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Binary black hole mergers: large kicks for generic spin orientations

Wolfgang Tichy, Pedro Marronetti

TL;DR

This work addresses whether large gravitational recoil kicks are a generic outcome of spinning binary black hole mergers. It uses numerical relativity with the moving-punctures approach on the BAM code to simulate equal-mass binaries with spin magnitude $S/m^2=0.8$ across arbitrary orientations, aiming to sample the kick velocity distribution. The authors find that kicks typically exceed $1000$ km s$^{-1}$, with a maximum near $2500$ km s$^{-1}$ when spins are anti-aligned in the orbital plane, and that most of the kick is accumulated during the final merger; higher spins would yield larger kicks. These results imply that merged black holes could routinely escape from a wide range of galaxy potentials, impacting models of supermassive and intermediate-mass black hole growth, though further parameter studies (including mass ratio variations) are needed for generalization.

Abstract

We present results from several simulations of equal mass black holes with spin. The spin magnitudes are $S/m^2=0.8$ in all cases, but we vary the spin orientations arbitrarily, in and outside the orbital plane. We find that in all but one case the final merged black hole acquires a kick of more than 1000 km/s, indicating that kicks of this magnitude are likely to be generic and should be expected for mergers with general spin orientations. The maximum kick velocity we find is 2500 km/s and occurs for initial spins which are anti-aligned in the initial orbital plane.

Binary black hole mergers: large kicks for generic spin orientations

TL;DR

This work addresses whether large gravitational recoil kicks are a generic outcome of spinning binary black hole mergers. It uses numerical relativity with the moving-punctures approach on the BAM code to simulate equal-mass binaries with spin magnitude across arbitrary orientations, aiming to sample the kick velocity distribution. The authors find that kicks typically exceed km s, with a maximum near km s when spins are anti-aligned in the orbital plane, and that most of the kick is accumulated during the final merger; higher spins would yield larger kicks. These results imply that merged black holes could routinely escape from a wide range of galaxy potentials, impacting models of supermassive and intermediate-mass black hole growth, though further parameter studies (including mass ratio variations) are needed for generalization.

Abstract

We present results from several simulations of equal mass black holes with spin. The spin magnitudes are in all cases, but we vary the spin orientations arbitrarily, in and outside the orbital plane. We find that in all but one case the final merged black hole acquires a kick of more than 1000 km/s, indicating that kicks of this magnitude are likely to be generic and should be expected for mergers with general spin orientations. The maximum kick velocity we find is 2500 km/s and occurs for initial spins which are anti-aligned in the initial orbital plane.

Paper Structure

This paper contains 4 sections, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. Numerical Techniques
  3. Results
  4. Discussion

Figures (3)

  • Figure 1: This plot shows the energy radiated during the merger of a binary where both initial spins ($S/m^2 =0.8$) are anti-aligned to the orbital angular momentum. The upper panel shows the energy radiated for the resolutions $h_1=M/56.9$, $h_2=M/61.6$, $h_3=M/66.4$. In the lower panel we show the differences scaled for fourth order convergence.
  • Figure 2: The z-component of the radiated momentum for the -1,135/-1,-135 run. The initial spins are perpendicular to each other and lie in the initial orbital plane. This plot shows that on our relatively small grids the location of the extraction radius plays a big role and is the main contributor to the error of $\pm$20% quoted here.
  • Figure 3: This plot shows the tracks of the black hole centers for the -1,180/89,180 run. Here one of the initial spins is in the initial orbital plane, and the other one is perpendicular to the orbital plane. One can see that the orbits do not stay in one plane.