2003A&A...409..193U

We present ISOCAM observations (5-17µm) of the HST M16 field, including the striking elephant trunk structures. The spectroscopic data have been obtained using the ISOCAM-CVF low-resolution mode, having a spectral resolution of ∼40 (λ/Δλ) and a pixel size of 6''. The spectra show a combination of Unidentified Infrared Band (UIB) emission centred at 6.2, 7.7, 8.6, 11.3 and 16.4µm, and atomic fine-structure lines of [ArII] 7.0µm, [ArIII] 9.0µm, [SIV] 10.5µm, [NeII] 12.8µm and [NeIII] 15.6µm. The spectra were decomposed by fitting a combination of Lorentzian line profiles and a blackbody curve to the data to produce maps of the spatial distribution of the fine-structure and UIB emission features. These maps have been compared with VLA radio, 450µm SCUBA maps, ¹²CO (J=1-0) maps, and broadband filters ISOCAM LW2 (5-8.5µm) and LW3 (12-18µm) observations. Decomposed spectral features have shown that the LW2 filter is dominated by UIB emission and the emission observed in the LW3 filter is a combination of Neon fine-structure lines and a steep continuum, with the continuum dominating. Using the photoionisation code CLOUDY we have produced theoretical values for the fine-structure ratio of [NeIII] 15.6µm/[NeII] 12.8µm, from which we have calculated the effective ionising flux impinging onto the tips of the pillars to be ∼1-3x10¹⁰photons/cm²/s, consistent with estimates made from VLA observations.