The Mid‐Infrared Spectrum of Star‐forming Galaxies: Global Properties of Polycyclic Aromatic Hydrocarbon Emission

We present a sample of low-resolution 5-38um Spitzer IRS spectra of the innerfew square kiloparsecs of 59 nearby galaxies spanning a large range of starformation properties. A robust method for decomposing mid-infrared galaxyspectra is described, and used to explore the behavior of PAH emission and theprevalence of silicate dust extinction. Evidence for silicate extinction isfound in ~1/8 of the sample, at strengths which indicate most normal galaxiesundergo A_V < ~3 magnitudes averaged over their centers. The contribution ofPAH emission to the total infrared power is found to peak near 10% and extendup to ~20%, and is suppressed at metallicities Z < ~Z_sun/4, as well as inlow-luminosity AGN environments. Strong inter-band PAH feature strengthvariations (2-5x) are observed, with the presence of a weak AGN and, to alesser degree, increasing metallicity shifting power to the longer wavelengthbands. A peculiar PAH emission spectrum with markedly diminished 5-8um featuresarises among the sample solely in systems with relatively hard radiation fieldsharboring low-luminosity AGN. The AGN may modify the emitting graindistribution and provide the direct excitation source of the unusual PAHemission, which cautions against using absolute PAH strength to estimate starformation rates in systems harboring active nuclei. Alternatively, the low starformation intensity often associated with weak AGN may affect the spectrum. Theeffect of variations in the mid-infrared spectrum on broadband infrared surveysis modeled, and points to more than a factor of two uncertainty in resultswhich assume a fixed PAH emission spectrum, for redshifts z=0-2.5.Comment: Accepted for publication in ApJ, 24 pages (abstract typo fixed, reference added