Late-time symmetry near black hole horizons
  It is expected that black holes are formed dynamically under the gravitational collapses and approach to the stationary states. In this paper, we show that the asymptotic Killing vector at late time should exist on the horizon and then that it can be extended to the out of black holes under the assumption of the analyticity of spacetimes. This fact implies that if there is another asymptotic Killing vector which becomes stationary Killing at far region and spacelike in "ergoregion", the rotating black holes may have the asymptotically axisymmetric Killing vector at late time. Thus, we may expect that the asymptotic rigidity of the black holes holds. 
Kentaro Tanabe, Tetsuya Shiromizu, Shunichiro Kinoshita
gr-qc astro-ph.CO