Understanding the 8 μm versus Paα Relationship on Subarcsecond Scales in Luminous Infrared Galaxies

This work explores in detail the relation between the 8 μm and the Paα emissions for 122 H II regions identified in a sample of 10 low-z LIRGs with nearly constant metallicity [12 + log (O/H) ∼ 8.8]. We use Gemini T-ReCS high spatial resolution (≲0.4″~120 pc for the average distance of 60 Mpc of our sample) mid-infrared imaging (at 8.7 or 10.3 μm), together with HST NICMOS continuum and Paα images. The LIRG H II regions extend the L8 μ m vs. LPa α relation found for H II knots in the high-metallicity SINGS galaxies by about 2 orders of magnitude to higher luminosities. Since the metallicity of the LIRG sample is nearly constant, we can rule out this effect as a cause for the scatter seen in the relationship. In turn, it is attributed to two effects: age and PAH features. The L8 μ m/LPa α ratio, which varies by a factor of 10 for the LIRG H II regions, is reproduced by a model with instantaneous star formation and ages ranging from ~4 to 7.5 Myr. The remaining dispersion around the model predictions for a given age is probably due to differential contributions of the PAH features (the 8.6 μm, in our case) to the 8 μm emission from galaxy to galaxy.