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

Henry A. Kobulnicky, Dennis Zaritsky

Research output: Contribution to journalArticlepeer-review

96 Scopus citations

Abstract

We measure the H II-region oxygen and nitrogen abundances for 14 star-forming emission-line galaxies (ELGs) at intermediate redshifts (0.11 < z < 0.5) using optical spectra obtained with the Keck II telescope and low-resolution imaging multiobject spectrograph. The target galaxies exhibit a range of metallicities from slightly metal poor, as in the LMC [12 + log (O/H) ≃ 8.4], to supersolar [12 + log (O/H) ≃ 9.05] where the solar value is 12 + log (O/H) ≃ 8.89. Oxygen abundances of the sample correlate strongly with rest-frame blue luminosities. The metallicity-luminosity relation based on these 14 objects is formally indistinguishable from the one obeyed by galaxies in the local universe, although there is marginal evidence (1.1 σ) that the sample is slightly more metal-deficient than local galaxies of the same luminosity. The observed galaxies exhibit smaller emission line widths than local galaxies of similar metallicity, but proper corrections for inclination angle and other systematic effects are unknown. For eight of the 14 objects we measure nitrogen-to-oxygen (N/O) ratios. Seven of the eight systems show evidence for secondary 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 objects contain substantial old stellar populations that were responsible for the bulk of the heavy elements presently seen in the ionized gas. Four of the 14 galaxies exhibit small half-light radii and narrow emission-line profiles (compact narrow emission-line galaxies [CNELGs]; Koo et al. 1995) consistent with small dynamical masses despite their large optical luminosities and high levels of chemical enrichment. We find that the four CNELGs are indistinguishable from the 10 other emission-line galaxies (ELGs) in the sample on the basis of their metallicity and luminosity alone. Because of their morphological similarity to H II and spheroidal galaxies, CNELGs have been proposed as the starburst progenitors of today's spheroidal galaxies. Our assessment of the stellar chemical abundances in nearby spheroidal galaxies reveals that the majority of the CNELGs are presently ∼ 4 mag brighter and ∼ 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 185. Two of the four CNELGs exhibit oxygen abundances higher than the planetary nebula oxygen abundances in NGC 205, making an evolution between these two CNELGs and metal-poor dwarf spheroidal galaxies highly improbable. However, the data are consistent with the hypothesis that more luminous and metal-rich spheroidal galaxies like NGC 3605 may become the evolutionary endpoints 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 ∼ 5 Gyr ago.

Original languageEnglish (US)
Pages (from-to)118-135
Number of pages18
JournalAstrophysical Journal
Volume511
Issue number1 PART 1
DOIs
StatePublished - Jan 20 1999

Keywords

  • Galaxies: Abundances
  • Galaxies: Evolution
  • Galaxies: ISM
  • Galaxies: Starburst

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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