Fluorescent molecular hydrogen in the Eagle nebula

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 2 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 2 number density of ∼ 10 4 cm ‐3 and are irradiated by a far‐UV field ∼10 4 × the ambient interstellar value. The H 2 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 2 emission from the source is consistent with purely fluorescent excitation, however a significant fraction of the H 2 emission (∼ 25 per cent) from the northernmost column shows evidence for `collisional fluorescence', i.e. redistribution of H 2 level populations through collisions. This emission is confined to clumps up to ∼ 0.01 pc in diameter, with densities ≥ 10 5 cm ‐3 , and perhaps > 10 6 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.