An X-ray view of gas and dust in the diffuse interstellar medium

Understanding the chemistry of the interstellar medium (ISM) is fundamental for the comprehension of the Galaxy evolution. Some of the refractory interstellar elements, such as Si, Mg and Fe, are known to be locked up into dust grains. Iron is known to be highly depleted from the gas phase into solids, but the exact composition of the iron-bearing grains is not yet well understood. Oxygen is one of the most abundant elements in the Galaxy and important for life on Earth. Nevertheless, the exact reservoirs of oxygen in different interstellar environments still remain an open question.
The X-ray energy band provide an ideal way to solve these mysteries. X-ray radiation from astronomical X-ray sources can be absorbed by atoms and solids in the ISM, producing distinct absorption features. By studying these features, we can investigate the dust chemical composition, crystallinity and grain size in different density environments of the ISM, along with the different gas phases.
This thesis demonstrates our newly calculated dust extinction cross sections in the oxygen K and iron L-edges using laboratory measurements. The models have been further applied to the high-resolution X-ray absorption spectra of bright background sources along the galactic plane. This enabled us to unveil the dust mineralogy in the diffuse lines of sight and also to study the different gas phases of the ISM. Moreover, a smaller fraction of this thesis is focused on the nature and structure of the photoionised gas detected in the vicinity of X-ray binaries.