PAHs and star formation in the HII regions of nearby galaxies M83 and
  M33
  We present mid-infrared (MIR) spectra of HII regions within star-forming
galaxies M83 and M33. Their emission features are compared with Galactic and
extragalactic HII regions, HII-type galaxies, starburst galaxies, and
Seyfert/LINER type galaxies. Our main results are as follows: (i) the M33 and
M83 HII regions lie in between Seyfert/LINER galaxies and HII-type galaxies in
the 7.7/11.3 - 6.2/11.3 plane, while the different sub-samples exhibiting
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
(Rg), 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
[NeIII]/[NeII] 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$\alpha$ luminosity; (vii) Based on the total luminosity
from PAH and FIR emission, we argue that extragalactic HII regions are more
suitable templates in modeling and interpreting the large scale properties of
galaxies compared to Galactic HII regions.
