The stellar kinematic signature of massive black hole binaries

The stalling radius of a merging massive binary black hole (BBH) is expected to be below 0.1 arcsec even in nearby galaxies, and thus BBHs are not expected to be spatially resolved in the near future. However, as we show below, a BBH may be detectable through the significantly anisotropic stellar velocity distribution it produces on scales five to 10 times larger than the binary separation. We calculate the velocity distribution of stable orbits near a BBH by solving the restricted three-body problem for a BBH embedded in a bulge potential. We present high-resolution maps of the projected velocity distribution moments, based on snapshots of similar to 10(8) stable orbits. The kinematic signature of a BBH in the average velocity maps is a counter rotating torus of stars outside the BBH Hill spheres. The velocity dispersion maps reveal a dip in the inner region, and an excess of 20-40 per cent further out, compared to a single black hole of the same total mass. More pronounced signatures are seen in the third and fourth Gauss-Hermite velocity moments maps. The detection of these signatures may indicate the presence of a BBH currently, or at some earlier time, which depends on the rate of velocity phase-space mixing following the BBH merger.