Determine the mechanisms governing PAH destruction, survival, and migration

Determine the physical mechanisms governing polycyclic aromatic hydrocarbon destruction, survival, and migration in and around extragalactic H II regions, particularly across different evolutionary stages and interstellar-medium conditions.

Background

The paper finds that regions with lower filling factors tend to exhibit higher PAH-to-dust ratios, consistent with less efficient PAH processing in more porous ionized-gas structures. The authors interpret this behavior within a tentative evolutionary scenario in which intense radiation fields initially destroy PAHs, while later gas dispersal weakens PAH destruction.

That interpretation is not expected to be uniform because PAH behavior may also depend on local density structure, radiation fields, shocks, grain processing, and transport. The authors therefore identify the underlying mechanisms of PAH destruction, survival, and migration as unresolved.

References

As several CN-functionalized PAHs have been observed in the interstellar medium with significant abundances \citep[e.g.,][]{Mc_Guire_2018,McGuire_2021,Gabi_2024,Cernicharo_2024,Wenzel_2025} and it has been shown that CN radicals react readily with PAHs \citep[e.g.,][]{Heitkamper_2022}, the question is whether this emission could originate from the ongoing destruction of CN-functionalized PAHs. This hypothesis would be consistent with the possible top-down formation mechanisms discussed in , which could produce small hydrocarbons in predominantly atomic PDR environments.

— Sub-arcsecond imaging of HCN and CN in the Orion Bar. UV-driven warm Nitrogen chemistry in dense environments  (2609.19361 - Zannese et al., 16 Sep 2026) in Section 4.4, “Evidence of destruction of CN-functionalized PAHs?”

This evolutionary scenario is not expected to be uniform, as it ultimately depends on the particular physical conditions of the ISM in each region because the mechanisms governing PAH destruction, survival, and migration remain poorly constrained.

— H II region filling factors in NGC 628: Luminosity-size relation and connection with polycyclic aromatic hydrocarbon emission  (2609.00902 - Riesco et al., 1 Sep 2026) in Section 5, subsection 5.4, Relation with the PAH-to-dust ratio

Determining whether this deficit is due to grain destruction or ionization, however requires observations of the larger PAH grains at MIR wavelengths.

— JWST/NIRSpec Reveals Diverse Nuclear Environments in Dwarf Galaxies Hosting AGN  (2609.04315 - Bohn et al., 3 Sep 2026) in Section 6, Conclusion