We present a sample of low-resolution 5–38 µm Spitzer IRS spectra of the inner few square kiloparsecs of 59 nearby galaxies spanning a large range of star formation properties .
A robust method for decomposing mid-infrared galaxy spectra is described , and used to explore the behavior of PAH emission and the prevalence of silicate dust extinction .
Evidence for silicate extinction is found in \sim 1/8 of the sample , at strengths which indicate most normal galaxies undergo A _ { V } \lesssim 3 magnitudes averaged over their centers .
The contribution of PAH emission to the total infrared power is found to peak near 10 % and extend up to \sim 20 % , and is suppressed at metallicities Z \lesssim Z _ { \sun } / 4 , as well as in low-luminosity AGN environments .
Strong inter-band PAH feature strength variations ( 2–5 \times ) are observed , with the presence of a weak AGN and , to a lesser degree , increasing metallicity shifting power to the longer wavelength bands .
A peculiar PAH emission spectrum with markedly diminished 5–8 µm features arises among the sample solely in systems with relatively hard radiation fields harboring low-luminosity AGN .
The AGN may modify the emitting grain distribution and provide the direct excitation source of the unusual PAH emission , which cautions against using absolute PAH strength to estimate star formation rates in systems harboring active nuclei .
Alternatively , the low star formation intensity often associated with weak AGN may affect the spectrum .
The effect of variations in the mid-infrared spectrum on broadband infrared surveys is modeled , and points to more than a factor of two uncertainty in results which assume a fixed PAH emission spectrum , for redshifts z = 0 - 2.5 .