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

Abstract

We present a sample of low-resolution 5-38um Spitzer IRS spectra of the inner\nfew square kiloparsecs of 59 nearby galaxies spanning a large range of star\nformation properties. A robust method for decomposing mid-infrared galaxy\nspectra is described, and used to explore the behavior of PAH emission and the\nprevalence of silicate dust extinction. Evidence for silicate extinction is\nfound in ~1/8 of the sample, at strengths which indicate most normal galaxies\nundergo A_V < ~3 magnitudes averaged over their centers. The contribution of\nPAH emission to the total infrared power is found to peak near 10% and extend\nup to ~20%, and is suppressed at metallicities Z < ~Z_sun/4, as well as in\nlow-luminosity AGN environments. Strong inter-band PAH feature strength\nvariations (2-5x) are observed, with the presence of a weak AGN and, to a\nlesser degree, increasing metallicity shifting power to the longer wavelength\nbands. A peculiar PAH emission spectrum with markedly diminished 5-8um features\narises among the sample solely in systems with relatively hard radiation fields\nharboring low-luminosity AGN. The AGN may modify the emitting grain\ndistribution and provide the direct excitation source of the unusual PAH\nemission, which cautions against using absolute PAH strength to estimate star\nformation rates in systems harboring active nuclei. Alternatively, the low star\nformation intensity often associated with weak AGN may affect the spectrum. The\neffect of variations in the mid-infrared spectrum on broadband infrared surveys\nis modeled, and points to more than a factor of two uncertainty in results\nwhich assume a fixed PAH emission spectrum, for redshifts z=0-2.5.\n