Galactic diffuse gamma-ray emission at very high energies

Diffuse Galactic γ-ray emission is the most prominent observable signature of celestial cosmic-ray interactions at high energies. While being studied at GeV energies for several decades, assessments of diffuse γ-ray emission at TeV energies is complicated by the low fluxes and the dominance of γ-ray sources in this energy regime. Measurements have so far been exclusively performed by ground-based particle detector arrays with their large fields of view. Imaging atmospheric Cherenkov telescopes with their superior angular resolution are hampered by few-degree-sized fields of view, which challenge the data collection on large scales and in particular the determination of the charged-cosmic-ray background, customarily done from within the field of view. The systematic survey of the central Galaxy performed with the High Energy Stereoscopic System (H.E.S.S.) is used in this work for the first measurement of large-scale diffuse γ-ray emission along the Galactic Plane using imaging atmospheric Cherenkov telescopes. The challenge of the charged-cosmic-ray background is met by two different approaches: On the one hand a standard determination from within the field of view demonstrates that TeV diffuse emission is measurable by imaging atmospheric Cherenkov telescopes but results in distortions of the measurable signal. On the other hand a modelling of the background using Monte Carlo simulations enables an absolute measurement essentially unaffected by background-subtraction issues. Applying this novel technique the first high-resolution (<0.1◦) measurement of Galactic diffuse γ-ray emission is presented. The other complication of TeV diffuse-emission measurements is the entanglement of γ-ray sources and diffuse emission. Unresolved sources below the detection threshold of the instrument form part of the measurable diffuse signal and discrete sources shape the distribution of the truly diffuse emission stemming from propagating cosmic rays. In this work, special focus is placed on this interplay. Concepts for the inclusion of γ-ray sources in the analysis and interpretation of TeV diffuse emission are developed and applied to the H.E.S.S. measurement: A population synthesis is invoked to characterise the overall properties of the Galactic TeV γ-ray source population. It is based on the strongly biased H.E.S.S. Galactic γ-ray source catalogue, requiring the inclusion of the complex details of the selection bias of the H.E.S.S. data set. Simulated source populations are then used for the derivation of the amount of unresolved γ-ray sources in the measurable diffuse signal and as input for the predictions of the diffuse emission stemming from cosmic-ray propagation. With the development of concepts and techniques for the measurement and interpretation of diffuse γ-ray emission at TeV energies, this work does not aim at providing final answers on this subject. It is meant to lay the foundation for a more detailed understanding of cosmic-ray transport at TeV energies based on high-resolution H.E.S.S. and future Cherenkov Telescope Array Observatory (CTAO) measurements.