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

3.5. Discussion 99
defined by Vilchez and Esteban (1996). We used the calibration done by Pérez-Montero and
Díaz (2005).
The combination of the S 23 and O 23 parameters gives S 23 /O 23 , defined by Díaz and Pérez-
Montero (2000), a parameter that increases monotonically with the oxygen abundance up to
oversolar regime and which is very useful to study variations over wide ranges of metallicity.
We applied the calibration found in Pérez-Montero and Díaz (2005).
The S 3 O 3 and Ar 3 O 3 parameters were defined and calibrated by Stasińska (2006). They
used the ratio of [Ariii] 7136 Å and [Oiii] 5007 Å, and [Siii] 9069 Å and [Oiii] 5007 Å emission
lines, respectively.
Finally, the N 2 O 2 parameter, defined by Pérez-Montero and Díaz (2005) as the ratio
between [Nii] and [Oii] emission lines, can be used to obtain the N/O ratio. In this case, we
have used the calibration by Díaz et al. (2007), which includes high metallicity Hii regions.
In Table 3.12 we present the oxygen and nitrogen abundances and their uncertainties for
each integrated spectra as derived using the empirical calibrators listed above. As it can be
seen, the estimated oxygen abundances coincides well with those obtained from the direct
method, all of them around the solar value.
The parameters involving the sulphur lines give slightly lower abundance values. In this
case, it should be noted that the near IR sulphur lines relative to the hydrogen recombination
lines are more affected by reddening than in the case of oxygen when no Paschen lines are
observed, which is the case. Moreover, as we showed in section 3.3.9, the near infrared spectra
are affected by atmospheric absorption. Although we corrected for this effect, we concluded
that only the [Siii] λ 9069 Å could be used. Even after the correction, if this line would still
be affected by some absorption, this would decrease the derived metallicity. Nevertheless, as
well as the rest of calibrators, the abundances are consistent taken into account the errors
in the calibrations and in the measurements.
Regarding the N 2 O 2 parameter, very little difference is found in N/O among the four
knots, which are also consistent with the ratio derived directly.
Both from the direct method and empirical calibrators, we can conclude that the four
knots have very similar abundances, with no evidence of abundance variations.
3.5.4 Ionization structure
Table 3.12 gives some general properties of the integrated spectra as the ratio of lines
representative of the ionization structure and derived ionization parameters.
Figure 3.27 shows the BPT diagrams as Figure 3.19, but including the five data from the
integrated spectra. The (red) circle points represent the values of the knots, while the (yellow)
square point the position in the diagram of the whole PPak field. As it can be appreciated,
the integrated measurements span over the point-to-point values, with the same large spread
in log([Oiii] λ 5007/Hβ) as compared with the relative small variation in the other two line