Fluorescent molecular hydrogen in the Eagle nebula

AB S TRACT 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 H2 1±0 S(1) and 2±1 S(1) lines at 2.122 and 2.248mm, respectively, and in the H i Br g line at 2.166mm. The trunk-like features have an average H2 number density of ,10 4 cm and are irradiated by a far-UV ®eld ,10 ́ the ambient interstellar value. The H2 intensity pro®le 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 ®nd that most of the H2 emission from the source is consistent with purely ̄uorescent excitation, however a signi®cant fraction of the H2 emission (,25 per cent) from the northernmost column shows evidence for `collisional ̄uorescence', i.e. redistribution of H2 level populations through collisions. This emission is con®ned to clumps up to ,0.01 pc in diameter, with densities >10 cm, and perhaps > 10 cm, ®lling 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.