Can black hole superradiance be induced by galactic plasmas?
  Highly spinning Kerr black holes with masses $M = 1 - 100\ M_{\odot}$ are
subject to an efficient superradiant instability in the presence of bosons with
masses $\mu \sim 10^{-10} - 10^{-12}\ {\rm eV}$. We observe that this matches
the effective plasma-induced photon mass in diffuse galactic or intracluster
environments ($\omega_{\rm pl} \sim 10^{-10} - 10^{-12}\ {\rm eV}$). 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 $\sim 10^{55} \ {\rm erg \, s}^{-1}$. We
discuss possible astrophysical effects this could give rise to, including a
speculative connection to Fast Radio Bursts.
