Apparent shape of super-spinning black holes

We consider the possibility that astrophysical black holes (BHs) can violate the Kerr bound; i.e., they can have angular momentum greater than BH mass, J > M. We discuss implications on the BH apparent shape. Even if the bound is violated by a small amount, the shadow cast by the BH changes significantly (it is similar to an order of magnitude smaller) from the case with J <= M and can be used as a clear observational signature in the search for super-spinning BHs. We discuss briefly recent observations in the mm range of the supermassive BH at the center of the Galaxy, speculating on the possibility that it might violate the Kerr bound.