The relationship of mid-infrared PAH emission to far-infrared luminosity in star-forming galaxies in the Spitzer Wide-area Infrared Extragalactic Survey

We examine the behavior of the 6-9 µ m PAH complex as a function of LIR, using integrated 8, 24, and 70 µ m fluxes for star-forming galaxies in the Spitzer Wide-area Infrared Extragalactic Survey (SWIRE). Using redshifts measured by the Sloan Digitized Sky Survey, we measure 8 µ m-FIR flux ratios for 520 galaxies at z < 0.3, with 8-1000 µ m bolometric luminosities between 8.0 < log10}(L{IR) < 11.8. We use spectral line indices from the MPA/JHU Value-Added Catalog to study the behavior of the (f8/fIR})-L_{IR relation as a function of metallicity and nuclear activity. Comparing with pre-Spitzer spectral templates (e.g., Chary & Elbaz 2001; Dale & Helou 2002) suggests that empirical refinements can be made which would improve the agreement of the mid- to far-infrared luminosity ratios. This low-redshift recalibration may be important when interpreting mid-infrared (e.g. MIPS 24 µ m) observations of galaxies at z ˜ 2.