  We investigate the bilayer Ruddlesden-Popper iridate Sr$_3$Ir$_2$O$_7$ by
temperature-dependent angle-resolved photoemission. We find a narrow-gap
correlated insulator, with spectral features indicative of a polaronic ground
state, strikingly similar to that observed previously for the parent compounds
of the cuprate superconductors. We additionally observe similar behaviour for
the single-layer cousin Sr$_2$IrO$_4$, indicating that strong electron-boson
coupling dominates the low-energy excitations of this exotic family of
materials, and providing a microscopic link between the insulating ground
states of the seemingly-disparate 3d cuprates and 5d iridates.
