The metallicity of a galaxy provides information about its star formation history. Moreover, the measurement of the metallicities of galaxies at high-z will provide clues to the star formation history of the early universe. Thus the development and understanding of infrared metallicity diagnostics are important for future missions such as SIRTF, NGST, and FIRST.
Madden (2000) has analyzed ISO and KAO (Kuiper Airborne Observatory) observations of the [CII] 158µm line in a sample of 15 dwarf galaxies with metallicities ranging from 0.1 to 0.5 solar. She finds very high values of [CII] / CO(1-0) and somewhat higher [CII] / FIR (Figure 1) in these low metallicity systems relative to other galaxies discussed in Stacey et al. (1991), but does not find an unambiguous direct correlation of the [CII] / CO ratio with metallicity within her sample. She interprets the enhanced [CII] / CO(1-0) to be the result of reduced dust abundance, which allows the UV radiation to penetrate further into the molecular core (Maloney & Black 1988; Israel et al. 1996), producing a geometric dilution effect.
Figure 1. A comparison of low-metallicity galaxies with galactic star-forming regions and normal and starburst galaxies. Lines of constant I[C II] / I(CO) ratios are shown and range from 2000 up to 70,000 for some dwarf galaxies. The ratios of both axes are normalized to the local interstellar radiation field (1.3 × 10-4 erg s-1 cm-2 sr-1). From Madden (2000).