Can black hole superradiance be induced by galactic plasmas?

Highly spinning Kerr black holes with masses M=1-100M⊙ are subject to an efficient superradiant instability in the presence of bosons with masses μ∼10−10-10−12eV. We observe that this matches the effective plasma-induced photon mass in diffuse galactic or intracluster environments (ωpl∼10−10-10−12eV). This suggests that bare Kerr black holes within galactic or intracluster environments, possibly even including the ones produced in recently observed gravitational wave events, are unstable to formation of a photon cloud that may contain a significant fraction of the mass of the original black hole. At maximal efficiency, the instability timescale for a massive vector is milliseconds, potentially leading to a transient rate of energy extraction from a black hole in principle as large as ∼1055ergs−1. We discuss possible astrophysical effects this could give rise to, including a speculative connection to Fast Radio Bursts.