  Massive bosonic fields in the background of a Kerr black hole can either
trigger superradiant instabilities (black-hole bombs) or form equilibrium
configurations corresponding to pure bound states, known as stationary scalar
clouds. Here, similar phenomena are shown to emerge in the fluctuation dynamics
of a rotating photon-fluid model. In the presence of suitable vortex flows, the
density fluctuations are governed by the massive Klein-Gordon equation on a
(2+1) curved spacetime, possessing an ergoregion and an event horizon. We
report on superradiant instabilities originating from quasi-bound phonon states
trapped by the vortex background and, remarkably, on the existence of
stationary modes in synchronous rotation with the horizon. These represent the
acoustic counterpart of astrophysical scalar clouds. Our system offers a
promising platform for analogue gravity experiments on superradiant
instabilities of massive bosons and black-hole-field equilibrium
configurations.
