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

We have studied the relationship between the star formation rate (SFR), surface density, and gas surface density in the spiral galaxy M51a ( NGC 5194), using multiwavelength data obtained as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS ). We introduce a new SFR index based on a linear combination of H emission-line and 24 m continuum luminosities, which provides reliable extinction-corrected ionizing fluxes and SFR densities over a wide range of dust attenuations. The combination of these extinction-corrected SFR densities with aperture synthesis H i and CO maps has allowed us to probe the form of the spatially resolved star formation law on scales of 0.5Y2 kpc. We find that the resolved SFR versus gas surface density relation is well represented by a Schmidt power law, which is similar in form and dispersion to the disk-averaged Schmidt law. We observe a comparably strong correlation of the SFR surface density with the molecular gas surface density, but no significant correlation with the surface density of atomic gas. The best-fitting slope of the Schmidt law varies from N ¼ 1:37 to 1.56, with zero point and slope that change systematically with the spatial sampling scale. We tentatively attribute these variations to the effects of areal sampling and averaging of a nonlinear intrinsic star formation law. Our data can also be fitted by an alternative parameterization of the SFR surface density in terms of the ratio of gas surface density to local dynamical time, but with a considerable dispersion.

We explore variations in dust emission within the edge-on Sd spiral galaxy NGC 4631 using 3.6Y160 m Spitzer Space Telescope data and 450Y850 m JCMT data with the goals of understanding the relation between PAHs and dust emission, studying the variations in the colors of the dust emission, and searching for possible excess submillimeter emission compared to what is expected from dust models extrapolated from far-infrared wavelengths. The 8 m PAH emission correlates best with 24 m hot dust emission on 1.7 kpc scales, but the relation breaks down on 650 pc scales, possibly because of differences in the mean free paths between photons that excite the PAHs and photons that heat the dust and possibly because the PAHs are destroyed by the hard radiation fields within some star formation regions. The ratio of 8 m PAH emission to 160 m cool dust emission appears to vary as a function of radius. The 70 m/160 m and 160 m /450 m flux density ratios are remarkably constant even though the surface brightnesses vary by factors of 25, which suggests that the emission is from dust heated by a nearly uniform radiation field. Globally, we find an excess of 850 Y1230 m emission relative to what would be predicted by dust models. The 850 m excess is highest in regions with low 160 m surface brightnesses, although the magnitude depends on the model fit to the data. We rule out variable emissivity functions or $4 K dust as the possible origins of this 850 m emission, but we do discuss the other possible mechanisms that could produce the emission.

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Using the new capabilities of Spitzer and extensive multiwavelength data from SINGS, it is now possible to study the infrared properties of star formation in nearby galaxies down to scales equivalent to large H ii regions. We are therefore able to determine what fraction of large, infrared-selected star-forming regions in normal galaxies are highly obscured and address how much of the star formation we miss by relying solely on the optical portion of the spectrum. Employing a new empirical method for deriving attenuations of infrared-selected star-forming regions, we investigate the statistics of obscured star formation on 500 pc scales in a sample of 38 nearby galaxies. We find that the median attenuation is 1.4 mag in H and that there is no evidence for a substantial subpopulation of uniformly highly obscured star-forming regions. The regions in the highly obscured tail of the attenuation distribution (A H k 3) make up only $4% of the sample of nearly 1800 regions, although very embedded infrared sources on the much smaller scales and lower luminosities of compact and ultracompact H ii regions are almost certainly present in greater numbers. The highly obscured cases in our sample are generally the bright, central regions of galaxies with high overall attenuation but are not otherwise remarkable. We also find that a majority of the galaxies show decreasing radial trends in H attenuation. The small fraction of highly obscured regions seen in this sample of normal, star-forming galaxies suggests that on 500 pc scales the timescale for significant dispersal or breakup of nearby, optically thick dust clouds is short relative to the lifetime of a typical star-forming region.