Optical and infrared emission of H II complexes as a clue to the PAH life cycle

TL;DR

This study investigates how the ratio of infrared emission features in H II complexes relates to their physical properties, revealing that PAH abundance is influenced by metallicity, age, and UV radiation, with complex formation and destruction processes involved.

Contribution

It provides new insights into the factors affecting PAH evolution in H II complexes, emphasizing the role of metallicity, age, and radiation in shaping PAH abundance.

Findings

Confirmed correlation between $F_{8}/F_{24}$ ratio and metallicity.

Found $F_{8}/F_{24}$ correlates with age in low metallicity complexes.

Suggested PAH abundance is affected by UV radiation, not just destruction.

Abstract

We present an analysis of optical spectroscopy and infrared aperture photometry of more than 100 H II complexes in nine galaxies. Spectra obtained with the 6-m telescope of SAO RAS are used along with archival data from Spitzer and several ground-based telescopes to infer a strength of polycyclic aromatic hydrocarbon (PAH) emission, age, properties of the UV radiation field, and metallicity of studied H II complexes. Physical properties (age, radiation field parameters, metallicity) are related to the $F_{8}/F_{24}$ ratio used as a proxy for the PAH abundance in order to reveal factors that may influence the PAH evolution in H II complexes. The well-known correlation between the $F_{8}/F_{24}$ ratio and metallicity is confirmed in the studied complexes. The infrared flux ratio also correlates with the [O III]$\lambda 5007/\mathrm{H\beta}$ ratio which is often considered as an indicator of the radiation field hardness, but this correlation seems to be a mere reflection of a correlation between [O III]$\lambda 5007/\mathrm{H\beta}$ and metallicity. In separate metallicity bins, the $F_{8}/F_{24}$ ratio is found to correlate with an age of an H II complex, which is estimated from the equivalent width of $\mathrm{H}\beta$ line. The correlation is positive for low metallicity complexes and negative for high metallicity complexes. Analysing various mechanisms of PAH formation and destruction in the context of found correlations, we suggest that PAH abundance is likely altered by the UV radiation within H II complexes, but this is not necessarily due to their destruction. If PAHs can also form in H II complexes due to some processes like aromatisation, photodestruction, shattering and sputtering of very small grains, the net $F_{8}/F_{24}$ ratio is determined by a balance between all these processes that can be different at different metallicities.