Metal abundances of KISS galaxies. I. Coarse metal abundances and the metallicity-luminosity relation

We derive metal abundance estimates for a large sample of starbursting emission-line galaxies (ELGs). Our sample is drawn from the KPNO International Spectroscopic Survey ( KISS), which has discovered over 2000 ELG candidates to date. Follow-up optical spectra have been obtained for approximately 900 of these objects. A three-step process is used to obtain metal abundances for these galaxies. We first calculate accurate nebular abundances for 12 galaxies whose spectra cover the full optical region from [O II] lambdalambda3726, 3729 to beyond [S II] lambdalambda6717, 6731 and include detection of [O III] lambda4363. Using secondary metallicity indicators R-23 and p(3), we calculate metallicities for an additional 59 galaxies with spectra that cover a similar wavelength range but lack [O II} lambda4363. The results are used to calibrate relations between metallicity and two readily observed emission-line ratios, which allow us to estimate coarse metallicities for 519 galaxies in total. The uncertainty in these latter abundance estimates is 0.16 dex. From the large, homogeneously observed sample of star-forming galaxies, we identify low-metallicity candidates for future study and investigate the metallicity-luminosity relation. We find a linear metallicity-luminosity relation of the form 12 + log(O/H) = 4.059 0.240 M-B, with an rms scatter of 0.252. This result implies that the slope of the metallicity-luminosity relation is steeper than when dwarf galaxies are considered alone and may be evidence that the relationship is not linear over the full luminosity range of the sample.