OXYGEN ABUNDANCES IN LOW SURFACE BRIGHTNESS DISK GALAXIES

The oxygen abundances in the H ii regions of a sample of low surface brightness (LSB) disk galaxies are presented. In general, LSB galaxies are found to be metal poor (Z < 1/3Z.). Indeed, some LSB galaxies rival the lowest abundance extragalactic objects known, and this sample greatly increases the number of very low metallicity systems known. These low metallicities indicate that LSB galaxies evolve slowly, forming relatively few stars over a Hubble time. The low metallicities of LSB galaxies occur even though many are comparable in size and mass to the prominent spirals which define the Hubble sequence. As well as being low in surface brightness, these galaxies tend to be isolated. This suggests that surface mass density and environment are more relevant to galaxy evolution than gross size. Despite the low surface brightness of the disks, massive (M > 60 M.) stars are inferred to be present and no abnormality of the IMF is indicated. Many low-excitation H II regions exist at low metallicity in LSB galaxies, and the ionization parameter is not tightly correlated with metallicity. However, there does seem to be a significant envelope of maximum ionization at a given metallicity.