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

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 Special Astrophysical Observatory of the Russian Academy of Sciences 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 F8/F24 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 F8/F24 ratio and metallicity is confirmed in the studied complexes. The infrared flux ratio also correlates with the [O iii]λ5007/Hβ 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]λ5007/Hβ and metallicity. In separate metallicity bins, the F8/F24 ratio is found to correlate with an age of an H ii complex, which is estimated from the equivalent width of Hβ 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 aromatization, photodestruction, shattering and sputtering of very small grains, the net F8/F24 ratio is determined by a balance between all these processes that can be different at different metallicities.