Interstellar polycyclic aromatic hydrocarbons (PAHs), omnipresent across the Universe, play an intrinsic part in the formation of stars, planets and possibly even life itself. While the generic PAH infrared (IR) emission features are now routinely used to trace star formation and as redshift indicators for distant, dust obscured, galaxies, the PAH spectra themselves contain a wealth of untapped information. For example, relative intensities of the different PAH features provide a qualitative, sensitive measure of the state of the PAH population and changing relative intensities track changes in local physical conditions. Here the PAH emission in the Spitzer-IRS spectral map of the northwest photon dominated region (PDR) in the Iris nebula (NGC 7023) is broken down into contributing PAH subclasses (charge, structure and size) using exclusively the tools and spectra made available through The NASA Ames PAH IR Spectroscopic Database (http://www.astrochem.org/pahdb) and an appropriate PAH emission model. The variations in the contributing PAH subclasses across NGC 7023 are then linked to variations in PAH band strength ratios, which traditionally are considered qualitative proxies for the different aspects of the state of the PAH population, i.e., charge, structure, and size. This enables, the first, quantitative calibration of the PAH proxies and, subsequently, a direct connection with local physical conditions via the PAH ionization parameter. Resulting maps of the northwest PDR show an evolution in contributing PAH subclasses that paint a coherent astrophysical picture and are able to trace subtle changes in morphology, radiation field, PDR boundary, etc., revealing the interplay between PAH (photo)chemistry and changing local physical conditions in the emission zones.

Tying interstellar PAH emission spectra and (photo)chemistry to local physical conditions in the emission zones

249th National Meeting (2015)

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