2.6 Other Methods
Another possible metallicity indicator is that of the cold (neutral) ISM in a galaxy, where problems with self pollution should be small or absent. By using background continuum sources, absorption lines from the neutral ISM may be used to estimate chemical abundances. Kunth et al. (1994) used UV-absorption lines arising in the neutral gas in the line of sight towards the young stellar association in IZw18 to derive oxygen abundances of the gaseous halo of IZw18. The method is similar to the one used for deriving abundances in QSO absorption line system. This method is rather unexplored for galaxies, but may prove to be of future use if the difficulties due to line saturation can be tackled or circumvented (Pettini and Lipman 1995)
X-ray observations have been used to derive metallicity of the hot intra cluster medium in rich galaxy clusters, and has recently been applied also to starburst galaxies (Persic et al. 1998). X-ray observations are interesting because they may provide a means of observing ``hot'' metals produced in the current star formation event that have not yet mixed with the photoionised gas, and to study abundances in gas expelled by ``superwinds''.
In principle, abundances may be derived also from radio observations. That has been done in the local ISM in our galaxy. For external galaxies they have not been applied much due to sensitivity problems. CO observations of dwarf galaxies are complicated by the fact that for low metallicities the CO flux depends both on the mass and metallicity of the molecular gas complex. These questions may be further addressed by space based infrared spectroscopy.
The chemical composition of the ISM has been studied in the ultraviolet (UV) with e.g. the IUE and HST satellite observatories. A relatively unexplored wavelength region is however the far UV. New space borne instruments like FUSE (far ultraviolet spectroscopic explorer) may be of importance here. Other methods include observations of supernova remnants (see Pagel 1997) and the concept of dynamical metallicity, cf. Haser et al. (1998).