Mass-Metallicity Relations for Nearby Dwarf Irregular Galaxies

For a sample of 25 dwarf irregular galaxies with distances D ∼ < 5 Mpc and measured oxygen abundances, we present results derived from galaxy luminosities at 4.5 μm and stellar masses from near-infrared imaging with IRAC on the Spitzer Space Telescope. We have constructed the appropriate luminosity-metallicity (L–Z) and mass-metallicity (M–Z) relations, and compared these relations with their corresponding relations from the Sloan Digital Sky Survey (SDSS). We obtain the following results. 1. The dispersion in the near-infrared L–Z relation is reduced with respect to the dispersion in the optical L–Z relation, which agrees with expectations for reduced variations of stellar mass-to-light ratios at longer wavelengths compared to optical wavelengths. 2. The dispersion in the optical L–Z relation is similar over approximately 11 mag in optical luminosity. 3. With our constructed M–Z relation, we have extended the SDSS M–Z relation to lower masses by about 2.5 dex in stellar mass. 4. The dispersion in the M–Z relation appears to be comparable over a range of 5.5 dex in stellar mass.