1 0 Ju l 2 00 9 The Aromatic Features in Very Faint Dwarf Galaxies

We present optical and mid-infrared photometry of a statistically complete sample of 29 very faint dwarf galaxies (Mr > −15 mag) selected from the SDSS spectroscopic sample and observed in the mid-infrared with Spitzer IRAC. This sample contains nearby (redshift z . 0.005) galaxies three magnitudes fainter than previously studied samples. We compare our sample with other star-forming galaxies that have been observed with both IRAC and SDSS. We examine the relationship of the infrared color, sensitive to PAH abundance, with star-formation rates, gas-phase metallicities and radiation hardness, all estimated from optical emission lines. Consistent with studies of more luminous dwarfs, we find that the very faint dwarf galaxies show much weaker PAH emission than more luminous galaxies with similar specific star-formation rates. Unlike more luminous galaxies, we find that the very faint dwarf galaxies show no significant dependence at all of PAH emission on starformation rate, metallicity, or radiation hardness, despite the fact that the sample spans a significant range in all of these quantities. When the very faint dwarfs in our sample are compared with more luminous (Mr ∼ −18 mag) dwarfs, we find that PAH emission depends on metallicity and radiation hardness. These two parameters are correlated; we look at the PAH–metallicity relation at fixed radiation hardness and the PAH–hardness relation at fixed metallicity. This test shows that the PAH emission in dwarf galaxies depends most directly on metallicity. Subject headings: galaxies: dwarf — galaxies: evolution — galaxies: ISM — infrared: galaxies — ISM: general — stars: formation