Gaseous halos of late-type spiral galaxies

This article reviews the most recent observational results on disk-halo interactions in nearby galaxies. The implications of these results on our understanding of the structure of the interstellar medium (ISM) in our Galaxy and external spiral galaxies, with particular emphasis on the halo ISM, are discussed. Disk-halo interactions occur only above the brightest H II regions, which are found almost exclusively in late-type spirals and dwarf and irregular galaxies. Circumnuclear starbursts arl the most energetic flavor of this phenomenon. The existence, the shapes, and the properties of gaseous halos depend on the level of energy input into the disk ISM per unit surface area, i.e., on the feedback of mass, momentum, and energy produced by vigorous star formation. Thus, current theoretical models of the ISM take into account that gaseous halos are natural extensions of disk ISMs and are heated by the winds and ionizing radiation of massive stars, and by the shock waves of supernovae and their remnants. Observationally, this is reflected by the fact that all phases of the ISM known to exist in galaxy disks have also been detected in halos. Disk-halo interactions are a very effective way of redistributing energy, metals, and magnetic fields in galaxies and of expelling them into intergalactic space when outflows reach escape velocity.