Infrared Emission from Interstellar Dust. IV. The Silicate‐Graphite‐PAH Model in the Post‐SpitzerEra

Infrared (IR) emission spectra are calculated for dust composed of mixturesof amorphous silicate and graphitic grains, including varying amounts ofpolycyclic aromatic hydrocarbon (PAH) particles. The models are constrained toreproduce the average Milky Way extinction curve. The calculations include theeffects of single-photon heating. Updated IR absorption properties for the PAHsare presented, that are consistent with observed emission spectra, includingthose newly obtained by Spitzer Space Telescope. We find a size distributionfor the PAHs that results in emission band ratios consistent with observedspectra of the Milky Way and other galaxies. Emission spectra are presented forvarious intensities of the illuminating starlight. We calculate how theefficiency of emission into different IR bands depends on PAH size; the strong7.7um emission feature is produced mainly by PAH particles containing <1000 Catoms. We show how the emission spectrum depends on U, the starlight intensityrelative to the local interstellar radiation field. The submm and far-infraredemission is compared to the observed emission from the local interstellarmedium. Using a simple distribution function, we calculate the emissionspectrum for dust heated by a distribution of starlight intensities, such asoccurs within galaxies. The models are parameterized by the PAH mass fractionqpah, the lower cutoff Umin, and the fraction gamma of the dust heated bystarlight with U>Umin. We present graphical procedures using IRAC and MIPSphotometry to estimate qpah, Umin, and gamma, the fraction f_PDR of the dustluminosity coming from photodissociation regions with U>100, and the total dustmass Mdust.Comment: revised, to appear in ApJ. 57 pages. Revised version has numerous corrections, small changes to spectra, and includes model emissivities for photometric bands of Akari (IRC, FIS) and Herschel (PACS, SPIRE