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The massive star forming region W49A represents one of the largest complexesof massive star formation present in the Milky Way and contains at least fiftyyoung massive stars still enshrouded in their natal molecular cloud. We employSpitzer/IRS spectral mapping observations of the northern part of W49A toinvestigate the mid-infrared (MIR) spatial appearance of the polycyclicaromatic hydrocarbon (PAH) bands, PAH plateau features, atomic lines andcontinuum emission. We examine the spatial variations of the MIR emissioncomponents in slices through two of the ultra compact-HII (UC-HII) regions. Wefind that the PAH bands reproduce known trends, with the caveat that the 6.2$\mu$m PAH band seems to decouple from the other ionized PAH bands in some ofthe UC-HII regions -- an effect previously observed only in one other object:the giant star forming region N66 in the LMC. Furthermore, we compare thenature of the emission surrounding W49A to that of `diffuse' sightlines. It isfound that the surrounding emission can be explained by line of sight emission,and does not represent true `diffuse' material. Additionally, we examine theMIR appearance of star formation on various scales from UC-HII regions tostarburst galaxies, including a discussion of the fraction of PAH emission inthe 8 $\mu$m IRAC filter. We find that the MIR appearance of W49A is that of astarburst on large scales yet its individual components are consistent withother galactic HII regions.

The mid-infrared appearance of the Galactic Mini-Starburst W49A