A Critical Examination of the PAH Hypothesis

TL;DR

This paper critically reexamines the PAH hypothesis for Aromatic Infrared Bands (AIBs), arguing that the remarkable spectral uniformity of the main bands across diverse astrophysical environments implies a small, well-defined PAH set rather than a vast, variable population. By leveraging high-quality JWST and ISO SWS data, it shows that the wings and peak positions of the 3.3 and 11.2 μm bands place stringent constraints on candidate PAHs, while also revealing tensions with standard PAH models and with the lack of UV/optical PAH absorption features. The authors discuss unresolved issues including the constancy of the wings, the 11.2/3.3 ratio, the unidentified 217 nm feature, and the plateau/continuum components, highlighting the need for quantitative formation/survival analyses and potentially alternative carrier materials. The work underscores a need for targeted laboratory and theoretical studies to determine whether gas-phase PAHs can consistently account for the AIBs or whether other carbonaceous materials must be invoked.

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 mic originate from gas-phase PAH molecules. These bands exhibit consistent peak wavelengths and profiles in diverse sources, and ISO SWS and JWST spectra show a nearly identical red wing of the 3.3 mic AIB and blue wing of the 11.2 mic 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 HII 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 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.

Figures (9)

• Figure 1: The normalized JWST spectra of the atomic PDR region and DF3 regions of the Orion Bar (black and red solid lines, respectively) after subtraction of the continuum and plateau emission. The units of the surface brightness are MJy sr$^{-1}$. For comparison the normalized ISO spectrum of NGC 7027 from Tokunaga21 is shown (black dashed line). The JWST spectra of IRAS 21282, M17, NGC 1333, and NGC 2023 (grey solid lines) are from Boersma23. Note that the ISO spectrum of NGC 7027 was taken with a 14$\times$ 20 aperture while the JWST spectra were taken with a 3 diameter aperture.
• Figure 2: Expanded view of the red wing of the 3.3 $\mu$m band. The one sigma error bars of the atomic PDR and DF3 spectra are shown as shaded areas around each profile. The inset shows that the error bars are overlapping and therefore $\lambda_R (3.3)_{1/2}$ for the atomic PDR and DF3 spectra is not significantly different within the uncertainty of the observations.
• Figure 3: Comparison of the normalized 11.2 $\mu$m AIB profile in the JWST atomic PDR and DF3 regions of the Orion Bar to the ISO SWS spectra of the Orion Bar H2S1, TY CrA, and NGC 7027. Emission lines have been removed. In the PAH hypothesis, the weaker emission band at 11.0 $\mu$m is attributed to PAH cations.
• Figure 4: Expanded view of the blue wing of the 11.2 $\mu$m band. The one sigma error bars of the ISO SWS spectra (Orion Bar H2S1, TY CrA, and NGC 7027) are shown as shaded regions on either side of the profiles. The error bars for the JWST profiles of the atomic PDR and DF3 regions are too small to be seen, but uncorrected residual fringes are present.
• Figure 5: Comparison of the 6.2 $\mu$m AIB. The continuum was removed as described by Peeters02. The peak wavelength is nearly the same in all of the sources. Although the FWHM varies, it is no wider than that of DF3.