PAH in extragalactic star-forming complexes

On the scales of entire galaxies, a smaller PAH abundance in lower metallicity galaxies is often observed. We study this relationship for star-forming regions in nearby galaxies, for a sample containing more than 200 HII complexes, using spatially resolved observations from the Herschel Space Observatory and Spitzer Space Telescope.

We use a model for the dust emission to estimate the physical parameters (PAH abundance, metallicity, ultraviolet radiation field, etc.) of these complexes. The same correlation of PAH abundance with metallicity, as seen for entire galaxies, is apparently preserved at smaller scales, at least when the Kobulnicky and Kewley metallicity calibration is used. We discuss possible reasons for this correlation, noting that traces of less effective PAH formation in low-metallicity asymptotic giant branch stars should be smeared out by radial mixing in galactic discs.