBH can receive a gravitational-wave recoil kick. We show that recoiling SMBHs induce two competing effects on the galaxy’s background stars. First, some stars become bound to the SMBH and comove with it, an effect strongest at low recoil velocities. Second, background stars are ejected as the recoiling SMBH falls back due to dynamical friction, an effect strongest at high recoil velocities. At intermediate recoil velocities (500–1000 km s−1), both effects become important, and the density of bound stars can exceed that of the background stellar core. This yields a central dense NSC that is clearly visible in the galaxy’s surface brightness profile. We show that NSCs formed in this way have realistic sizes, masses, and velocity dispersions when measured similarly to observed systems. This provides a route for even giant ellipticals containing SMBHs to host an NSC. We predict such NSCs should have indistinguishable colors, ages, and chemistry from non-NSC central stars, combined with low ellipticities.