PAHs and star formation in the H II regions of nearby galaxies M83 and M33

We present mid-infrared (MIR) spectra of H II regions within star-forming galaxies M83 and M33. Their emission features are compared with Galactic and extragalactic HII regions, H II-type galaxies, starburst galaxies, and Seyfert/LINER type galaxies. Our main results are as follows: (i) the M33 and M83 H II regions lie in between Seyfert/LINER galaxies and H II-type galaxies in the 7.7/11.3-6.2/11.3 plane, while the different sub-samples exhibit different 7.7/6.2 ratios; (ii) Using the NASA Ames PAH IR Spectroscopic database, we demonstrate that the 6.2/7.7 ratio does not effectively track PAH size, but the 11.3/3.3 PAH ratio does; (iii) variations on the 17 mu m PAH band depends on object type; however, there is no dependence on metallicity for both extragalactic HII regions and galaxies; (iv) the PAH/VSG intensity ratio decreases with the hardness of the radiation field and galactocentric radius (R-g), yet the ionization alone cannot account for the variation seen in all of our sources; (v) the relative strength of PAH features does not change significantly with increasing radiation hardness, as measured through the [Ne III]/[Ne II] ratio and the ionization index; (vi) We present PAH SFR calibrations based on the tight correlation between the 6.2, 7.7, and 11.3 mu m PAH luminosities with the 24 mu m luminosity and the combination of the 24 mu m and H a luminosity; (vii) Based on the total luminosity from PAH and FIR emission, we argue that extragalactic HII regions are more suitable templates in modelling and interpreting the large-scale properties of galaxies compared to Galactic HII regions.