Comparing infrared star formation rate indicators with optically derived quantities

We examine the UV reprocessing efficiencies of warm dust and polycyclic aromatic hydrocarbons (PAHs) through an analysis of the mid- and far-infrared surface luminosity densities of 85 nearby Hα-selected star-forming galaxies detected by the volume-limited KPNO (Kitt Peak National Observatory) International Spectroscopic Survey (KISS). Because Hα selection is not biased towards continuum-bright objects, the KISS sample spans a wide range in stellar masses (10^8-10¹²M_⊙), as well as Hα luminosity (10^39-10⁴³ erg s^-1), mid-infrared 8.0 μm luminosity (10⁴¹-10⁴⁴ erg s^-1), and [Bw - R] colour (-0.1-2.2). We find that mid-infrared PAH emission in the Spitzer InfraRed Array Camera (IRAC) 8.0 μm band correlates with star formation, and that the efficiency with which galaxies reprocess UV energy into PAH emission depends on metallicity. We also find that the relationship between far-infrared luminosity in the Spitzer Multiband Imaging Photometer for Spitzer 24 μm band pass and Hα-measured star formation rate varies from galaxy to galaxy within our sample; we do not observe a metallicity dependence in this relationship.We use optical colours and established mass-to-light relationships to determine stellar masses for the KISS galaxies; we compare these masses to those of nearby galaxies as a confirmation that the volume-limited nature of KISS avoids strong biases. We also examine the relationship between IRAC 3.6 μm luminosity and galaxy stellar mass, and find a colour-dependent correlation between the two.