3 The observational material and the reduction technique

Figure 2: Detailed structure (in logarithmic scale) of some representative emissions at the nine observed PA of NGC 6818. The blue-shifted gas is to the left. The slit orientation at each PA is indicated in the corresponding [O I] frame. The $\lambda$6300.304 Å night sky line has been removed from the [O I] spectral image.

Nine echellograms of NGC 6818 (exposure time 600 s; spectral resolution 60 000 with a slit 1.0 arcsec wide) have been obtained on July 29, 2000 at the ESO NTT in photometric sky conditions and seeing between 0.7 and 1.0 arcsec. We have used a 40 arcsec long slit centered on the exciting star for all the selected PA, ranging from 10 $\hbox {$^\circ $ }$ to 170 $\hbox {$^\circ $ }$ with a constant step of 20 $\hbox {$^\circ $ }$. Since we do not insert an interference filter (as normally done by the other observers to isolate a single order), each spectrum covers 80 echelle orders ($\lambda$3967 Å of [Ne III] to $\lambda$7751 Å of [Ar III]), and provides the spatio-kinematical structure of a good two dozen nebular emissions, representing all the main ionic species.

The reduction method follows conceptually the standard procedure, including bias, flat field, distortion correction, wavelength and flux calibration, and is carefully described in Paper IV.

Figure 2 illustrates the detailed structure in [O I] ($\lambda$6300 Å), [N II] ($\lambda$6584 Å), H I ($\lambda$6563 Å), [O III] ($\lambda$5007 Å) and He II ($\lambda$4686 Å) at the observed PA. NGC 6818 exhibits a complex ionization structure:
- [O I] is only seen in the outermost regions, in the form of distinct condensations, which in some cases are symmetrically arranged (the moustaches), otherwise single (like the cometary knot in PA $=150\hbox{$^\circ$ }$, southern edge);
- the low ionization [N II] emission forms a double envelope of inhomogeneous structure from PA $=50\hbox{$^\circ$ }$ to PA $=130\hbox{$^\circ$ }$, and a single, elongated and distorted ring from PA $=150\hbox{$^\circ$ }$ to PA $=30\hbox{$^\circ$ }$. The presence of FLIERS (fast, low ionization emitting regions), as introduced by Balick et al. (1993, see also Corradi et al. 1996), is not confirmed here: although some isolated spikes present a faint, high velocity tail, the overall [N II] emission of NGC 6818 can be understood in terms of "normal'' nebular regions simply shadowed by some inner and dense layer causing the ionization drop in the outer plasma;
- the H$\alpha$ line (central row in Fig. 2) is characterized by a very blurred appearance (due to a combination of thermal motions plus fine structure plus expansion velocity gradient), masking the detailed distribution of the ionized gas. The same effect is also present in He I and He II, but it is particularly damaging for H$\alpha$, the reference emission in both the radial electron density and ionization determinations (Paper IV);
- $\lambda$5007 $\AA\/$ of [O III] (a mean-high excitation ion), by far the strongest line in the optical region, highlights the double envelope structure of NGC 6818: the inner shell (better seen between PA $=50\hbox{$^\circ$ }$ and PA $=130\hbox{$^\circ$ }$) is an irregular ellipsoid broken along the major axis (N-S direction) whose equatorial, denser regions are identified by the "moustaches''. The line-tilt between PA $=90\hbox {$^\circ $ }$ and PA $=110\hbox{$^\circ$ }$ suggests that we are misaligned with both the intermediate and the minor axes of the inner ellipsoid, and that the line of the nodes is at PA $\simeq 60\hbox{$^\circ$ }$. The outer shell is almost un-tilted, spherical, holed at North and South, and circumscribes the internal one;
- the He II emission (bottom row in Fig. 2) marks the highest excitation nebular regions, mainly constituted by the inner shell. Note the blurred appearance of $\lambda$4686 Å, essentially due to the thirteen fine structure components.

The first, qualitative picture of the spatial structure coming from the echellograms confirms the indications already obtained from the imaging (Sect. 2).

 

Table 1: Peak separation in the cspl of NGC 6818.

Ion	IP (eV)			2 $V{\rm exp}$ (km s-1)
		Wilson (1950)	Sabbadin (1984)	Meatheringham et al. (1988)	Hyung et at. (1999)	this paper
[O I]	0.0	-	-	-	-	72:
[S II]	10.4	-	-	-	60.7	71
[O II]	13.6	60.2	-	59.1	58.9	69:
H I	13.6	55.5	52	-	42.6	55
[N II]	14.5	-	64	-	61.8	70
[S III]	23.4	-	-	-	50.8	65:
He I	24.6	-	-	-	57.3	66
[Ar III]	27.6	-	-	-	48.9	64
[O III]	35.1	56.2	54	55.1	51.3	62
[Ar IV]	40.7	-	-	-	30.7	48
[Ne III]	41.0	58.0	-	-	52.2	63
N III	47.4	-	-	-	41.5	-
He II	54.4	42.4	-	41.9	31.5	45
[Ar V]	59.8	-	-	-	20.2	36
[Ne V]	97.1	32.6	-	-	-	-