科研之友
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摘要
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] lambda 4363 line at a strength of 4 sigma or greater, and thus determine oxygen abundances using the %26quot;direct%26quot; method. Our results provide the first %26quot;direct%26quot; estimates of oxygen abundance for 19 of these galaxies. %26quot;Direct%26quot; oxygen abundances were compared to B-band luminosities, 4.5 mu m luminosities, and stellar masses in order to characterize the luminosity-metallicity and mass-metallicity relationships at low luminosity. We present and analyze a %26quot;Combined Select%26quot; sample composed of 38 objects (drawn from a sub-set of our parent sample and the literature) with %26quot;direct%26quot; 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 mu m 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-star), 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 %26quot;strong-line%26quot; methods, so significant departures from the L-Z relationships may indicate that caution is prudent in such cases. With these new %26quot;direct%26quot; metallicities we also revisit the 70/160 mu m 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 less than or similar to B - V less than or similar to 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.
- 出版日期2012-8-1
