Core Formation in Galactic Nuclei due to Recoiling Black Holes

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Abstract
Anisotropic gravitational radiation from a coalescing black hole binary can impart a recoil velocity of up to several hundred kilometers per second to the remnant black hole. We examine the effects of recoiling massive black holes on their host stellar bulges, both for holes that escape their host and for those that return to the galactic center via dynamical friction. We show that removal of a black hole via radiation recoil generally results in a rapidly formed central core in the stellar system, with the effect being largest when the hole stays bound to the bulge and the recoil velocity is comparable to the bulge velocity dispersion. Black hole recoil therefore provides a mechanism for producing cores in some early-type galaxies, but it is expected to be most efficient in faint elliptical galaxies that are known to have steep density profiles. We argue that these results may hint at a significant role for gas in facilitating the coalescence of binary black holes in faint (power-law) early-type galaxies.