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

Abstract

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