The Nature of Infrared Emission in the Local Group Dwarf Galaxy NGC 6822 as Revealed bySpitzer

We present Spitzer imaging of the metal-deficient (Z ~30% Z_sun) Local Groupdwarf galaxy NGC 6822. On spatial scales of ~130 pc, we study the nature of IR,H alpha, HI, and radio continuum emission. Nebular emission strength correlateswith IR surface brightness; however, roughly half of the IR emission isassociated with diffuse regions not luminous at H alpha (as found in previousstudies). The global ratio of dust to HI gas in the ISM, while uncertain at thefactor of ~2 level, is ~25 times lower than the global values derived forspiral galaxies using similar modeling techniques; localized ratios of dust toHI gas are about a factor of five higher than the global value in NGC 6822.There are strong variations (factors of ~10) in the relative ratios of H alphaand IR flux throughout the central disk; the low dust content of NGC 6822 islikely responsible for the different H alpha/IR ratios compared to those foundin more metal-rich environments. The H alpha and IR emission is associated withhigh-column density (> ~1E21 cm^-2) neutral gas. Increases in IR surfacebrightness appear to be affected by both increased radiation field strength andincreased local gas density. Individual regions and the galaxy as a whole fallwithin the observed scatter of recent high-resolution studies of the radio-farIR correlation in nearby spiral galaxies; this is likely the result of depletedradio and far-IR emission strengths in the ISM of this dwarf galaxy.