THE CALIBRATION OF MONOCHROMATIC FAR-INFRARED STAR FORMATION RATE INDICATORS

Spitzer data at 24, 70, and 160 mu m and ground-based Ha images are analyzed for a sample of 189 nearby star-forming and starburst galaxies to investigate whether reliable star formation rate (SFR) indicators can be defined using the monochromatic infrared dust emission centered at 70 and 160 mu m. We compare recently published recipes for SFR measures using combinations of the 24 mu m and observed Ha luminosities with those using 24 mu m luminosity alone. From these comparisons, we derive a reference SFR indicator for use in our analysis. Linear correlations between SFR and the 70 mu m and 160 mu m luminosity are found for L(70) greater than or similar to 1.4 x 10(42) erg s(-1) and L(160) greater than or similar to 2 x 10(42) erg s(-1), corresponding to SFR greater than or similar to 0.1-0.3 M-circle dot yr(-1), and calibrations of SFRs based on L(70) and L(160) are proposed. Below those two luminosity limits, the relation between SFR and 70 mu m (160 mu m) luminosity is nonlinear and SFR calibrations become problematic. A more important limitation is the dispersion of the data around the mean trend, which increases for increasing wavelength. The scatter of the 70 mu m (160 mu m) data around the mean is about 25% (factor similar to 2) larger than the scatter of the 24 mu m data. We interpret this increasing dispersion as an effect of the increasing contribution to the infrared emission of dust heated by stellar populations not associated with the current star formation. Thus, the 70 (160) mu m luminosity can be reliably used to trace SFRs in large galaxy samples, but will be of limited utility for individual objects, with the exception of infrared-dominated galaxies. The nonlinear relation between SFR and the 70 and 160 mu m emission at faint galaxy luminosities suggests a variety of mechanisms affecting the infrared emission for decreasing luminosity, such as increasing transparency of the interstellar medium, decreasing effective dust temperature, and decreasing filling factor of star-forming regions across the galaxy. In all cases, the calibrations hold for galaxies with oxygen abundance higher than roughly 12 + log(O/H) similar to 8.1. At lower metallicity, the infrared luminosity no longer reliably traces the SFR because galaxies are less dusty and more transparent.