Simulations of the effect of black hole recoil on surrounding gas disks

General Relativistic simulations show that the final coalescence of black hole binaries typically results in a kick or recoil of the merged black hole away from the center of mass of the pre-merger binary. The kick - which arises because gravitational waves carry away linear momentum - can have a magnitude of several thousands of km / s, although smaller values of the order of a few hundred km / s may be more typical. These kicks are of particular interest for supermassive black hole binaries that merge within gaseous disks. The kick will perturb the surrounding gas, and this can result in an electromagnetic signature of the merger process. Electromagnetic counterparts - if they exist - would be important for a variety of reasons, not least since they would allow localization of mergers that could otherwise be detected only via their gravitational wave emission. The animations on this page show how a geometrically thin circumbinary accretion disk responds to a kick following the merger of a supermassive black hole binary.

Kick perpendicular to the disk plane

Visualization of the response of a surrounding gas disk to a black hole kick that is perpendicular to the disk plane. The problem is axisymmetric, so the movie depicts gas density (log scale) in the (r,z) plane of cylindrical polar co-ordinates, in the frame of the black hole. The kick unbinds gas in the outer disk, and perturbs gas at smaller radii. The perturbed gas oscillates before settling back into the original disk plane.

Computed using ZEUS. Click on the image for a movie in mpeg format.

Kick directed into the disk plane

Response of the gas disk when the kick is directed directly into the disk plane. The movie shows gas density in the (r,phi) plane of cylindrical polar co-ordinates. For clarity, the disk shown in the movie had a constant surface density prior to the kick. In this geometry, some of the gas that remains bound to the black hole suffers a large reduction in its specific angular momentum as a result of the recoil. Subsequent infall of this gas provides the dominant source of energy release.

Computed using ZEUS. Click on the image for a movie in mpeg format. Note that the color scheme artificially emphasizes non-axisymmetric structure.

Kick at an arbitrary angle (here 60 degrees) to the disk plane

The response of the gas disk to a kick directed at an angle of 60 degrees from the disk plane. Unlike the two cases shown above this is a fully three-dimensional problem, but it shares many of the features of the special cases. In particular, accretion is very important for kicks directed close to the disk plane, but less important for kicks that are close to perpendicular.

Computed using SPH, and visualized using SPLASH. Click on the image to view a movie in Quicktime format.

Simulations and visualization by: Elena Rossi, Phil Armitage and Giuseppe Lodato
The simulations, and related analytic calculations of the observability of electromagnetic counterparts to black hole mergers, are described more fully in the paper: Black hole mergers: the first light, Elena M. Rossi, G. Lodato, P.J. Armitage, J.E. Pringle and A.R. King, MNRAS, 401, 2021 (2010)