Infrared Spectral Energy Distributions of Nearby Galaxies

Dale, D. A., Bendo, G. J., Engelbracht, C. W., Gordon, K. D., Regan, M. W., Armus, L., Cannon, J. M., Calzetti, D., Draine, B. T., Helou, G., Joseph, R. D., Kennicutt, R. C., Li, A., Murphy, E. J., Roussel, H., Walter, F., Hanson, H. M., Hollenbach, D. J., Jarrett, T. H., Kewley, L. J., Lamanna, C. A., Leitherer, C., Meyer, M. J., Rieke, G. H., Rieke, M. J., Sheth, K., Smith, J. D. T., & Thornley, M. D.
2005, The Astrophysical Journal, 633, 857

The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out a comprehensive multiwavelength survey on a sample of 75 nearby galaxies. The 1-850 ЮМm spectral energy distributions (SEDs) are presented using broadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. The infrared colors derived from the globally integrated Spitzer data are generally consistent with the previous generation of models that were developed using global data for normal star-forming galaxies, although significant deviations are observed. Spitzer's excellent sensitivity and resolution also allow a detailed investigation of the infrared SEDs for various locations within the three large, nearby galaxies NGC 3031 (M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapes is found within each galaxy, especially for NGC 3031, the closest of the three targets and thus the galaxy for which the smallest spatial scales can be explored. Strong correlations exist between the local star formation rate and the infrared colors fЮН(70 ЮМm)/fЮН(160 ЮМm) and fЮН(24 ЮМm)/fЮН(160 ЮМm), suggesting that the 24 and 70 ЮМm emission are useful tracers of the local star formation activity level. Preliminary evidence indicates that variations in the 24 ЮМm emission, and not variations in the emission from polycyclic aromatic hydrocarbons at 8 ЮМm, drive the variations in the fЮН(8.0 ЮМm)/fЮН(24 ЮМm) colors within NGC 3031, NGC 5194, and NGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sample are representative of the range present at high redshift, then extrapolations of total infrared luminosities and star formation rates from the observed 24 ЮМm flux will be uncertain at the factor of 5 level (total range). The corresponding uncertainties using the redshifted 8.0 ЮМm flux (e.g., observed 24 ЮМm flux for a z=2 source) are factors of 10-20. Considerable caution should be used when interpreting such extrapolated infrared luminosities.

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