Investigating the origin of gas flows at the disc-halo interface of local galaxies

The evolution of galaxies is largely affected by interactions with the surrounding environment. For example, galaxies are believed to sustain their star formation, a process which consumes clouds of cold gas, by accreting gas from their external environment. On the other hand, stellar feedback processes, such as stellar winds and supernova explosions, can expel gas from the galaxy to large distances. The disc-halo interface (the region relatively close to the galactic discs but still thousands of light years from the disc plane) is the crucial transition point of the stellar feedback and gas accretion, and therefore a window to the complicated gas exchange processes. The gas layer located at the disc-halo interface is known as extraplanar gas. Understanding the origin of this gas layer is essential for a complete view of the gas cycle between galaxies and their surrounding environment, and is the main focus of this thesis. In this thesis, I have presented a detailed investigation of the extraplanar gas in a sample of four spiral galaxies, including the Milky Way. Both radio and optical data have been obtained to trace extraplanar gas at different temperatures. We found that the motion of extraplanar gas is similar to the rotating disc, although with a slower rotation velocity. The properties are broadly consistent with an internal origin of the galactic fountain (gas cycle triggered by stellar feedback) combined with gas accretion. For the galaxy NGC 2403, in particular, the above scenario predicted a gas accretion rate to support the star formation activity of this galaxy