The Calibration of Mid‐Infrared Star Formation Rate Indicators

Abstract

With the goal of investigating the degree to which the mid-infrared emission\ntraces the star formation rate (SFR), we analyze Spitzer 8 um and 24 um data of\nstar-forming regions in a sample of 33 nearby galaxies with available\nHST/NICMOS images in the Paschen-alpha (1.8756 um) emission line. The galaxies\nare drawn from the Spitzer Infrared Nearby Galaxies Survey (SINGS) sample, and\ncover a range of morphologies and a factor ~10 in oxygen abundance. Published\ndata on local low-metallicity starburst galaxies and Luminous Infrared Galaxies\nare also included in the analysis. Both the stellar-continuum-subtracted 8 um\nemission and the 24 um emission correlate with the extinction-corrected\nPa-alpha line emission, although neither relationship is linear. Simple models\nof stellar populations and dust extinction and emission are able to reproduce\nthe observed non-linear trend of the 24 um emission versus number of ionizing\nphotons, including the modest deficiency of 24 um emission in the low\nmetallicity regions, which results from a combination of decreasing dust\nopacity and dust temperature at low luminosities. Conversely, the trend of the\n8 um emission as a function of the number of ionizing photons is not well\nreproduced by the same models. The 8 um emission is contributed, in larger\nmeasure than the 24 um emission, by dust heated by non-ionizing stellar\npopulations, in agreement with previous findings. Two SFR calibrations, one\nusing the 24 um emission and the other using a combination of the 24 um and\nH-alpha luminosities (Kennicutt et al. 2007), are presented. No calibration is\npresented for the 8 um emission, because of its significant dependence on both\nmetallicity and environment. The calibrations presented here should be directly\napplicable to systems dominated by on-going star formation.\n