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.