DIRECT OXYGEN ABUNDANCES FOR LOW-LUMINOSITY LVL GALAXIES

Abstract

We present MMT spectroscopic observations of HII regions in 42 low luminosity\ngalaxies in the LVL. For 31 galaxies, we measured the temperature sensitive [O\nIII] line at a strength of 4 sigma or greater, and thus determine direct oxygen\nabundances. Our results provide the first direct estimates of oxygen abundance\nfor 19 galaxies. Oxygen abundances were compared to B-band and 4.5 micron\nluminosities and stellar masses in order to characterize the\nluminosity-metallicity (L-Z) and mass-metallicity (M-Z) relationships at\nlow-luminosity.\n We present and analyze a "Combined Select" sample composed of 38 objects\n(drawn from our parent sample and the literature) with direct oxygen abundances\nand reliable distance determinations (TRGB or Ceph). Consistent with previous\nstudies, the B-band and 4.5 micron L-Z relationships were found to be\n12+log(O/H)=(6.27+/-0.21)+(-0.11+/-0.01)M_B and\n12+log(O/H)=(6.10+/-0.21)+(-0.10+/-0.01)M_[4.5] (sigma=0.15 and 0.14). For this\nsample, we derive a M-Z relationship of\n12+log(O/H)=(5.61+/-0.24)+(0.29+/-0.03)log(M*), which agrees with previous\nstudies; however, the dispersion (sigma=0.15) is not significantly lower than\nthat of the L-Z relationships. Because of the low dispersions in these\nrelationships, if an accurate distance is available, the luminosity of a\nlow-luminosity galaxy is often a better indicator of metallicity than that\nderived using certain strong-line methods, so significant departures from the\nL-Z relationships may indicate that caution is prudent in such cases. We also\nrevisit the 70/160 micron color metallicity relationship.\n Additionally, we examine N/O abundance trends with respect to oxygen\nabundance and B-V color. We find a positive correlation between N/O ratio and\nB-V color for 0.05\\lesssimB-V\\lesssim0.75:\nlog(N/O)=(1.18+/-0.9)x(B-V)+(-1.92+/-0.08), with a dispersion of sigma=0.14,\nthat is in agreement with previous studies.\n