Fluorescent molecular hydrogen in the Eagle nebula

We used the University of New South Wales infrared Fabre-Perot (UNSWIRF) to investigate the photodissociation region (PDR) associated with the 'elepha nt trunk' features in the M16 H II region (the Eagle nebula). Images were m ade in the H-2 1-0 S(1) and 2-1 S(1) lines at 2.122 and 2.248 mu m, respect ively, and in the H I Br gamma line at 2.166 mu m. The trunk-like features have an average H-2 number density of similar to 10(4) cm(-3) and are irrad iated by a far-UV held similar to 10(4) x the ambient interstellar value. T he H-2 intensity profile across the trunks is consistent with a simple mode l in which cylindrical columns of gas are illuminated externally, primarily by a direct component (the stars of NGC 6611), with an additional contribu tion from an isotropic component (scattered light). We find that most of th e H-2 emission from the source is consistent with purely fluorescent excita tion, however a significant fraction of the H-2 emission (similar to 25 per cent) from the northernmost column shows evidence far 'collisional fluores cence', i.e, redistribution of H-2 level populations through collisions. Th is emission is confined to clumps up to similar to 0.01 pc in diameter, wit h densities greater than or equal to 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 in tensities and ratios are consistent with steady-state and not time-dependen t PDR models.