Recoiling super-massive black holes, a search in the nearby universe

Davide Lena

September 03, 2013

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Abstract: Anisotropic emission of gravitational waves during the coalescence of supermassive black hole binaries can deliver a large “kick” velocity to the new BH (v~10^3 km/s). N-body simulations predict that, for kick velocities larger than 40% of the galactic escape velocity, the BH may experience long lived (few Gyr) oscillations with amplitudes comparable with the size of the galactic core (~10^2 pc), suggesting that BHs offset from the galactic centers may be common, even in nearby ellipticals. In order to search for such offsets, we perform a hotometric analysis of a sample of “core” elliptical galaxies using HST observations. Typical recovered offsets are within 1% of the core radius. In contrast, statistical arguments based on N-body simulations show that the typical probability to observe an offset larger than 0.1 core radii (~10 pc) is of order of 70% for a single galaxy. This is derived under the assumption that the BH binary coalescence produced a moderately large kick (v ~ 250 km/s) at the time of the last merger. Assuming that all galaxies in the sample experienced at least one such merger during their lifetime, the probability to observe no offsets larger than 1% the core radius over the sample is of order of 10^-8.