Polycyclic Aromatic Hydrocarbon emission in Spitzer/IRS maps II: A direct link between band profiles and the radiation field strength

Abstract

We decompose the observed 7.7 $\mu$m polycyclic aromatic hydrocarbon (PAH) emission complexes in a large sample of over 7000 mid-infrared spectra of the interstellar medium (ISM) using spectral cubes observed with the Spitzer/IRS-SL instrument. In order to fit the 7.7 $\mu$m PAH emission complex we invoke four Gaussian components which are found to be very stable in terms of their peak positions and widths across all of our spectra, and subsequently define a decomposition with fixed parameters which gives an acceptable fit for all the spectra. We see a strong environmental dependence on the inter-relationships between our band fluxes - in the HII regions all four components are inter-correlated, while in the reflection nebulae (RNe) the inner and outer pairs of bands correlate in the same manner as previously seen for NGC~2023. We show that this effect arises because the RNe maps are dominated by strongly irradiated PDR emission, while the much larger HII region maps are dominated by emission from regions much more distant from the exciting stars, leading to subtly different spectral behavior. Further investigation of this dichotomy reveals that the ratio of two of these components (centered at 7.6 and 7.8 $\mu$m) is linearly related to the UV field intensity (log G${}_0$). We find that this relationship does not hold for sources consisting of circumstellar material, which are known to have variable 7.7 $\mu$m spectral profiles.

20 pages, 17 figures, ApJ Accepted

References in corpus (6)

• Variations of the Mid-IR Aromatic Features Inside and Among Galaxies
• The unusual hydrocarbon emission from the early carbon star HD 100764: The connection between aromatics and aliphatics
• Carbon-rich dust past the asymptotic giant branch: aliphatics, aromatics, and fullerenes in the Magellanic Clouds
• The PAH emission characteristics of the reflection nebula NGC2023
• The mid-infrared appearance of the Galactic Mini-Starburst W49A
• A bimodal dust grain distribution in the IC 434 HII region

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