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

December 2006 • 2006ApJ...652.1170C

Authors • 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.

Abstract • We present Spitzer imaging of the metal-deficient (Z~=30% Z_solar) 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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IPAC Authors
(alphabetical)

Lee Armus

Senior Scientist

George Helou

IPAC Executive Director