The Galactic ecosystem: Outflow and infall in the halo of the Milky Way

Abstract

The evolution, dynamics and eventual fate of galaxies is largely determined by access to and distribution of their primary fuel: atomic neutral hydrogen. Neutral hydrogen is not only pervasive in the disk of galaxies such as the Milky Way, but can also be found in the hot halo surrounding galaxies. The signatures of hydrogen detected in galactic haloes are caused by three key processes: outflow, due to energetic events associated with the galactic disk; infall, due to interactions with other nearby galaxies or the intergalactic medium; and circulation within the galactic ecosystem. In this thesis, a nearby Galactic supershell GSH 006-15+7 is studied, in order to understand how supershells are able to circulate cold gas between the disk and halo. By analysing HI self-absorption in the shell wall, the spin temperature of the gas is constrained to be cold and dense. Based on the morphology of the supershell and its estimated distance, GSH 006-15+7 has a likely origin in the Sagittarius OB 1 association of young stars. There is also evidence that the shell is transitioning into a chimney structure based on fragmentation at high Galactic latitudes, with an associated ionised hydrogen feature indicating a potential position of break-out into the Milky Way halo. This result is supported by findings in optical emission lines of high energy activity. Anomalous velocity gas deviates from that expected of Galactic rotation, and as such pinpoints hydrogen that is part of the cycle of outflow, infall and circulation. The Galactic All Sky Survey (GASS) of southern-sky neutral hydrogen is used to catalogue anomalous velocity gas in the halo of the Milky Way. Both classical high-velocity clouds and anomalous velocity clouds are included in the catalogue. With their lower velocities, anomalous velocity clouds are intrinsically closer to Galactic rotation and hence can be expected to probe the bridge between the Galactic disk and the halo. The GASS catalogue features unprecedented combination of high sensitivity with high angular and spectral resolution in the southern sky, and will be useful for studies of anomalous velocity gas on various scales. Several GASS clouds are followed up, some of known origin, at high angular resolution with the Australia Telescope Compact Array in order to determine the relative influences of origin and environment in clouds showing evidence of interaction. By combining the population of GASS high-velocity clouds with a very sensitive survey of neutral hydrogen in the halo, a Milky Way halo hidden from typical surveys of neutral hydrogen due to sensitivity limits is revealed, where the brightest neutral hydrogen merges with a diffuse prevalent medium that is likely to contribute just as much gas content as the bright high-velocity clouds. These results are consistent with findings in other wavelengths indicating the presence of more hydrogen in the halo than detected in the high-velocity cloud population. If the detected diffuse gas follows predicted supernova-driven models of cooling, then these two populations of neutral hydrogen combined can potentially account for the entire Galactic star formation rate. Overall, the studies in this thesis have revealed an active and dynamic Galaxy that maintains strong connections between its disk and the surrounding halo environment, in which neutral hydrogen remains a pivotal and powerful key to unlocking its evolutionary past and star-forming future.