On Extending the Mass-Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass

We report 4.5 μm luminosities for 27 nearby (D ≲ 5 Mpc) dwarf irregular galaxies measured with the Spitzer Infrared Array Camera. We have constructed the 4.5 μm luminosity-metallicity (L-Z) relation for 25 dwarf galaxies with secure distance and interstellar medium oxygen abundance measurements. The 4.5 μm L-Z relation is 12 + log(O/H) = (5.78 ± 0.21) + (-0.122 ± 0.012)M[4.5], where M[4.5] is the absolute magnitude at 4.5 μm. The dispersion in the near-infrared L-Z relation is smaller than the corresponding dispersion in the optical L-Z relation. The subsequently derived stellar mass-metallicity (M*-Z) relation is 12 + log(O/H) = (5.65 ± 0.23) + (0.298 ± 0.030) log M*, and extends the SDSS M*- Z relation to lower mass by about 2.5 dex. We find that the dispersion in the M*-Z relation is similar over 5 orders of magnitude in stellar mass, and that the relationship between stellar mass and interstellar medium metallicity is similarly tight from high-mass to low-mass systems. We find a larger scatter at low mass in the relation between effective yield and total baryonic mass. In fact, there are a few dwarf galaxies with large yields, which is difficult to explain if galactic winds are ubiquitous in dwarf galaxies. The low scatter in the L-Z and M*-Z relationships are difficult to understand if galactic superwinds or blowout are responsible for the low metallicities at low mass or luminosity. Naively, one would expect an ever increasing scatter at lower masses, which is not observed.