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

3.4. Results 89
correction, and we have propagated them through our calculations.
Electron densities have been derived from the [Sii] λ 6717,6731 Å line ratio, which is representative
of the low-excitation zone of the ionized gas. In all cases, they were found to be
lower than 100 particles per cubic centimetre, well below the critical density for collisional
de-excitation. As compared with the map of Figure 3.18, the densities derived from the
integrated spectra match well the ones obtained from the map for each particular region.
Several electron temperatures for three spectra have been measured: t([Oii]), t([Oiii])
and t([Siii]). For knot D, no auroral line was found in its integrated spectrum.
The spectral resolution is at limit to resolve the [Oii] λλ 7319,7330 Å lines, so we measured
the integrated flux of the doublet. This lines can have a contribution by direct recombination
which increases with temperature (see Appendix B). Using the calculated [Oiii] electron
temperatures, we have estimated this contribution to be less than 3 per cent in all cases and
therefore we have not corrected for this effect. For knot B, the [Oii] λλ 7319,7330 Å lines did
not have enough intensity to be measured with precision, so we derived its [Oii] temperature
from t([Oiii]) using the relations based on the photoionization models described in Pérez-
Montero and Díaz (2003), which take into account explicitly the dependence of t([Oii]) on
the electron density:
t([Oii]) =
1.2 + 0.002 · n + 4.2
n
t([Oiii]) −1 + 0.08 + 0.003 · n + 2.5
n
Although the [Oiii] λ 4363 Å auroral line was detected in the one dithering exposure, the
signal-to-noise of the mosaic was higher and the estimated errors lower. For this reason, we
have used the t([Oiii]) derived from the mosaic in the abundance analysis. At any rate, the
[Oiii] temperatures calculated from both sets of data coincide within the errors.
In the case of the sulphur, the [Siii] λ 6312 Å auroral line could only be measured with
enough precision in the integrated spectra of knot A. For the rest of the knots, the [Siii]
temperature has been estimated from the empirical relation:
t([Siii]) = (1.19 ± 0.08)t([Oiii]) − (0.32 ± 0.10)
found by Hägele et al. (2006).
The electron densities and temperatures derived for the integrated spectra of the PPakfield
and the four knots are listed in Table 3.7 along with their corresponding errors.
Chemical abundances
We have derived the ionic chemical abundances of the different species using the stronger
emission lines available detected in the spectra and the task ionic of the STSDAS package
in IRAF (see Appendix B). The total abundances have been derived by taking into account,