Polycyclic aromatic hydrocarbons in spatially resolved extragalactic star forming complexes

TL;DR

This study investigates how the abundance of polycyclic aromatic hydrocarbons (PAHs) varies in star-forming regions of nearby galaxies, revealing a consistent correlation with metallicity at smaller scales using infrared data.

Contribution

It provides spatially-resolved analysis of PAH abundance in over 200 extragalactic star-forming complexes, confirming the metallicity-PAH relationship at smaller scales.

Findings

PAH abundance correlates with metallicity in star-forming regions.

Lower-metallicity complexes have less effective PAH formation.

Ultraviolet radiation fields are weaker in low-metallicity environments.

Abstract

The abundance of polycyclic aromatic hydrocarbons (PAHs) in low- and high-metallicity galaxies has been widely discussed since the time when detailed infrared data for extragalactic objects were first obtained. 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 & Kewley metallicity calibration is used. We discuss possible reasons for this correlation, noting that traces of less-effective PAH formation in low-metallicity AGB stars should be smeared out by radial mixing in galactic disks. Effective destruction by the harder and more intensive ultraviolet field in low-metallicity environments is qualitatively consistent with our data, as the ultraviolet field intensity, derived from the infrared photometry, is indeed smaller in HII complexes with lower metallicity.