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

76 3 • IFS of a GEHR in NGC 6946
1.0
0.8
0.6
0.4
0.2
0.0
log [OIII]5007/Hβ
−0.8−0.6−0.4−0.2
−1.2 −0.8 −0.4 0.0
log [NII]6584/Hα
−1.2 −0.8 −0.4 0.0
log [SII]6717,31/Hα
Figure 3.19: Diagnostic diagrams derived from different line ratios. Left panel: Relation between
[Oiii] λ 5007/Hβ and [Nii] λ 6584/Hα. Right panel: Relation between [Oiii] λ 5007/Hβ and
[Sii] λλ 6717,6731/Hα. The solid curve represents the theoretical maximum starburst line (Kewley
et al., 2001).
It should be noted the variation of the [Oiii] λ 5007/Hβ ratio across the region, of
about 0.8 dex. This is comparable with the spread found in Hii galaxies, as in the case
of IIZw70 (Kehrig et al., 2008). On the other hand, the variation in the [Nii] λ 6584/Hα and
[Sii] λλ 6717,6731/Hα ratios are lower. Numerically, the field average ratio of log([Oiii] λ 5007/-
Hβ) = -0.08 ± 0.16, log([Nii] λ 6584/Hα) = -0.58 ± 0.07 and log([Sii] λλ 6717,6731/Hα) =
-0.54 ± 0.10 (1σ errors).
A spatial representation of these three line ratios is shown in Figure 3.20. For the sake of
clarity and comparison, Hα is also included in the upper left panel, corrected for reddening.
Unlike the reddening map, the structures that appear in the excitation maps are much
simpler. In terms of the main four knots, particularly knot A, there is a very good correlation
between the value of the intensity of Hα and the value of the ratio, being negative the sign
of the correlation for ratios log([Sii] λλ 6717,6731/Hα) and log([Nii] λ 6584/Hα) and positive
for the ratio log([Oiii] λ 5007/Hβ). As it was mentioned before, this is the expected behaviour
if in the central zones of the Hii region the sulphur, nitrogen and, oxygen were twice ionized,
while only once in the surroundings due to the increasing distance from the young stellar
cluster or ionizing source.