Gaseous Haloes: Linking Galaxies to the IGM

Recent observations show that the discs of spiral galaxies are surrounded by massive and extended haloes of both neutral and ionized gas. Such haloes have peculiar kinematics with respect to the discs: they rotate more slowly and they show an overall inflow motion. The origin of the gaseous haloes is still unknown. They can be generated by supernova outflows from the discs and/or by accretion from the surrounding Intergalactic Medium (IGM). They are probably analogous to the Intermediate and High Velocity Clouds of the Milky Way. The study of the gaseous haloes gives crucial information on the circulation of material within a galaxy, which is fundamental for its life-cycle. Moreover, these haloes are the interfaces between galaxies and their surrounding environment therefore providing a new and efficient way to probe the properties and the content of the IGM. After reviewing recent observational results, I will present a dynamical model for the formation of the gaseous haloes. This model attempts to reproduce the gas circulation in a spiral galaxy taking into account both internal processes (galactic fountain) and accretion from the IGM.