ON EXTENDING THE MASS-METALLICITY RELATION OF GALAXIES BY 2.5 DECADES IN STELLAR MASS

We report 4.5 m luminosities for 27 nearby (DP 5 Mpc) dwarf irregular galaxies measured with the Spitzer Infrared Array Camera. We have constructed the 4.5 m luminosity-metallicity (L-Z) relation for 25 dwarf galaxies with secure distance and interstellar medium oxygen abundance measurements. The 4.5 m L-Z relation is 12þ log (O/H) (5:78 0:21)þ (0:122 0:012)M4:5, where M[4.5] is the absolute magnitude at 4.5 m. The dis-persion 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 12þ log (O/H) (5:65 0:23)þ (0:298 0:030)logM, and extends the SDSSM- 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 metal-licities at low mass or luminosity. Naively, one would expect an ever increasing scatter at lower masses, which is not observed