  Strong electron interactions in solids increase effective mass, and shrink
the electronic bands [1]. One of the most unique and robust experimental facts
about iron-based superconductors [2-4] is the renormalization of the conduction
band by factor of 3 near the Fermi level [5-9]. Obviously related to
superconductivity, this unusual behaviour remains unexplained. Here, by
studying the momentum-resolved spectrum of the whole valence band in a
representative material, we show that this phenomenon originates from
electronic interaction on a much larger energy scale. We observe an abrupt
depletion of the spectral weight in the middle of the Fe $3d$ band, which is
accompanied by a drastic increase of the scattering rate. Remarkably, all
spectral anomalies including the low-energy renormalization can be explained by
coupling to excitations, strongly peaked at about 0.5 eV. Such high-energy
interaction distinguishes all unconventional superconductors from common
metals.
