Quantification of the Age Dependence of Mid-infrared Star Formation Rate Indicators

We combine James Webb Space Telescope images of the nearby galaxy NGC 5194 in the hydrogen recombination line Paα (1.8756 μm) from the Cycle 1 program JWST-FEAST with 21 μm dust continuum images from the Cycle 2 Treasury program JWGT to quantify the difference in the calibration of mid-infrared star formation rates (SFRs) between H II regions and galaxies. We use archival Hubble Space Telescope Hα imaging to correct the Paα emission for the effects of dust attenuation. Our data confirm previous results that the dust-corrected Paα flux is tightly correlated with the 21 μm emission at the scales of H II regions. When combined with published JWST data for the H II regions of the galaxy NGC 628 and Spitzer Space Telescope 24 μm data for whole galaxies and for kiloparsec-size galaxy regions, we show that the L(24)-L(Paα) relation has exponent > 1 across six decades in luminosity. In addition, the hybrid 24 μm + Hα SFR indicator has a scaling constant about 4.4 times higher for H II regions than for whole galaxies, also in agreement with previous results. Models of stellar populations with a range of star formation histories reveal that the observed trends can be entirely ascribed to and quantified with the contribution to the infrared emission by stellar populations older than ∼5-6 Myr. Based on the models’ results, we provide (1) a calibration for the infrared SFR across 6 orders of magnitude in L(24), from H II regions to luminous galaxies, and (2) a prescription for the scaling constant of the hybrid infrared SFR indicators as a function of the star formation timescale.