Understanding the 8 μm versus Paα relationship on subarcsecond scales in luminous infrared galaxies

This work explores in detail the relation between the 8 mu m and the Pa alpha emissions for 122 H II regions identified in a sample of 10 low-z LIRGs with nearly constant metallicity [12 + log (O/H) similar to 8.8]. We use Gemini T-ReCS high spatial resolution (less than or similar to 0.4 '' similar to 120 pc for the average distance of 60 Mpc of our sample) mid-infrared imaging (at 8.7 or 10.3 mu m), together with HST NICMOS continuum and Pa alpha images. The LIRG H II regions extend the L-8 (mu m) vs. L-Pa alpha 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 L-8 mu m/L-Pa alpha 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 similar to 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 mu m, in our case) to the 8 mu m emission from galaxy to galaxy.