The PAH 3.4 micron feature as a tracer of shielding in the Orion Bar and NGC 6240

TL;DR

This study analyzes the 3.4 micron PAH spectral feature using JWST data, identifying two distinct components linked to different aliphatic chains and their relation to shielding by molecular gas, providing new diagnostics for interstellar medium conditions.

Contribution

First identification of two spectral components of the PAH 3.4 micron feature in an external galaxy, linking them to molecular shielding and photon energy environments.

Findings

Distinct spectral components at 3.395 and 3.405 microns linked to different aliphatic chains.

Strong correlation between spectral features and molecular gas or aromatic PAH emission.

Flux ratio indicates regions shielded from energetic UV photons.

Abstract

We have carried out a detailed analysis of the 3.4 micron spectral feature arising from Polycyclic Aromatic Hydrocarbons (PAH), using JWST archival data. For the first time in an external galaxy (NGC 6240), we have identified two distinct spectral components of the PAH 3.4 micron feature: a shorter wavelength component at 3.395 micron, which we attribute to short aliphatic chains tightly attached to the aromatic rings of the PAH molecules; and a longer wavelength feature at 3.405 microns that arises from longer, more fragile, aliphatic chains that are weakly attached to the parent PAH molecule. These longer chains are more easily destroyed by far-ultraviolet photons (>5eV) and PAH thermal emission only occurs where PAH molecules are shielded from more energetic photons by dense molecular gas. We see a very strong correlation in the morphology of the PAH 3.395 micron feature with the PAH 3.3 micron emission, the latter arising from robust aromatic PAH molecules. We also see an equally strong correlation between the PAH 3.405 micron morphology and the warm molecular gas, as traced by H2 vibrational lines. We show that the flux ratio PAH_3.395/PAH_3.405 < 0.3 corresponds strongly to regions where the PAH molecules are shielded by dense molecular gas, so that only modestly energetic UV photons penetrate to excite the PAHs. Our work shows that PAH 3.405 micron and PAH 3.395 micron emission features can provide robust diagnostics of the physical conditions of the interstellar medium in external galaxies, and can be used to quantify the energies of the photon field penetrating molecular clouds.