Effects of post-Newtonian spin alignment on the distribution of black-hole recoils

Abstract

Recent numerical relativity simulations have shown that the final black hole produced in a binary merger can recoil with a velocity as large as $5000\text{ }\text{ }\mathrm{km}/\mathrm{s}$. Because of enhanced gravitational-wave emission in the so-called ``hang-up'' configurations, this maximum recoil occurs when the black-hole spins are partially aligned with the orbital angular momentum. We revisit our previous statistical analysis of post-Newtonian evolutions of black-hole binaries in the light of these new findings. We demonstrate that despite these new configurations with enhanced recoil velocities, spin alignment during the post-Newtonian stage of the inspiral will still significantly suppress (or enhance) kick magnitudes when the initial spin of the more massive black hole is more (or less) closely aligned with the orbital angular momentum than that of the smaller hole. We present a preliminary study of how this post-Newtonian spin alignment affects the ejection probabilities of supermassive black holes from their host galaxies with astrophysically motivated mass ratio and initial spin distributions. We find that spin alignment suppresses (enhances) ejection probabilities by $\ensuremath{\sim}40%$ (20%) for an observationally motivated mass-dependent galactic escape velocity, and by an even greater amount for a constant escape velocity of $1000\text{ }\text{ }\mathrm{km}/\mathrm{s}$. Kick suppression is thus at least a factor two more efficient than enhancement.