With the goal of investigating the degree to which the mid-infrared emission traces the star formation rate (SFR), we analyze Spitzer 8 µm and 24 µm

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 estimate the conversion factor relating CO emission to H 2 mass, α CO , in five Local Group galaxies that span approximately an order of magnitude in metallicity -M 31, M 33, the Large Magellanic Cloud (LMC), NGC 6822, and the Small Magellanic Cloud (SMC). We model the dust mass along the line of sight from infrared (IR) emission and then solve for the α CO that best allows a single gas-to-dust ratio (δ GDR ) to describe each system. This approach remains sensitive to COdark envelopes H 2 surrounding molecular clouds. In M 31, M 33, and the LMC we find α CO ≈ 3-9 M ⊙ pc −2 (K km s −1 ) −1 , consistent with the Milky Way value within the uncertainties. The two lowest metallicity galaxies in our sample, NGC 6822 and the SMC (12 + log(O/H) ≈ 8.2 and 8.0), exhibit a much higher α CO . Our best estimates are α NGC6822 CO ≈ 30 M ⊙ pc −2 (K km s −1 ) −1 and α SMC CO ≈ 70 M ⊙ pc −2 (K km s −1 ) −1 . These results are consistent with the conversion factor becoming a strong function of metallicity around 12 + log(O/H) ∼ 8.4 − 8.2. We favor an interpretation where decreased dust-shielding leads to the dominance of CO-free envelopes around molecular clouds below this metallicity. 11 Hubble Fellow 12 We work with α CO , the conversion from integrated CO intensity to mass of molecular gas. A linear scaling relates α CO

We combine Hα emission-line and infrared continuum measurements of two samples of nearby galaxies to derive dust attenuation-corrected star formation rates (SFRs). We use a simple energy balance based method that has been applied previously to HII regions in the Spitzer Infrared Nearby Galaxies Survey (SINGS), and extend the methodology to integrated measurements of galaxies. We find that our composite Hα + IR based SFRs are in excellent agreement with attenuation-corrected SFRs derived from integrated spectrophotometry, over the full range of SFRs (0.01 -80 M ⊙ yr −1 ) and attenuations (0 -2.5 mag) studied. We find that the combination of Hα and total infrared luminosities provides the most robust SFR measurements, but combinations of Hα measurements with monochromatic luminosities at 24 µm and 8 µm perform nearly as well. The calibrations differ significantly from those obtained for HII regions (Calzetti et al. 2007), with the difference attributable to a more evolved population of stars heating the dust. Our results are consistent with a significant component of diffuse dust (the 'IR cirrus' component) that is heated by a non-star-forming -2population. The same methodology can be applied to [O II]λ3727 emissionline measurements, and the radio continuum fluxes of galaxies can be applied in place of IR fluxes when the latter are not available. We assess the precision and systematic reliability of all of these composite methods.-4 -The advent of large sets of multi-wavelength observations of nearby galaxies now provides us with the opportunity to derive attenuation-corrected Hα and UV continuum luminosities of galaxies by combining these fluxes with various components of the IR emission. Moreover the availability of integrated optical spectra (and in some cases Pa α maps) of the same galaxies allows us to derive additional attenuation estimates, and test the precision and systematic reliability of the respective attenuation-corrected SFR measurements.The Spitzer Infrared Nearby Galaxies Survey (SINGS; Kennicutt et al. 2003) offers an ideal dataset for testing and calibrating such multi-wavelength SFR estimators. The survey includes imaging of a diverse sample of 75 galaxies within 30 Mpc, with wavelength coverage extending from the UV to the radio, including ultraviolet imaging at 150 and 230 nm, Hα, and 7 infrared wavelengths over 3.6 -160 µm. In addition, drift-scanned spectra over the wavelength range 3600 -6900Å are available, which complement matching infrared spectra over the range 10 -40 µm. We first applied these data to calibrate and test the combined use of Hα and 24 µm infrared fluxes of individual HII regions to derive attenuation-corrected emission-line fluxes (Calzetti et al. 2007, hereafter denoted C07;Kennicutt et al. 2007;Prescott et al. 2007). These studies revealed that the Spitzer 24 µm sources were highly correlated in position and flux with those of optical HII region counterparts. Kennicutt et al. (2007) and C07 found that the ratio of 24 µm to Hα fluxes yielded attenuation-corrected Hα lum...

Physical dust models are presented for 65 galaxies in SINGS that are strongly detected in the four IRAC bands and three MIPS bands. For each galaxy we estimate (1) the total dust mass, (2) the fraction of the dust mass contributed by PAHs, and (3) the intensity of the starlight heating the dust grains. We find that spiral galaxies have dust properties resembling the dust in the local region of the Milky Way, with similar dust-to-gas ratio and similar PAH abundance. The observed SEDs, including galaxies with SCUBA photometry, can be reproduced by dust models that do not require ''cold'' (T P 10 K) dust. The dust-to-gas ratio is observed to be dependent on metallicity. In the interstellar media of galaxies with A O 12 þ log 10 (O/H ) > 8:1, grains contain a substantial fraction of interstellar Mg, Si, and Fe. Galaxies with A O < 8:1 and extended H i envelopes in some cases appear to have global dust-to-gas ratios that are low for their measured oxygen abundance, but the dust-to-gas ratio in the regions where infrared emission is detected generally appears to be consistent with a substantial fraction of interstellar Mg, Si, and Fe being contained in dust. The PAH index q PAH , the fraction of the dust mass in the form of PAHs, correlates with metallicity. The nine galaxies in our sample with A O < 8:1 have a median q PAH ¼ 1:0%, whereas galaxies with A O > 8:1 have a median q PAH ¼ 3:55%. The derived dust masses favor a value X CO % 4 ; 10 20 cm À2 (K km s À1 ) À1 for the CO-to-H 2 conversion factor. Except for some starbursting systems (Mrk 33, Tol 89, NGC 3049), dust in the diffuse ISM dominates the IR power.