  Electron tunneling is associated with light emission. In order to elucidate
its generating mechanism, we provide a novel experimental ansatz that employs
fixed-distance epitaxial graphene as metallic electrodes. In contrast to
previous experiments, this permits an unobscured light spread from the tunnel
junction, enabling both a reliable calibration of the visible to infrared
emission spectrum and a detailed analysis of the dependence of the parameters
involved. In an open, non-resonant geometry, the emitted light is perfectly
characterized by a Planck spectrum. In an electromagnetically resonant
environment, resonant radiation is added to the thermal spectrum, both being
strictly proportional in intensity. In full agreement with a simple heat
conduction model, we provide evidence that in both cases the light emission
stems from a hot electronic subsystem in interaction with its linear
electromagnetic environment. These very clear results should resolve any
ambiguity about the mechanism of light emission in nano contacts.
