We present a sample of resolved galactic H ii regions and photodissociation regions ( PDRs ) observed with the Spitzer infrared spectrograph ( IRS ) in spectral mapping mode between the wavelengths of 5–15 µm .
For each object we have spectral maps at a spatial resolution of \sim 4″ in which we have measured all of the mid-infrared emission and absorption features .
These include the PAH emission bands , primarily at 6.2 , 7.7 , 8.6 , 11.2 and 12.7 µm , as well as the spectral emission lines of neon and sulfur and the absorption band caused by silicate dust at around 9.8 µm .
In this work we describe the data in detail , including the data reduction and measurement strategies , and subsequently present the PAH emission band intensity correlations for each of the objects and the sample as a whole .
We find that there are distinct differences between the sources in the sample , with two main groups , the first comprising the H ii regions and the second the reflection nebulae ( RNe ) .
Three sources , the reflection nebula NGC 7023 , the Horsehead nebula PDR ( an interface between the H ii region IC 434 and the Orion B molecular cloud ) and M 17 , resist this categorization , with the Horsehead PDR points mimicking the RNe and the NGC 7023 fluxes displaying unique bifurcated appearance in our correlation plots .
These discrepancies seem to be due to the very low radiation field experienced by the Horsehead PDR and the very clean separation between the PDR environment and a diffuse environment in the NGC 7023 observations .