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

TL;DR

This study uses JWST/NIRSpec IFU data to spatially resolve the PAH 3.4 μm feature in the Orion Bar and NGC 6240, revealing two distinct sub-features at 3.395 μm and 3.405 μm. Through Drude-profile decomposition and DFT-based theoretical spectra, the authors assign the 3.395 μm emission to robust aromatic-aliphatic PAHs and the 3.405 μm emission to fragile aliphatic chains that survive only when shielded by dense molecular gas. They demonstrate a strong morphological correlation between the 3.405 μm component and warm H$_2$ emission, and show that the ratio $PAH_{3.395}/PAH_{3.405} < 0.3$ marks shielded regions where only modest UV photons penetrate. The work provides a new, quantitative diagnostic for ISM conditions and the energy distribution of the photon field within molecular clouds, applicable to external galaxies as well as Galactic environments, and highlights the influence of shielding on PAH chemistry and evolution.

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.

Figures (10)

• Figure 1: Rest-frame spectrum of a representative single spaxel in the nuclear region of NGC 6240 (0 1 E, 0 4 S of the southern nucleus). Plotted flux is per spectral pixel. Median 1--$\sigma$ error across the spectral range is 4.4% of plotted flux. Observed continuum subtracted spectrum in black, fits to the PAH features in red, fits to H$_2$ features in green, post-fit residual in blue. PAH Drude profiles overlaid in magenta (shifted for clarity, see Sec. \ref{['obs_pah']} for details).
• Figure 2: Intensity maps of the PAH 3.395 $\mu$m, PAH 3.405 $\mu$m, PAH 3.3 $\mu$m and H$_2$ 0$-$0 S(8) emission features in the Orion Bar region (1$^{\prime\prime}$=0.002 pc). The three dissociation fronts (DF) are indicated in each map. Spaxel fluxes are in units of 10$^{-15}$ ergs s$^{-1}$ cm$^{-2}$. Typical errors on fluxes are in the range 10--20%, except for H$_2$ 0$-$0 S(8), where median error is 26%.
• Figure 3: DFT theoretical emission spectra of the 3-4 $\mu$m region showing aliphatic and aromatic C-H stretching vibrational modes for molecules with aliphatic side groups. From bottom left and going clockwise, Coronene C$_{24}$H$_{12}$, Coronene with methyl side-group C$_{24}$H$_{11}$-CH$_{3}$, Coronene with ethyl side-group C$_{24}$H$_{11}$-CH$_{2}$-CH$_{3}$, Coronene with vinyl side-group C$_{24}$H$_{11}$CH=CH$_{2}$.
• Figure 4: PAH 3.395$/$3.405 as function of PAH3.3$/$H$_{2}$ flux ratio for the rotational H$_{2}$ 0-0 S(8) emission line at $\lambda$ = 5.05 $\mu$m (left panel) and the Pfund${\gamma}/$H$_{2}$ ratio (right panel) for the Orion Bar. We assign those points with PAH3.395$/$3.405<0.3 to "shielded" PAHs (green circles) and the remaining points with values above this ratio as "iraddiated" PAHs (red circles), as discussed in the text.
• Figure 5: The distribution of the spatially resolved points for the PAH 3.3 (black), 3.395 (red) and 3.405 (green) $\mu$m features as a function of the H$_{2}$ 0-0 S(8) line, for the Orion Bar.