Fluorescent molecular hydrogen in the Eagle nebula

Abstract

We used the University of New South Wales Infrared Fabry-Perot (UNSWIRF) to investigate the photodissociation region (PDR) associated with theelephant 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 2.248mm, respectively, and in the Hi Brg line at 2.166mm. The trunk-like features have an average H2 number density of ,10 4 cm y3 and are irradiated by a far-UV ®eld ,10 4 ´ 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 purelyuorescent excitation, however a signi®cant fraction of the H2 emission (,25 per cent) from the northernmost column shows evidence forcollisionaluorescence', 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 5 cm y3 , and perhaps > 10 6 cm y3 , ®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.