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 HII region (the Eagle nebula). Images were made in the H₂ 1-0 S(1) and 2-1 S(1) lines at 2.122 and 2.248 μm, respectively, and in the HI Brγ line at 2.166 μm. The trunk-like features have an average H₂ number density of ∼10⁴cm^-3 and are irradiated by a far-UV field ∼10⁴x the ambient interstellar value. The H₂ 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 H₂ emission from the source is consistent with purely fluorescent excitation, however a significant fraction of the H₂ 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 ≥ 10⁵cm^-3, and perhaps > 10⁶cm^-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.