Spitzermapping of Polycyclic Aromatic Hydrocarbon and H₂features in Photodissociation Regions

The mid-infrared spectra of dense photodissociation regions (PDRs) are typically dominated by emission from polycyclic aromatic hydrocarbons (PAHs) and the lowest pure rotational states of molecular hydrogen (H 2 ), the two species which are probes of the physical properties of gas and dust in intense UV radiation fields. We utilize the high angular resolution of the Infrared Spectrograph on the Spitzer Space Telescope to construct spectral maps of the PAH and H 2 features for three of the best-studied PDRs in the galaxy, NGC 7023, NGC 2023, and IC 63. We present spatially resolved maps of the physical properties, including the H 2 ortho-to-para ratio, temperature, and G o /n H . We also present evidence for PAH dehydrogenation, which may support theories of H 2 formation on PAH surfaces, and a detection of preferential self-shielding of ortho-H 2 . All PDRs studied exhibit average temperatures of ∼500–800 K, warm H 2 column densities of ∼10 20 cm −2 , G o /n H ∼ 0.1–0.8, and ortho-to-para ratios of ∼1.8. We find that while the average of each of these properties is consistent with previous single value measurements of these PDRs, when available, the addition of spatial resolution yields a diversity of values with gas temperatures as high as 1500 K, column densities spanning ∼2 orders of magnitude, and extreme ortho-to-para ratios ( R OP ) of R OP < 1 and R OP > 3.