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

TL;DR

This study analyzes mid-infrared spectra of HII regions in nearby galaxies M83 and M33, revealing how PAH features relate to star formation, radiation fields, and galaxy properties, and establishing new PAH-based star formation rate calibrations.

Contribution

It provides a comparative analysis of PAH features in extragalactic HII regions and introduces PAH-based star formation rate calibrations, highlighting differences from Galactic regions.

Findings

M33 and M83 HII regions occupy an intermediate position between Seyfert/LINER and HII-type galaxies.

The 11.3/3.3 PAH ratio better tracks PAH size than the 6.2/7.7 ratio.

PAH/VSG ratio decreases with radiation hardness and galactocentric radius.

Abstract

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.