Metallicity Determinations

Knowledge of the ionization mechanism and the element abundances of the diffuse ionized medium (hot and warm) is crucial for understanding the metal enrichment and the chemical evolution history of galaxies. Moreover, it is still unclear, e.g., depending on energy input and galaxy potential, how much quiescent or superwind/starburst galaxies contribute to the ionization and metal enrichment of the intergalactic medium (IGM). Consequently, investigating the physics of the different phases of the ISM in different types of galaxies and different galactic environments (disk and halo) is of high importance. Here, the widespread diffuse halo gas is of particular interest since it might help to understand the possible mass and energy transfer between the disk and the halo. This transport process is thought to be most important with regard to the ballance of metals in evolutionary models.

Between 30% and 50% of the H$alpha$ flux of star forming late-type galaxies is emitted by the diffuse ionized gas (DIG) (Kennicutt et al. 1989) . As the name already implies, DIG has a diffuse morphology and represents an extended low surface brightness component of the ISM. It often forms large shells, (super)bubbles, and filamentary or extended layer-like structures. The energy source to ionize this gas phase and to drive significant amounts of gas to large distances above the plane of the parent galaxy is not fixed yet. Natural candidates would be OB stars and SNe which could provide sufficient ionizing flux and kinetic energy. Still, there are problems with this hypothesis:

• How can "hard" photons travel far out into the halo without being absorbed by the high density filter of the disk plane (free mean path length >1kpc)?
• Why show measurements in our galaxy limits of HeI emission well below all theoretical predictions (e.g. Domgörgen & Mathis 1994), implying a softer ionizing spectrum than provided by O stars?
• Why do the currently available spectra imply only small effects of shock ionization for the extraplanar DIG (e.g. Rand 1996 , Tüllmann & Dettmar 2000) and low mass systems (Martin 1998) , if SNe drive the gas outward as implied by the expansion velocity of some of the large shells?

To answer the questions it is necessary to determine the temperature and density of the diffuse gas in galaxies spanning a large range in current star formation rate and burst age. Because the detection of the temperature sensitive, but very weak auroral lines of [OIII]4363Å, or [NII]5755Å in the DIG is beyond the capabilities of 4m class telescopes, this research project represents even a serious challenge for 10m class telescopes, such as Keck or the VLT .

Fig. 2.1: This continuum subtracted Ha-image of NGC1963 was taken with the ESO 2.2m telescope in combination with EFOSC2 in 1993 (Rossa & Dettmar 2000) . Despite its relatively faint DIG layer, this galaxy provides sufficient spectral information to be discussed for the first time in the framework of diagnostic diagrams Tüllmann & Dettmar 2000) . Beside the regions covered by slits s1 and s2 several other extraplanar features are marked by boxes.

Provided SNe are the primary energy source, the very existence of gas flowing out of these galaxies implies, that a significant fraction of metals are pumped into the halo or even got lost to the intergalactic medium (e.g. Mac Low & Ferrara 1999) .
While these galactic winds are proposed theoretically to explain the chemical evolution of dwarf galaxies (Pilyugin 1992) direct observational prove is scarce. Problems are caused by the faintness of the gas and the need of high quality element abundance determinations to detect the small differences between enriched halo gas and gas in the disk.
For a metallicity determination of the diffuse ionized gas one needs again a high quality electron temperature determination, which can only be derived by the detection of the faint auroral emission lines ([OIII]4363Å, [NII]5755Å). Up to now, this line was observed only for DIG located in the Milky Way (Reynolds et al. 2001) . Although an ambitious VLT-based study of the gaseous environment of NGC55 yielded the up to now most reliable elemental abundances in gaseous high-density regions (Tüllmann et al. 2003) , the data turned out to be not sensitive enough to detect significant [OIII]4363Å emission in the DIG. However, reasonable upper limits resulted in abundances which undoubtledly revealed that DIG is significantly less metalrich than the gas within the disk. For further details see also this page .

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