The Calibration of Mid-Infrared Star Formation Rate Indicators

Abstract

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.