The nature of infrared emission in the Local Group dwarf galaxy NGC 6822 as revealed by Spitzer

We present Spitzer imaging of the metal-deficient (Z similar or equal to 30% Z circle dot) Local Group dwarf galaxy NGC 6822. On spatial scales of similar to 130 pc, we study the nature of IR, H alpha, H-I, and radio continuum emission. Nebular emission strength correlates with IR surface brightness; however, roughly half of the IR emission is associated with diffuse regions not luminous at H alpha (as found in previous studies). The global ratio of dust to H-I gas in the ISM, while uncertain at the actor of similar to 2 level, is similar to 25 times lower than the global values derived for spiral galaxies using similar modeling techniques; localized ratios of dust to H-I gas are about a factor of 5 higher than the global value in NGC 6822. There are strong variations (factors of similar to 10) in the relative ratios of H alpha and IR flux throughout the central disk; the low dust content of NGC 6822 is likely responsible for the different H alpha/IR ratios compared to those found in more metal-rich environments. The H alpha and IR emission is associated with high column density (greater than or similar to 10(21) cm(-2)) neutral gas. Increases in IR surface brightness appear to be affected by both increased radiation field strength and increased local gas density. Individual regions and the galaxy as a whole fall within the observed scatter of recent high-resolution studies of the radio - far-IR correlation in nearby spiral galaxies; this is likely the result of depleted radio and far-IR emission strengths in the ISM of this dwarf galaxy.