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

4.4. Discussion 133
2.2
2.0
HII Galaxies
Hagele et al. (2006,2008)
J1657 Knots
1.8
t e
([SII])
1.6
1.4
1.2
1.0
n e
= 10 cm -3
n e
= 100 cm -3
0.8
0.6
0.4
0.8 1.0 1.2 1.4 1.6 1.8
t e
([OII])
Figure 4.13: Relation between t e ([Sii]) and t e ([Oii]) for knots of J1657, the objects studied by Hägele
et al. (2006) and Hägele et al. (2008) and Hii regions from the literature. The solid line represents
the one to one relation. The red dashed lines correspond to the photo-ionization models from Pérez-
Montero and Díaz (2003) for electron densities N e = 10 and 100 cm −3 . Temperatures are in units of
10 4 K.
ing a scatter which is larger than observational errors. As it was mentioned in Chapter §3, the
[Oii] temperature is somewhat dependent on density. The density effect can be seen by looking
at the model sequences which are overplotted and which correspond to photo-ionization
models from Pérez-Montero and Díaz (2003) for electron densities N e = 10, 100 and 500 cm −3 .
This relationship was used to derived the [Oii] temperature for Knot B in NGC 6946 and
for IIZw71. Higher density models show lower values of t e ([Oii]) for a given t e ([Oiii]). The
effect is more noticeable at high electron temperatures. The data points populate the region
of the diagram spanned by model sequences with most objects located between the model
sequences corresponding to N e 100 and 500 cm −3 . The three knots of J1657, however, lie
between the model sequences for N e 10 and 100 cm −3 (in fact, very close to the first one),
which is consistent with the derived values of the density.
The standard procedure in principle adopted for the analysis of low resolution and poor
signal-to-noise data is to use the (not unique) relation between the [Oii] and [Oiii] temperatures
which is now extended to data of much higher quality. The solid line in Figure 4.12
shows the relation based on the photo-ionization models by Stasińska (1990), adopted in