Influence of the Environment on PAH Emission in Star-Forming Regions

TL;DR

This study examines how environmental factors like metallicity influence PAH emission in star-forming regions using Spitzer data, revealing PAH destruction in ionized gas and consistent photodestruction laws across different environments.

Contribution

It provides new insights into PAH behavior across various metallicity environments and clarifies the role of ionized gas in PAH destruction in star-forming regions.

Findings

PAHs are destroyed in ionized gas when [NeIII]/[NeII]>3

PAH emission sources follow a consistent photodestruction law regardless of metallicity

Starburst galaxies show PAH contributions not solely from PDRs

Abstract

We investigate the emission properties of polycyclic aromatic hydrocarbons (PAHs) in various metallicity environments with the Infrared Spectrograph on board Spitzer. Local giant HII regions are used as references as they enable access to the distinct interstellar medium components that contribute to the mid-infrared spectrum of star-forming galaxies: photodissociation regions (PDRs), photoionized gas, stellar clusters, and embedded regions. Three objects are considered, NGC3603 in the Milky Way, 30Doradus in the Large Magellanic Cloud, and N66 in the Small Magellanic Cloud. From the variations of the PAH/14um ratio, we find that PAHs are destroyed in the ionized gas for a radiation field such that [NeIII]/[NeII]>3. From the variations of the PAH/Hu-alpha ratio, we find that the PAH emission sources in the giant HII regions follow the same photodestruction law regardless of metallicity. We then compare these results with observations of starburst galaxies, HII galaxies, and blue compact dwarf galaxies (BCDs). While the integrated mid-infrared spectra of BCDs are reminiscent of a warm dusty ionized gas, we observe a significant contribution to the PAH emission in starburst galaxies that is not arising from PDRs.