Direct Oxygen Abundances for Low Luminosity LVL Galaxies

Danielle A. Berg (1), Evan D. Skillman (1), Andrew R. Marble (2,3), Liese van Zee (4), Charles W. Engelbracht (2), Janice C. Lee (5), Robert C. Kennicutt, Jr. (6,7), Daniela Calzetti (8), Daniel A. Dale (9), and Benjamin D. Johnson (10) ((1)Institute for Astrophysics, University of Minnesota, 116 Church St. SE, Minneapolis, MN 55455; (2)Steward Observatory, University of Arizona, 933 N Cherry Ave, Tucson, AZ 85721;(3)National Solar Observatory, 950 N Cherry Ave, Tucson, AZ 85719; (4) Astronomy Department, Indiana University, 727 East 3rd Street, Bloomington, IN 47405; (5) STScI, 3700 San Martin Drive, Baltimore, MD 21218; (6) Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK; (7) Steward Observatory, University of Arizona, Tucson, AZ 85721; (8) Department of Astronomy, University of Massachusetts, 710 North Pleasant Street, Amherst, MA, 01003; (9) Department of Physics and Astronomy, University of Wyoming, 1000 E. University, Laramie, WY 82071;
(10) Institut d’Astrophysique de Paris, UMR 7095, 98 bis Bvd Arago, 75014 Paris, France)

We present MMT spectroscopic observations of HII regions in 42 low luminosity galaxies in the LVL. For 31 galaxies, we measured the temperature sensitive [O III] line at a strength of 4 sigma or greater, and thus determine direct oxygen abundances. Our results provide the first direct estimates of oxygen abundance for 19 galaxies. Oxygen abundances were compared to B-band and 4.5 micron luminosities and stellar masses in order to characterize the luminosity-metallicity (L-Z) and mass-metallicity (M-Z) relationships at low-luminosity. We present and analyze a “Combined Select” sample composed of 38 objects (drawn from our parent sample and the literature) with direct oxygen abundances and reliable distance determinations (TRGB or Ceph). Consistent with previous studies, the B-band and 4.5 micron L-Z relationships 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] (sigma=0.15 and 0.14). For this sample, we derive a M-Z relationship of 12+log(O/H)=(5.61±0.24)+(0.29±0.03)log(M*), 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. 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\lesssimB-V\lesssim0.75: log(N/O)=(1.18±0.9)x(B-V)+(-1.92±-0.08), with a dispersion of sigma=0.14, that is in agreement with previous studies.

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