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

EHT constraint on the ultralight scalar hair of the M87 supermassive black hole