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

4.4. Discussion 135
same region and therefore the temperatures of [Oii], [Sii] and [Nii] are expected to show
similar values, once allowance is made for a possible density effect for the first two. In Figure
4.13 we show the relation between t e ([Sii]) and t e ([Oii]) for the knots of J1657, the objects
studied by Hägele et al. (2006) and Hägele et al. (2008) and those from the literature.
The relations derived by Pérez-Montero and Díaz (2003) using photo-ionization models for
electron densities N e = 10 and 100 cm −3 are also shown. Any dependence on density is difficult
to appreciate. The measurements for the knots of J1657 of the [Sii] and [Oii] temperatures
are located below the model sequence for N e = 100 cm −3 . Nevertheless, due to the large errors
associated to knots B and C, the values for both t e ([Oii]) and t e ([Sii]) are consistent with a
similar temperature for each ion for the three knots.
The situation, in general, seems to be better for the [Siii] temperature. Figure 4.14 shows
the relation between t e ([Siii]) and t e ([Oiii]) for the knots of J1657 and the compilation of
published data on Hii galaxies for which measurements of the nebular and auroral lines
of [Oiii] and [Siii] exist, thus allowing the simultaneous determination of T e ([Oiii]) and
T e ([Siii]). The solid line in the figure shows the model sequence from Pérez-Montero and
Díaz (2005), which differs slightly from the semi-empirical relation by Garnett (1992), while
the other two lines correspond to the relations given by Izotov et al. (2006) for low and
intermediate metallicity Hii regions. The three model sequences are coincident for temperatures
in the range from 12000 and 17000 K, and very little, if any, metallicity dependence
is predicted. The three knots of J1657, however, lie at both sides of the one to one relation.
As can be seen, although Knots B and C have similar T e ([Oiii]) temperature, they have a
difference in T e ([Siii]) of 2500 K. Since the number of objects is small, more high quality
data are needed in order to confirm the relation between the [Oiii] and [Siii] temperatures
and obtain an empirical fit with well determined rms errors.
Abundances
The abundances derived for the three knots of J1657 and knots B and C of IIZw71 show
the characteristic low values found in strong line Hii galaxies. These values are between
7.78 and 7.99. From the high quality data of J1657 the mean error values for the oxygen
and neon abundances are 0.05 dex, 0.12 for sulphur and 0.07 for argon. Knots B and C of
J1657 show a similar value of 12+log(O/H), while Knot A is almost 0.2 dex higher than
the other pair. In the case of IIZw71, the derived ionic abundances in knots B and C are
very similar, leading to total O/H abundances of 12+log(O/H) = 7.90 in knot C and 7.95 in
knot B. These values deviate slightly from the metallicity reported in the literature for the
integrated object, which is 12+log(O/H) = 8.24 (Shi et al., 2005; Kewley et al., 2005) and
the difference is even greater than the value for the nuclear region, which could be considered
as knot C, given by Kewley et al. (2005), which is 12+log(O/H) = 8.55. The value reported
by Shi et al. (2005) are based on the direct method, while those reported by Kewley et al.