Diffuse Ionized Gas and the Disk-Halo Interaction in Spiral Galaxies

Abstract

Observational evidence for the presence of diffuse ionized gas (DIG) in the halos of star forming disk galaxies is discussed in the context of a disk-halo connection of the interstellar medium driven by multiple and clustered supernovae. New results from a survey for H⁺ halos of edge-on galaxies support this picture and allow us to establish a minimum energy release per unit area that is required to start the disk-halo mass exchange. The spectroscopically derived emission line ratios of halo DIG are difficult to explain consistantly by pure photoionization models; an additional heating source seems to be required to match the observations in external galaxies as well as in the Milky Way. The halo DIG is typically correlated with the presence of other gaseous phases of the ISM in the halo. This includes cosmic rays and magnetic fields. Synchrotron polarization maps thus provide evidence for a large scale magnetic field in the gaseous halos. Finally, the possible rôle of charged dust in the the disk-halo interface for the formation of structures is also briefly discussed.