The Calibration of Mid-Infrared Star Formation Rate Indicators
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With the goal of investigating the degree to which the MIR emission traces the SFR, we analyze Spitzer 8 and 24 m data of star- forming regions in a sample of 33 nearby galaxies with available HST NICMOS images in the Pa (1.8756 m) emission line. The galaxies are drawn fromthe SINGS sample and cover a range of morphologies and a factor 10 in oxygen abundance. Published data on local low-metallicity starburst galaxies and LIRGs are also included in the analysis. Both the stellar continuumYsubtracted 8 m emission and the 24 m emission correlate with the extinction-corrected Pa line emission, although neither relationship is linear. Simple models of stellar populations and dust extinction and emission are able to reproduce the observed nonlinear trend of the 24 m emission versus number of ionizing photons, including the modest deficiency of 24 m emission in the low-metallicity regions, which results from a combination of decreasing dust opacity and dust temperature at low luminosities. Conversely, the trend of the 8 m emission as a function of the number of ionizing photons is not well reproduced by the same models. The 8 m emission is contributed, in larger measure than the 24 m emission, by dust heated by nonionizing stellar populations, in addition to the ionizing ones, in agreement with previous findings. Two SFR calibrations, one using the 24 m emission and the other using a combination of the 24 m and H luminosities (Kennicutt and coworkers), are presented. No calibration is presented for the 8 m emission because of its significant dependence on both metallicity and environment. The calibrations presented here should be directly applicable to systems dominated by ongoing star formation.
Physics and Astronomy publications (MU)
The Astrophysical Journal, 666:870Y895, 2007 September 10