Infrared (IR) emission spectra are calculated for dust heated by starlight, for mixtures of amorphous silicate and graphitic grains, including varying amounts of polycyclic aromatic hydrocarbon (PAH) particles. The models are constrained to reproduce the average Milky Way extinction curve.The calculations include the effects of single-photon heating. Updated IR absorption properties for the PAHs are presented, that are consistent with observed emission spectra, including those newly obtained by Spitzer Space Telescope. We find a size distribution for the PAHs giving emission band ratios consistent with the observed spectra of the Milky Way and other galaxies.Emission spectra are presented for a wide range of starlight intensities. We calculate how the efficiency of emission into different IR bands depends on PAH size; the strong 7.7 µm emission feature is produced mainly by PAH particles containing < 10 3 C atoms. We also calculate how the emission spectrum depends on U , the starlight intensity relative to the local interstellar radiation field. The submm and far-infrared emission is compared to the observed emission from the local interstellar medium.Using a simple distribution function, we calculate the emission spectrum for dust heated by a distribution of starlight intensities, such as occurs within galaxies. The models are parameterized by the PAH mass fraction q PAH , the lower cutoff U min of the starlight intensity distribution, and the fraction γ of the dust heated by starlight with U > U min . We present graphical procedures using Spitzer IRAC and MIPS photometry to estimate the parameters q PAH , U min , and γ, the fraction f PDR of the dust luminosity coming from photodissociation regions with U > 100, and the total dust mass M dust .When we observe emission from a region in the Milky Way or another galaxy, there will always be a mixture of dust types and sizes present. We consider the emission from a specific set of dust mixtures, as described in §5. All the dust mixtures considered here are consistent with the observed "average" extinction curve for diffuse regions in the local Milky Way, but they differ from one another in the assumed abundance of small PAH particles. In §6 we show that the far-IR and submm emission calculated for the model is consistent with the COBE-FIRAS observations of emission from dust in the local Milky Way. In §7 we show that the calculated IRAC band ratios appear to be consistent with the spectrum of diffuse emission from the Milky Way, as determined by Flagey et al. (2006) from Spitzer Space Telescope observations.The long wavelength emission from the dust model depends on the intensity of the starlight heating the dust. It will often be the case that the region observed (e.g., an entire star-forming galaxy) will include dust heated by a wide range of starlight intensities. In §8 we describe a simple parametric model for the distribution of the dust mass between regions with starlight intensities ranging from low to very high. We show in §9 how observations in the 3 MIPS...

We present a quantitative model for the infrared emission from dust in the diffuse interstellar medium. The model consists of a mixture of amorphous silicate grains and carbonaceous grains, each with a wide size distribution ranging from molecules containing tens of atoms to large grains 1 micron in diameter. We assume that the carbonaceous grains have polycyclic aromatic hydrocarbon (PAH)-like properties at very small sizes, and graphitic properties for radii a 50Å. On the basis of recent laboratory studies and guided by astronomical observations, we propose "astronomical" absorption cross sections for use in modeling neutral and ionized PAHs from the far ultraviolet to the far infrared. We also propose modifications to the far-infrared emissivity of "astronomical silicate".We calculate energy distribution functions for small grains undergoing "temperature spikes" caused by stochastic absorption of starlight photons, using realistic heat capacities and optical properties. Using a grain size distribution consistent with the observed interstellar extinction, we are able to reproduce the near-IR to submillimeter emission spectrum of the diffuse interstellar medium, including the PAH emission features at 3.3, 6.2, 7.7, 8.6, and 11.3µm. The model is compared with the observed emission at high Galactic latitudes as well as in the Galactic plane, as measured by the COBE/DIRBE, COBE/FIRAS, IRTS/MIRS, and IRTS/NIRS instruments. The model has 60 × 10 −6 of C (relative to H) locked up in PAHs, with 45 × 10 −6 of C in a component peaking at ∼ 6Å (N C ≈ 100 carbon atoms) to account for the PAH emission features, and with 15 × 10 −6 of C in a component peaking at ∼ 50Å to account for the 60µm flux. The total infrared emission is in excellent agreement with COBE/DIRBE observations at high galactic latitudes, just as the albedo for our grain model is in accord with observations of the diffuse galactic light. The aromatic absorption features at 3.3µm and 6.2µm predicted by our dust model are consistent with observations. We calculate infrared emission spectra for our dust model heated by a range of starlight intensities, from 0.3 to 10 4 times the local interstellar radiation field, and we tabulate the intensities integrated over the SIRTF/IRAC and MIPS bands. We also provide dust opacities tabulated from the extreme ultraviolet to submillimeter wavelengths.

The SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging and spectroscopic survey of 75 nearby galaxies. Its primary goal is to characterize the infrared emission of galaxies and their principal infrared-emitting components, across a broad range of galaxy properties and star formation environments. SINGS will provide new insights into the physical processes connecting star formation to the interstellar medium properties of galaxies, and provide a vital foundation for understanding infrared observations of the distant universe and ultraluminous and active galaxies. The galaxy sample and observing strategy have been designed to maximize the scientific and archival value of the data set for the SIRTF user community at large. The SIRTF images and spectra will be supplemented by a comprehensive multiwavelength library of ancillary and complementary observations, including radio continuum, HI, CO, submillimeter, BV RIJHK, Hα, Paschen-α, ultraviolet, and X-ray data. This paper describes the main astrophysical issues to be addressed by SINGS, the galaxy sample and the observing strategy, and the SIRTF and other ancillary data products.

We present a method for calculating the infrared emission from a population of dust grains heated by starlight, including very small grains for which stochastic heating by starlight photons results in high temperature transients. Because state-to-state transition rates are generally unavailable for complex molecules, we consider model PAH, graphitic, and silicate grains with realistic vibrational mode spectra and realistic radiative properties. The vibrational density of states is used in a statistical-mechanical description of the emission process. Unlike previous treatments, our approach fully incorporates multiphoton heating effects, important for large grains or strong radiation fields.We discuss how the "temperature" of the grain is related to its vibrational energy. By comparing with an "exact" statistical calculation of the emission process, we determine the conditions under which the "thermal" and the "continuous cooling" approximations can be used to calculate the emission spectrum.We present results for the infrared emission spectra of PAH grains of various sizes heated by starlight. We show how the relative strengths of the 6.2, 7.7, and 11.3µm features depend on grain size, starlight spectrum and intensity, and grain charging conditions. We show results for grains in the "cold neutral medium", "warm ionized medium", and representative conditions in photodissociation regions. Our model results are compared to observed ratios of emission features for the Milky Way and other galaxies, and for the M17 and NGC 7023 photodissociation regions.

We present the initial results from the Spitzer Survey of the Small Magellanic Cloud (S 3 MC), which imaged the star-forming body of the Small Magellanic Cloud (SMC) in all seven MIPS and IRAC wavebands. We find that the F 8 /F 24 ratio (an estimate of PAH abundance) has large spatial variations and takes a wide range of values that are unrelated to metallicity but anticorrelated with 24 µm brightness and F 24 /F 70 ratio. This suggests that photodestruction is primarily responsible for the low abundance of PAHs observed in star-forming low-metallicity galaxies. We use the S 3 MC images to compile a photometric catalog of ∼ 400, 000 mid-and far-infrared point sources in the SMC. The sources detected at the longest wavelengths fall into four main categories: 1) bright 5.8 µm sources with very faint optical counterparts and very red mid-infrared colors ([5.8] − [8.0] > 1.2), which we identify as YSOs. 2) Bright mid-infrared sources with mildly red colors (0.16 [5.8] − [8.0] < 0.6), identified as carbon stars. 3) Bright mid-infrared sources with neutral colors and bright optical counterparts, corresponding to oxygen-rich evolved stars. And, 4) unreddened early B stars (B3 to O9) with a large 24 µm excess. This excess is reminiscent of debris disks, and is detected in only a small fraction of these stars ( 5%). The majority of the brightest infrared point sources in the SMC fall into groups one to three. We use this photometric information to produce a catalog of 282 bright YSOs in the SMC with a very low level of contamination (∼ 7%).