Karl D. Gordon
University of Arizona

Spitzer Observations of the Aromatic Features
in M101 HII Regions and Starbursts

I will discuss the study of the Aromatic dust emission features in
M101 HII regions and a large sample of starburst galaxies using the
Spitzer Space Telescope.  M101 is unique in that it is face-on and has
large metallicity gradient ranging from log(O/H) of ~8.8 in the
nucleus to 7.5 at its edge.  The sample of 44 starburst galaxies was
picked to have metallicities ranging from the lowest known (i.e., I Zw
18) to super solar (7.2 < 12+log(0/H) < 9.3).  Spitzer spectroscopy
and imaging is used to probe the behavior of the Aromatic emission
features (also commonly called the PAH features) as a function of HII
region diagnostics.  The Aromatics are seen to decrease in strength
with decreasing metallicity (increasing excitation) in both M101 HII
regions and starburst galaxies.  In addition, the feature strength
ratios change systematically from the high metallicity, low excitation
regions to the low metallicity, high excitation regions.  The feature
variations at face value do not follow the predictions of the PAH
model (neutral versus ionized PAH molecules) and may be more
consistent with a solid state carrier (e.g., Hydrogenated Amorphous
Carbon, Quenched Carbonaceous Composite, coal, etc.).  Thus, studies
of the aromatic features in star forming regions give us important
clues to identifying their carrier which, in turn, helps to more
accurately model these features in studies ranging from objects in the
Milky Way to number counts in deep infrared surveys.