A spatially resolved study of ionized regions in galaxies at different ...

102 3 • IFS of a GEHR in NGC 6946
log(S23)
0.30
60¡DEC (arcsec)
50
0.25
40
0.20
30
20
0.15
10
0.10
0
0 10 20 30 40 50 60 70
RA (arcsec)
0.05
Figure 3.28: Map of the parameter S 23 .
temperatures for the ionizing stars. As expected, the highest ionization parameter is given
by Knot A.
The values of both ratios found for each knot are typical of high metallicity Hii regions,
which are found to be between -0.3 and 0.2 for log([Sii]/[Siii]) and between 0.1 and 1 for
log([Oii]/[Oiii]) (see the distribution of these ratios for high metallicity disc Hii regions in
Figure 17 of Díaz et al., 2007).
As mentioned in section 3.4.8, the Hα luminosity of the emission knots amounts to about
50% of the total PPak field of view. Knot A is the brightest of them and provides 75% of
the combined flux from the knots and 38% of the total Hα flux. This knot has the highest
excitation as characterised by the [Oii]/[Oiii] ratio and dominates the excitation deduced for
the PPak field integrated spectrum. On the other hand the measured Hβ equivalent width
for this entire region is about 40 Å, much lower than the ones measured for the individual
knots that range from 60 to 100 Å. That is, the integrated spectra shows high excitation
and a relatively evolved age which does not correspond to the individual knot evolutionary
stages.
The ionization structure of a nebula depends essentially on the shape of the ionizing
continuum and the nebular geometry. Figure 3.29 shows the point-to-point behaviour of the