Spitzer Mapping of PAHs and H2 in Photodissociation Regions

TL;DR

This study uses high-resolution infrared spectroscopy to map PAHs and H2 in three galactic PDRs, revealing detailed physical properties, evidence of PAH dehydrogenation, and spatial variations in gas conditions.

Contribution

First spatially resolved spectral maps of PAHs and H2 in multiple PDRs, providing new insights into their physical conditions and molecular processes.

Findings

Average gas temperatures of 500-800K, with some regions reaching 1500K.

H2 column densities around 10^20 cm^-2, varying by two orders of magnitude.

Detection of PAH dehydrogenation and preferential ortho-H2 self-shielding.

Abstract

The mid-infrared (MIR) 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 (H2); 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 H2 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 H2 ortho-to-para ratio, temperature, and G_o/n_H. We also present evidence for PAH dehydrogenation, which may support theories of H2 formation on PAH surfaces, and a detection of preferential self-shielding of ortho-H2. All PDRs studied exhibit average temperatures of ~500 - 800K, warm H2 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 of < 1 and > 3.