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Context. The High Energy Stereoscopic System Galactic plane survey (HGPS) is to date the most comprehensive census of Galactic gamma -ray sources at very high energies (VHE; 100 GeV <= E <= 100 TeV). As a consequence of the limited sensitivity of this survey, the 78 detected gamma -ray sources comprise only a small and biased subsample of the overall population. The larger part consists of currently unresolved sources, which contribute to large-scale diffuse emission to a still uncertain amount.Aims. We study the VHE gamma -ray source population in the Milky Way. For this purpose population-synthesis models are derived based on the distributions of source positions, extents, and luminosities.Methods. Several azimuth-symmetric and spiral-arm models are compared for spatial source distribution. The luminosity and radius function of the population are derived from the source properties of the HGPS data set and are corrected for the sensitivity bias of the HGPS. Based on these models, VHE source populations are simulated and the subsets of sources detectable according to the HGPS are compared with HGPS sources.Results. The power-law indices of luminosity and radius functions are determined to range between -1.6 and -1.9 for luminosity and -1.1 and -1.6 for radius. A two-arm spiral structure with central bar is discarded as spatial distribution of VHE sources, while azimuth-symmetric distributions and a distribution following a four-arm spiral structure without bar describe the HGPS data reasonably well. The total number of Galactic VHE sources is predicted to be in the range from 800 to 7000 with a total luminosity and flux of (1.6-6.3) x 10(36) ph s(-1) and (3-15) x 10(-10) ph cm(-2) s(-1), respectively.Conclusions. Depending on the model, the HGPS sample accounts for (68-87)% of the emission of the population in the scanned region. This suggests that unresolved sources represent a critical component of the diffuse emission measurable in the HGPS. With the foreseen jump in sensitivity of the Cherenkov Telescope Array, the number of detectable sources is predicted to increase by a factor between 5 and 9.
The field of gamma-ray astronomy opened a new window into the non-thermal universe that allows studying the acceleration sites of cosmic rays and the role of cosmic rays on evolutionary processes in galaxies. The detection of almost one hundred Galactic very-high-energy (VHE: 0.1−100TeV) gamma-ray sources in the Milky Way demonstrates that particle acceleration up to tens of TeV energies is a common phenomenon. Furthermore, the detection of VHE gamma rays from other galaxies has confirmed that cosmic rays are not exclusively accelerated in the Milky Way. The rapid development of gamma-ray astronomy in the past two decades has led to a transition from the detection and study of individual sources to source population studies. To answer the question, whether the VHE gamma-ray source population of the Milky Way is unique, observations of galaxies, for which individual sources can be resolved, are required. Such galaxies are the Magellanic Clouds, two satellite galaxies of the Milky Way, which have been surveyed by the H.E.S.S. experiment in the last decade. In this thesis, data from a total of 450 hours of H.E.S.S. observations towards the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are presented. During the analysis of the data sets, special emphasis is put on the evaluation of systematic uncertainties of the experiment in order to assure an unbiased flux estimation of the potential VHE gamma-ray sources of the Magellanic Clouds. A detailed analysis of the survey data revealed the detection of the gamma-ray binary LMCP3, the most powerful gamma-ray binary known so far, that is located in the LMC, and thus, increases the number of known VHE gamma-ray sources in the LMC to four. No other VHE gamma-ray source is detected in the Magellanic Clouds and integral flux upper limits are estimated. These flux upper limits are used to perform a source population study based on known VHE source classes and existing multi-wavelength catalogues. A comparison of the source populations of the Magellanic Clouds and the Milky Way revealed that no other source in the Magellanic Clouds is as bright as the most luminous VHE gamma-ray source in the LMC: the pulsar wind nebula N 157B, and that one-third of the source population of the Magellanic Clouds is less luminous than the other known VHE gamma-ray sources in the LMC. For only a couple of sources luminosity levels of Galactic VHE sources, that are more than one order of magnitude fainter than the detected sources in the LMC, are constrained. Based on the flux upper limits, differences on the TeV source populations in the Magellanic Clouds and the Milky Way as well as the importance of the source environments will be discussed.