Small-scale studies of the Milky Way disc and halo gas with absorption-line spectroscopy

Abstract

Progressive studies in the last decades have shown that the interstellar medium (ISM) is more complex and dynamic than previously thought. The presence of the small-scale structures, structures on scales below 1 pc, has required a deeper understanding of the ISM and the physical processes involved in its evolution. Despite the number of detections, the nature of these small-scale structures is still poorly understood, and a systematic study on the subject is very difficult, as the sensitivity of absorption line spectroscopy is often required. The far-UV range of the spectrum in particular, provides a unique opportunity to study a number of present atoms and ions, and most importantly molecular hydrogen, which enable us to derive the physical properties of the gas.<br /> As the main results in this thesis, I present lines of sight of small angular separations in the direction of the Large Magellanic Cloud, where we find indication to small-scale structures in the Milky Way disc and halo gas. The molecular hydrogen structures, detected in this work as well as by others, are further compared with the low-column density halo structures detected in optical NaI absorption towards a number of quasars. The distribution of the detected molecular hydrogen as well as the NaI absorbers throughout the Galaxy further emphasises the role of turbulences and dynamical processes driven by Galactic fountains, which can produce high-density transient regions during the cooling and fragmentation phase.