6.1.3 How metal-poor galaxies can be found?
Clearly, despite their strong emission lines metal-poor compact dwarf galaxies are difficult to detect simply because they are fainter than L* (the characteristic galaxy luminosity for a Schecter type luminosity function, cf. Schechter 1976, Binggeli et al. 1988) galaxies (a kind of Malmqvist bias). Only metal-poor galaxies with metallicity of the order of 0.1 solar that undergo starbursts are easy to pick out just because oxygen is the major cooling species, hence the [O III] lines at 4959 and 5007 Å are particularly strong. But as one moves to more deficient objects, say below 0.01 solar, forbidden lines fade and the dominant cooling agents are H and He (Kunth and Sargent 1986). Therefore a combination of H objective-prism spectroscopy and UV-excess searches should be promising. Comte (1998) suggests a mid-UV imaging survey from balloon borne or orbiting instruments. So far the SBS and UM surveys has given a handful of new galaxies but never with metallicity below 1/50 solar. Why this is so? It may be that extreme metal-poor star-forming galaxies are very rare or do not exist locally.