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Date and Time of the Query: 2019-03-25 T11:49:04 PDT
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For refcode 2012ApJ...754...98B:
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2012ApJ...754...98B Direct Oxygen Abundances for Low-luminosity LVL Galaxies Berg, Danielle A.; Skillman, Evan D.; Marble, Andrew R.; van Zee, Liese; Engelbracht, Charles W.; Lee, Janice C.; Kennicutt, Robert C., Jr.; Calzetti, Daniela; Dale, Daniel A.; Johnson, Benjamin D. Abstract. We present MMT spectroscopic observations of H II regions in 42 low luminosity galaxies in the Spitzer Local Volume Legacy survey. For 31 of the 42 galaxies in our sample, we were able to measure the temperature sensitive [O III] lambda4363 line at a strength of 4sigma or greater, and thus determine oxygen abundances using the "direct" method. Our results provide the first "direct" estimates of oxygen abundance for 19 of these galaxies. "Direct" oxygen abundances were compared to B-band luminosities, 4.5 micron luminosities, and stellar masses in order to characterize the luminosity-metallicity and mass-metallicity relationships at low luminosity. We present and analyze a "Combined Select" sample composed of 38 objects (drawn from a sub-set of our parent sample and the literature) with "direct" oxygen abundances and reliable distance determinations (based on the tip of the red giant branch or Cepheid variables). Consistent with previous studies, the B band and 4.5 micron luminosity-metallicity relationships for the 38 objects were found to be 12 + log(O/H) = (6.27 +/- 0.21) + (-- 0.11 +/- 0.01)M_B_ and 12 + log(O/H) = (6.10 +/- 0.21) + (-- 0.10 +/- 0.01)M_[4.5]_ with dispersions of sigma = 0.15 and 0.14, respectively. The slopes of the optical and near-IR L-Z relationships have been reported to be different for galaxies with luminosities greater than that of the LMC. However, the similarity of the slopes of the optical and near-IR L-Z relationships for our sample probably reflects little influence by dust extinction in the low luminosity galaxies. For this sample, we derive a mass-metallicity relationship of 12 + log(O/H) = (5.61 +/- 0.24) + (0.29 +/- 0.03)log (M_sstarf_), which agrees with previous studies; however, the dispersion (sigma = 0.15) is not significantly lower than that of the L-Z relationships. Because of the low dispersions in these relationships, if an accurate distance is available, the luminosity of a low luminosity galaxy is often a better indicator of metallicity than that derived using certain "strong-line" methods, so significant departures from the L-Z relationships may indicate that caution is prudent in such cases. With these new "direct" metallicities we also revisit the 70/160 micron color metallicity relationship. Additionally, we examine N/O abundance trends with respect to oxygen abundance and B -- V color. We find a positive correlation between N/O ratio and B -- V color for 0.05 <~ B -- V <~ 0.75: log (N/O) = (1.18 +/- 0.9) x (B -- V) + (-- 1.92 +/- 0.08), with a dispersion of sigma = 0.14, which is in agreement with previous studies. Observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. Key words: galaxies: abundances, galaxies: dwarf, galaxies: evolution
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