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

Abstract We have carried out a detailed analysis of the 3.4 μm 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 μm feature: a shorter wavelength component at 3.395 μm, 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 μm 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 μm feature with the PAH 3.3 μm emission, the latter arising from robust aromatic PAH molecules. We also see an equally strong correlation between the PAH 3.405 μm 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 μm and PAH 3.395 μm 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.