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

Cannon, John M., Walter, Fabian, Armus, Lee, Bendo, George J., Calzetti, Daniela, Draine, Bruce T., Engelbracht, Charles W., Helou, George, Kennicutt, Robert C., Jr., Leitherer, Claus, Roussel, Hélène, Bot, Caroline, Buckalew, Brent A., Dale, Daniel A., de Blok, W. J. G., Gordon, Karl D., Hollenbach, David J., Jarrett, Thomas H., Meyer, Martin J., Murphy, Eric J., Sheth, Kartik, & Thornley, Michele D.
2006, The Astrophysical Journal, 652, 1170

We present Spitzer imaging of the metal-deficient (Z~=30% Zsolar) Local Group dwarf galaxy NGC 6822. On spatial scales of ~130 pc, we study the nature of IR, Hα, 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 Hα (as found in previous studies). The global ratio of dust to H I gas in the ISM, while uncertain at the factor of ~2 level, is ~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 ~10) in the relative ratios of Hα and IR flux throughout the central disk; the low dust content of NGC 6822 is likely responsible for the different Hα/IR ratios compared to those found in more metal-rich environments. The Hα and IR emission is associated with high column density (>~1021 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.


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