Low-resolution , mid-infrared Spitzer /IRS spectral maps are presented for three nearby , low-metallicity dwarf galaxies ( NGC 55 , NGC 3109 and IC 5152 ) for the purpose of examining the spatial distribution and variation of polycyclic aromatic hydrocarbon ( PAH ) emission .
The sample straddles a metallicity of 12 + log ( O/H ) \approx 8 , a transition point below which PAH intensity empirically drops and the character of the interstellar medium changes .
We derive quantitative radiances of PAH features and atomic lines on both global and spatially-resolved scales .
The Spitzer spectra , combined with extensive ancillary data from the UV through the mid-infrared , allow us to examine changes in the physical environments and in PAH feature radiances down to a physical scale of \sim 50 pc .
We discuss correlations between various PAH emission feature and atomic line radiances .
The ( 6.2 \mu m ) / ( 11.3 \mu m ) , ( 7.7 \mu m ) / ( 11.3 \mu m ) , ( 8.6 \mu m ) / ( 11.3 \mu m ) , ( 7.7 \mu m ) / ( 6.2 \mu m ) , and ( 8.6 \mu m ) / ( 6.2 \mu m ) PAH radiance ratios are found to be independent of position across all three galaxies , although the ratios do vary from galaxy to galaxy .
As seen in other galaxies , we find no variation in the grain size distribution as a function of local radiation field strength .
Absolute PAH feature intensities as measured by a ratio of PAH/ ( 24 \mu m ) radiances are seen to vary both positionally within a given galaxy , and from one galaxy to another when integrated over the full observed extent of each system .
We examine direct comparisons of CC mode PAH ratios ( 7.7 \mu m ) / ( 6.2 \mu m ) and ( 8.6 \mu m ) / ( 6.2 \mu m ) to the mixed ( CC/CH ) mode PAH ratio ( 7.7 \mu m ) / ( 11.3 \mu m ) .
We find little variation in either mode , and no difference in trends between modes .
While the local conditions change markedly over the observed regions of these galaxies , the properties of PAH emission show a remarkable degree of uniformity .