Gas Accretion via Condensation and Fountains

For most of their lives, galaxies are surrounded by large and massive coronae of hot gas, which constitute vast reservoirs for gas accretion. This Chapter describes a mechanism that allows star-forming disc galaxies to extract gas from their coronae. Stellar feedback powers a continuous circulation (galactic fountain) of gas from the disc into the halo, producing mixing between metal-rich disc material and metal-poor coronal gas. This mixing causes a dramatic reduction of the cooling time of the corona making it condense and accrete onto the disc. This fountain- driven accretion model makes clear predictions for the kinematics of the extraplanar cold/warm gas in disc galaxies, which are in good agreement with a number of independent observations. The amount of gas accretion predicted by the model is of the order of what is needed to sustain star formation. Accretion is expected to occur preferentially in the outer parts of discs and its efficiency drops for higher coronal temperatures. Thus galaxies are able to gather new gas as long as they do not become too massive nor fall into large halos and maintain their star-forming gaseous discs.