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Session 31 - The Diffuse Interstellar Medium, Progress and Puzzles - I.
Oral session, Tuesday, June 11
Over the past 8 years, optical observations on mostly modestly sized telescopes have provided a wealth of information on the properties of Diffuse Ionized Gas (DIG), also referred to as the Warm Ionized Medium (WIM), in spiral galaxies. Deep H\alpha images, reaching sensitivities that approach the level at which the Galactic DIG in the solar neighborhood emits, have allowed us to map out the distribution of the DIG at the disk-halo interface in edge-on systems and across galactic disks in face-on galaxies. Observations of forbidden emission lines have provided constraints for the various ionization models in a variety of environments. The studies of extragalactic systems complement the Galactic observations in many important ways. In particular, this paper will address the most recent information on the following topics:
What is the H\alpha luminosity of the DIG, in absolute sense and as a fraction of the total? Is this fraction similar to estimates for the Galactic DIG? Does the fraction vary from one galaxy to another? Surprisingly, the answer to the latter question seems to be negative.
(2) How large is the extent of the DIG in the direction perpendicular to the plane in different galaxies? What is the connection with the star formation rate in the disk? Are there other factors which may influence the properties of the DIG at the disk-halo interface?
(3) What constraints do the observations of extragalactic systems provide on the various ionization models for the DIG? In particular, is helium ionized? How do the forbidden lines from nitrogen, oxygen, and sulphur vary across the DIG, as a function of intensity and distance away from nearby HII regions? Do HII regions leak ionizing photons, or can the DIG be ionized by field OB stars?
Program listing for Tuesday