6 Conclusions

We have obtained narrow band imaging of the bipolar outflow source NGC 2818 in the transitions [NII] $\lambda 6584$ Å, [OIII] $\lambda 5007$ Å, HI $\lambda 6563$ Å, and [SII] $\lambda\lambda 6717$ Å and 6731 Å. As a result, it has proved possible to confirm the overall complexity in outflow structure noted by previous authors, with filaments and condensations in the direct imaging having their counterparts in line-ratio mapping.

The map of projected electron density appears to be rather complex, and suggests the presence of higher density condensations and filaments within a lower density structure. Several of the density enhancements appear to be related to corresponding features in the excitation maps, and might constitute evidence for jet activity.

Of particular interest is an apparent extension in the [SII], [NII] minor axis structure towards the south, in a region where shocked H₂ S(1) emission has previously been observed by Schild (1995). This (together with correspondingly enhanced [SII]/H$\alpha$, [NII]/H$\alpha$ line ratios) may constitute evidence for shock excitation of optical transitions towards the nebular periphery. Comparison between the ratios [OIII]/H$\beta$, [SII]/H$\beta$ and planar shock modelling implies that shock velocities are likely to be of order $V_{\rm s} \geq 110$ km s^-1, whilst pre-shock densities (neutrals + ions) are in the range 10² cm$^{-3} < n_{\rm p} < 10^3$ cm^-3, consistent with our density mapping.

Although an outflow with such velocities has not yet been observed in this source, it is conceivable that some such component explains the overall nebular morphology.