Chemical properties of star-forming emission-line galaxies at z = 0.1-0.5

We measure the H II-region oxygen and nitrogen abundances for 14 star-formi ng emission-line galaxies (ELGs) at intermediate redshifts (0.11 < z < 0.5) using optical spectra obtained with the Keek II telescope and low-resoluti on imaging multiobject spectrograph. The target galaxies exhibit a range of metallicities from slightly metal poor, as in the LMC [:12 + log (O/H)simi lar or equal to 8.4], to supersolar [12 + log (O/H) similar or equal to.9.0 5] where the solar value is 12 + log (O/H)similar or equal to 8.89. Oxygen abundances of the sample correlate strongly with rest-frame blue luminositi es. The metallicity-luminosity relation based on these 14 objects is formal ly indistinguishable from the one obeyed by galaxies in the local universe, although there is marginal evidence (1.1 sigma) that the sample is slightl y more metal-deficient than local galaxies of the same luminosity. The obse rved galaxies exhibit smaller emission line widths than local galaxies of s imilar metallicity, but proper corrections for inclination angle and other systematic effects are unknown. For eight of the 14 objects we measure nitr ogen-to-oxygen (N/O) ratios. Seven of the eight systems show evidence for s econdary nitrogen production, with log(N/O)> -1.4, similar to local spiral galaxies. These chemical properties are inconsistent with unevolved objects undergoing a first burst of star formation. Comparison with local galaxies showing similar chemical properties suggests that these intermediate-z obj ects contain substantial old stellar populations that were responsible for the bulk of the heavy elements presently seen in the ionized gas. Four of t he 14 galaxies exhibit small half-light radii and narrow emission-line prof iles (compact narrow emission-line galaxies [CNELGs]; Koo et al. 1995) cons istent with small dynamical masses despite their large optical luminosities and high levels of chemical enrichment. We find that the four CNELGs are i ndistinguishable from the 10 other emission-line galaxies (ELGs) in the sam ple on the basis of their metallicity and luminosity alone. Because of thei r morphological similarity to H II and spheroidal galaxies, CNELGs have bee n proposed as the starburst progenitors of today's spheroidal galaxies. Our assessment of the stellar chemical abundances in nearby spheroidal galaxie s reveals that the majority of the CNELGs are presently similar to 4 mag br ighter and similar to 0.5 dex more metal rich than the bulk of the stars in well-known metal-poor dwarf spheroidal galaxies such as NGC 205 and NGC 18 5. Two of the four CNELGs exhibit oxygen abundances higher than the planeta ry nebula oxygen abundances in NGC 205, making an evolution between these t wo CNELGs and metal-poor dwarf spheroidal galaxies highly improbable. Howev er, the data are consistent with the hypothesis that more luminous and meta l-rich spheroidal galaxies like NGC 3605 may become the evolutionary endpoi nts of some CNELGs after 1-3 mag of fading. We suggest that the z = 0.1-0.4 ELGs, and perhaps some of the CNELGs, are the precursors to today's spiral galaxies during an episode of vigorous bulge star formation similar to 5 G yr ago.