1999MNRAS.304...98A

We used the University of New South Wales Infrared Fabry-Perot (UNSWIRF) to investigate the photodissociation region (PDR) associated with the `elephant trunk' features in the M16 H ii region (the Eagle nebula). Images were made in the H<sub>2</sub> 1–0 S(1) and 2–1 S(1) lines at 2.122 and 2.248 µ m, respectively, and in the H i Br γ line at 2.166 µ m. The trunk-like features have an average H<sub>2</sub> number density of ∼ 10<sup>4</sup> cm<sup>–3</sup> and are irradiated by a far-UV field ∼ 10<sup>4</sup>xthe ambient interstellar value. The H<sub>2</sub> intensity profile across the trunks is consistent with a simple model in which cylindrical columns of gas are illuminated externally, primarily by a direct component (the stars of NGC 6611), with an additional contribution from an isotropic component (scattered light). We find that most of the H<sub>2</sub> emission from the source is consistent with purely fluorescent excitation, however a significant fraction of the H<sub>2</sub> emission (∼ 25 per cent) from the northernmost column shows evidence for `collisional fluorescence', i.e. redistribution of H₂ level populations through collisions. This emission is confined to clumps up to ∼ 0.01 pc in diameter, with densities ≥ 10⁵ cm^–3, and perhaps > 10⁶ cm^–3, filling at most a few per cent of the volume of the trunks. The line intensities and ratios are consistent with steady-state and not time-dependent PDR models.