A critical examination of the PAH hypothesis

ABSTRACT The polycyclic aromatic hydrocarbon (PAH) hypothesis proposes that the aromatic infrared bands (AIBs) observed at 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 $\mu$m originate from gas-phase PAH molecules. These bands exhibit consistent peak wavelengths and profiles in diverse sources, and Infrared Space Observatory (ISO) Short-Wavelength Spectrometer (SWS) and James Webb Space Telescope (JWST) spectra show a nearly identical red wing of the 3.3 $\mu$m AIB and blue wing of the 11.2 $\mu$m AIB in the dominant Class A sources. This spectral uniformity suggests that the AIBs arise from a small, well-defined set of gas-phase PAH species, regardless of the excitation conditions or the nature of the source such as H ii regions, reflection nebulae, planetary nebula, young stellar objects, or the diffuse interstellar medium. However, a small number of gas-phase PAH species is inconsistent with current modeling of the AIBs that require a wide range of PAH types and sizes. It is also inconsistent with the lack of observed ultraviolet (UV) and optical absorption bands from gas-phase PAH molecules. Furthermore, there is no plausible formation pathway to produce only a small number of specific PAH molecules in the interstellar medium. These issues require quantitative investigation in order to definitively establish gas-phase PAH molecules as the carrier of the AIBs.