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

Abstract

We presentSpitzerimaging of the metal-deficient (Z ’ 30%Z ) Local Group dwarf galaxy NGC 6822. On spatial scales of130 pc, we study the nature of IR, H, Hi, 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 (as found in previous studies). The global ratio of dust to H i gas in the ISM, while uncertain at the factor of2 level, is25 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 10) in the relative ratios of H and IR flux throughout the central disk; the low dust content of NGC 6822 is likely responsible for the different H/ IR ratios compared to those found in more metal-rich environments. The Hand IR emission is associated with high column density (k1021 cm2 ) 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 radioYfar-IR correlation in nearby spiral