3 Excitation structure

Emission maps of NGC 2818 are shown in Fig. 1 for a variety of ionic transitions. It is clear that the source possesses a bipolar structure reminiscent of certain other type I outflows, including Mz 1, IC 4406, and M 1-8 (e.g. Corradi & Schwartz 1995). Similarly, although the resolution of the images is only moderate, it is apparent that the lower-excitation [NII] and [SII] structures appear significantly more complex than for HI and [OIII].

Such variations are also evident in Fig. 2, where we display spatial profiles along the minor axes (PA = $85^\circ$). The minor axis emission has a double-peaked structure. The southern low excitation peaks (i.e. those in [NII] and [SII]) appear broader than their higher excitation counterparts. This disparity, as we shall note later, may reflect local shock-enhancement of the lower excitation transitions.

Line ratio variations along the minor axis are illustrated in Fig. 3, whilst line-ratio maps are presented in Figs. 4 and 5. The [OIII]/H$\alpha$ emission peaks in an annulus, whilst lower excitation conditions prevail towards the nucleus and in the outer shell.

The ratios to H$\alpha$ of the low-excitation lines ([NII] and [SII]: Fig. 4) show the same complexity as in direct imaging (Fig. 1), with barely resolved line-ratio enhancements occurring at various locations within the projected shell structure. Note, particularly, the high [SII]/H$\alpha$ ratios near the northern limits of the source.

  

Figure 2: Minor axis profiles in line intensity for various ionic transitions, whence it may be noted that the low and higher excitation transitions are characterised by markedly differing structures. Note the greater extension of the lower excitation transitions towards the south (positive axial displacements), close to a region where H2 S(1) emission appears also to be enhanced (Schild 1995)

  

Figure 3: Spatial variations in line ratio along the nebular minor axis, together (bottom) with the corresponding trend in electron density $n_{\rm e}$