2016ApJ...818...60S

We calibrate the integrated luminosity from the polycyclic aromatic hydrocarbon (PAH) features at 6.2, 7.7, and 11.3 µm in galaxies as a measure of the star formation rate (SFR). These features are strong (containing as much as 5%-10% of the total infrared luminosity) and suffer minimal extinction. Our calibration uses Spitzer Infrared Spectrograph (IRS) measurements of 105 galaxies at 0 < z < 0.4, infrared (IR) luminosities of 10⁹-10¹² L_☉, combined with other well-calibrated SFR indicators. The PAH luminosity correlates linearly with the SFR as measured by the extinction-corrected Hα luminosity over the range of luminosities in our calibration sample. The scatter is 0.14 dex, comparable to that between SFRs derived from the Paα and extinction-corrected Hα emission lines, implying that the PAH features may be as accurate an SFR indicator as hydrogen recombination lines. The PAH SFR relation depends on gas-phase metallicity, for which we supply an empirical correction for galaxies with 0.2 < Z ≲ 0.7 Z_☉. We present a case study in advance of the James Webb Space Telescope (JWST), which will be capable of measuring SFRs from PAHs in distant galaxies at the peak of the SFR density in the universe (z ∼ 2) with SFRs as low as ∼10 M_☉/yr. We use Spitzer/IRS observations of the PAH features and Paα emission plus Hα measurements in lensed star-forming galaxies at 1 < z < 3 to demonstrate the ability of the PAHs to derive accurate SFRs. We also demonstrate that because the PAH features dominate the mid-IR fluxes, broadband mid-IR photometric measurements from JWST will both trace the SFR and provide a way to exclude galaxies dominated by an active galactic nucleus.