Diffuse Ionized Gas in Halos of Spiral Galaxies

Abstract

The presence of gaseous halos in star forming disk galaxies is reviewed in the context of a proposed disk-halo connection of the interstellar medium (ISM). Results from a new survey for H⁺ halos of edge-on galaxies are presented. The data confirm that the presence of diffuse ionized gas (DIG) in the disk-halo interface of spiral galaxies is related to star formation processes in the underlying disk. A discussion allows us to establish a minimum energy release per unit area that is required to start the disk-halo mass exchange. By comparing some recent observational results for diagnostic emission lines with model predictions from photoionization we demonstrate that the origin and excitation of the ionized halo gas is still not completely understood and that a discussion gives important constraints for models of the ISM. In comparison with similar findings for the Milky Way the need for an additional heating source is established. Special emphasis is given to some recent developments. In particular, new kinematical information for the DIG layer in NGC 5775 from ESO/VLT long-slit spectra is discussed in connection with the magnetic field structure in the halo of this object as deduced from VLA radio-continuum polarization data (Tüllmann et al., 2001). Finally, the rôle of dust for the physical processes in the disk-halo interface is briefly addressed.