  Hypothetical ultralight bosonic fields will spontaneously form macroscopic
bosonic halos around Kerr black holes, via superradiance, transferring part of
the mass and angular momentum of the black hole into the halo. Such process,
however, is only efficient if resonant: when the Compton wavelength of the
field approximately matches the gravitational scale of the black hole. For a
complex-valued field, the process can form a stationary, bosonic field-black
hole equilibrium state - a black hole with synchronised hair. For sufficiently
massive black holes, such as the one at the centre of the M87 supergiant
elliptic galaxy, the hairy black hole can be robust against its own
superradiant instabilities, within a Hubble time. Studying the shadows of such
scalar hairy black holes, we constrain the amount of hair which is compatible
with the Event Horizon Telescope (EHT) observations of the M87 supermassive
black hole, assuming the hair is a condensate of ultralight scalar particles of
mass $\mu\sim 10^{-20}$ eV, as to be dynamically viable. We show the EHT
observations set a weak constraint, in the sense that typical hairy black holes
that could develop their hair dynamically, are compatible with the
observations, when taking into account the EHT error bars and the black hole
mass/distance uncertainty.
