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

136 4 • Long-slit spectrophotometry of multiple knots of Hii galaxies
(2005) were derived using empirical calibrations since no temperature sensitive lines were
observed.
The logarithmic N/O ratios found for J1657 are -1.23 ± 0.11, -1.35 ± 0.16 and -1.36 ±
0.15 for knots A, B, and C, respectively, which are remarkably constant. The derived values
are on the high log(N/O) side of the distribution for this kind of objects (see Figure 4.15).
For IIZw71, although the difference between knots B and C is of 0.2 dex, due to the large
errors, it can be consider as well fairly constant.
In general, the common procedure of obtaining t e ([Oii]) from t e ([Oiii]) using Stasińska
(1990) relation and assuming t e ([Oii]) = t e ([Nii]), yields N/O ratios larger than using the
measured t e ([Oii]) values since, in most cases, the model sequence over-predicts t e ([Oii]). In
our particular case, as we have seen, for the three knots of J1657 t e ([Oii]) is under-predicted
by Stasińska (1990) models; for Knot B the difference is more than 2000 K. This would
decrease the N/O ratio by more than 0.1 dex.
The log(S/O) ratios found are the quite similar for Knots A and B of J1657, with -1.53
and -1.57, respectively, and higher for Knot C, with -1.32. The average error is 0.12. For
IIZw71, both knots shows a very similar value. They are consistent with log(S/O) ⊙ = -1.36,
the solar value (Grevesse and Sauval, 1998) within the observational errors (see Figure 4.15).
The logarithmic Ne/O ratio is remarkably constant of both objects, with a mean value
of 0.75, despite, for example, the differences in oxygen abundances between knot A and the
other two in J1657. They are consistent with solar value, log(Ne/O) = -0.61 dex 4 , as shown
in Figure 4.16.
Finally, the Ar/O ratios found show a very similar value for Knot A and B, while Knot
C has a ratio higher by 0.2 dex. Both knots of IIZw71 have the same ratio. The mean value
is consistent with solar 5 (see Figure 4.16).
4.4.2 Chemical abundances from empirical calibrators for Knots A and
D of IIZw71
The emission-line spectra of the four star-forming knots in IIZw71 are remarkably similar,
implying similar values for ionization parameter, ionization temperature, and chemical
abundances. We derived the ionization parameters from the ratio of the [Oii] and [Oiii] lines
according to the expression given in Díaz et al. (2000) (see Chapter §3). They are similar
in all the knots ranging from 6.42×10 −4 for knot A to 9.67×10 −4 for knot C. Using these
values, the corrected Hα fluxes, and the sizes of the regions from Hα images we can calculate
the density of the emitting gas (Diaz et al., 1991). This is similar for the four knots with a
value of about 20 particles · cm −3 , consistent with the upper limits we derived from the ratio
of the [Sii] lines and providing filling factor for the gas of a few times 10 −2 , values common
4 Oxygen from Allende Prieto et al. (2001) and neon from Grevesse and Sauval (1998).
5 Oxygen from Allende Prieto et al. (2001) and argon from Grevesse and Sauval (1998).