Diffuse TeV gamma-ray emission in the H.E.S.S. Galactic plane survey

Diffuse γ-ray emission has long been established as the most prominent feature in the GeV sky. Although the imaging atmospheric Cherenkov technique has been successful in revealing a large population of discrete TeV γ-ray sources, a thorough investigation of diffuse emission at TeV energies is still pending. Data from the Galactic Plane Survey (GPS) obtained by the High Energy Stereoscopic System (H.E.S.S.) have now achieved a sensitivity and coverage adequate for probing signatures of diffuse emission in the energy range of ∼100 GeV to a few TeV. γ-rays are produced in cosmic-ray interactions with the interstellar medium (aka “sea of cosmic rays”) and in inverse Compton scattering on cosmic photon fields. This inevitably leads to guaranteed γ-ray emission related to the gas content along the line-of-sight. Further contributions relate to those γ-ray sources that fall below the current detection threshold and the aforementioned inverse Compton emission. Based on the H.E.S.S. GPS, we present the first observational assessment of diffuse TeV γ-ray emission. The observation is compared with corresponding flux predictions based on the HI (LAB data) and CO (as a tracer of H₂, NANTEN data) gas distributions. Consequences for unresolved source contributions and the anticipated level of inverse Compton emission are discussed.