1998A&AS..133..381P

We report the results of narrow band imaging of the bipolar outflow source NGC 2818 in transitions of [NII], [OIII], [SII], and HI. As a consequence, we are able to assess the overall excitation properties of the shell, and determine that line ratios towards the nebular periphery are anomalous, and characteristic of planar shocks with velocity V_s≥110km/s. Pre-shock densities are likely to be modest, and of order 10²<n_p<10³cm^–3. The [SII] imaging is further used to derive a fully-sampled map of electron density, whence it is clear that the density structure is rather complex and related to various features within the excitation maps. Peak values n_e∼1.5x10³cm^–3 occur close to the nucleus, although densities subsequently decline for major axis displacements >7arcsec. The minor axis distribution, on the other hand, appears to be somewhat more extended and bilobal. It is probable that complexities in the density mapping reflect the presence of condensations and filaments within the primary shell structure, and that these are also responsible for apparent variations in density as a function of forbidden and permitted line transition.