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D.C. Jackson, J.M. Cannon, E.D. Skillman, R.D. Gehrz, C.E. Woodward, E. Polomski (University of Minnesota)
We present Spitzer/IRAC 4.5 and 8.0 micron imaging of 12 Local Group dwarf galaxies at unprecedented sensitivity and angular resolution in the mid-infrared. Our sample spans a range of more than one dex in nebular metallicity and over three orders of magnitude in current star formation rate, allowing us to examine the dependence of PAH emission on these important parameters. By scaling the 4.5 micron images and subtracting them from the 8.0 micron images, we can isolate the 7.7 micron PAH feature as well as any hot (350 K) dust in these objects. Overall, we observe an almost total absence of diffuse emission in these images; only three of the most luminous galaxies in the sample, NGC 55, NGC 3109, and IC 1613 show significant diffuse emission. Generally, the absence of PAH emission can be attributed to either a deficiency in the amount of PAH present or to a dearth of the UV radiation which is required to stimulate these macro-molecules into emission. The three targets in our sample which exhibit significant PAH emission have the highest current star formation rates, and systems of comparable metallicity but lower star formation rates show no such emission. This suggests the local radiation field is as important as metallicity in the generation of this PAH emission feature.
This work is supported in part by NASA (JPL/SSC-1256406, 1215746)
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Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.