PAH and Mid-Infrared Luminosities as Measures of Star Formation Rate in Spitzer First Look Survey Galaxies

We present evidence that the 8 μm (dust) and 24 μm luminosities of star-forming galaxies are both strongly correlated with their 1.4 GHz and Hα luminosities over a range in luminosity of 2-3 orders of magnitude. At the bright end, the correlations are found to be essentially linear over a luminosity range of about 2 orders of magnitude (corresponding to star formation rates of several tenths to several tens of solar masses per year). However, at the faint end there appears to be a slope change for dwarf galaxies, possibly due to the lower dust-to-gas ratios and lower metallicities of the dwarfs. The correlations suggest that PAH features and mid-IR continuum emissions are good measures of the star formation rates of galaxies, and we present calibrations of star formation rates based on existing radio and Hα relations. Our findings are based on a sample of star-forming galaxies selected from the main field of the Spitzer First Look Survey with the aid of spectroscopic data from the Sloan Digital Sky Survey and VLA 1.4 GHz data.