The role of X-ray and extreme-UV emission in shaping the interstellar medium: Insights from high-mass X-ray binaries and ultraluminous X-ray sources

The work presented in this thesis addresses the origin of high-energy ionising photons in low-metallicity galaxies, with the broader aim of understanding the sources that drive nebular He ii emission. This emission, widely observed in extremely metal-poor galaxies, requires photons with energies above 54.4 eV, yet the production of such hard radiation by standard stellar populations remains insufficient. Because direct observations of the extreme-ultraviolet (EUV) regime are impossible due to absorption, indirect diagnostics are required. Here, I combine stellar atmosphere models, photoionisation simulations, and multi-wavelength observations to examine how massive stars and high-mass X-ray binaries (HMXBs) contribute to the ionising budget, and how their presence can be identified from nebular emission lines. The first part of the thesis evaluates the impact of X-rays from O- and B-type stars. Using detailed stellar and nebular models, I test whether their intrinsic X-ray emission modifies the observable nebular spectrum. While soft X-rays are shown to raise the nebular temperature and shift the ionisation balance, they do not produce unique spectral features. In practice, X-rays from single OB stars are undetectable in the emissionline spectra of classical Hii regions, and their importance is limited to the cumulative contribution of large stellar populations. The second part focuses on HMXBs and ultraluminous X-ray sources (ULXs), where accretion onto compact objects generates luminous X-ray and EUV continua. Through systematic modelling, I show that the donor star and the accretion luminosity jointly determine the ionisation structure, but in different regimes. At low X-ray luminosity, donor type is still reflected in the nebula, with WR stars producing the hardest spectra. At extremelly high X-ray luminosities, by contrast, the nebula is completely X-ray dominated: stellar signatures vanish, while high-ionisation lines and strong He ii emission become characteristic features. One of the main findings of this work is that integrated line ratios are not always sufficient to separate these scenarios, as different combinations of donors and luminosities can produce degenerate results. To overcome this, I explore a method based on projected surface brightness profiles, which provides information about the radia structure of nebular lines. These profiles expose systematic differences between models that appear indistinguishable in integrated spectra. Importantly, the spatial scales of these differences are well matched to the resolving power of IFU spectrographs such as MUSE and MEGARA, meaning that this technique can be applied to Galactic and Local Group binaries today. In the final part of the thesis, I apply this approach to the case of Holmberg ii X-1, a prototypical ULX surrounded by a large ionised nebula. By combining archival X-ray and optical data with new UV spectroscopy, I show that the observed nebular spectrum can be reproduced with the models developed in this thesis, without invoking strong winds from the accretion disc. The UV spectrum further identifies the donor as a B supergiant and implies a black hole mass exceeding 60 𝑀•. These results highlight the utility of nebular analysis in constraining both the unobservable EUV spectrum and the physical nature of the binary. This thesis therefore provides a coherent framework for using nebular gas as a diagnostic of high-energy sources. It demonstrates that OB-star X-rays are negligible at the scale of individual Hii regions, while HMXBs and ULXs imprint clear nebular signatures, both in line ratios and in spatial profiles. These findings have direct implications for interpreting He ii emission in low-metallicity galaxies, and establish a pathway to test whether ULXs are indeed the missing contributors to the high-energy ionising background in the early Universe.