High-Velocity Clouds and the Local Intergalactic Medium

In this article I review recent observations of the gaseous halos of galaxies and the intergalactic medium at low redshift. In the first part I discuss distribution, metal content, and physical properties of the Galactic intermediate- and high-velocity clouds and the hot halo of the Milky Way. Recent absorption and emission measurements show that the Galaxy's tidal interaction with the Magellanic Clouds, the infall of low-metallicity gas, as well as the circulation of gas as part of the galactic fountain contribute to the observed distribution of gas in the halo of the Milky Way. In the second part I give a short overview on the circumgalactic gaseous environment of other nearby spiral galaxies. Multi-wavelength observations demonstrate that neutral and ionized gaseous halos of galaxies are common, and that they extend deep into intergalactic space. These studies suggest that the gaseous material around spiral galaxies is tightly connected to the on-going hierarchical formation and evolution of these galaxies. In the last part of this article I summarize recent quasar absorption-line measurements of the local intergalactic medium. In accordance with cosmological simulations, absorption-line studies in the far-ultraviolet indicate that both the photoionized Ly alpha forest and the shock-heated warm-hot intergalactic medium harbor a substantial fraction of the baryons in the local Universe.