Physics
Scientific paper
Aug 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999pcim.conf...18d&link_type=abstract
The Physics and Chemistry of the Interstellar Medium, Proceedings of the 3rd Cologne-Zermatt Symposium, held in Zermatt, Septemb
Physics
2
Scientific paper
Understanding the evolution of galaxies requires to understand the interstellar medium (ISM) in galaxies, its role in the star formation (SF) process, and the feedback of energy from SF regions into the medium. This energy input is related to physical processes associated with young stars such as winds, the interstellar radiation field, or (multiple) supernova explosions. It is believed that - as a result of these processes - the interstellar medium in the disk can connect to the halo. This is described in terms of galactic fountains, chimneys, or superbubble-outbreak by theory. As a consequence, the properties of gaseous halos could give a global measure of the overall physical state of the ISM in a galaxy. Observational support for the idea of such a large scale matter exchange between disk and halo comes from observations in the Milky Way as well as from observations of gaseous halos in external galaxies. While several phases of the ISM - from cold HI to X-ray coronae - have meanwhile been found in the halos of spiral galaxies, the H ^+ represents a particularly important constituent, since it is relatively easily observed in the optical. Therefore, imaging in and spectroscopy of optical emission lines allow us to study the distribution and excitation of this Diffuse Ionized Gas (DIG, or WIM for Warm Ionized Medium) with a spatial resolution not achievable for other phases of the ISM in external galaxies. The data available now demonstrate that the presence of DIG in the disk-halo interface of spiral galaxies is related to star formation processes in the underlying disk. This contribution summarizes some recent observational results and compares them with model predictions. It is demonstrated that the origin and ionization of the DIG is still not completely understood and that a discussion could give important constraints for models of the ISM in general and on the disk-halo connection in particular.
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