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

4.3. Results 127
4.3.3 Chemical abundance derivation
We have derived the ionic chemical abundances of the different species using the strongest
available emission lines detected in the analyzed spectra and the task ionic of the STSDAS
package in IRAF, as described in Chapter §3. For details on the derived equations, see
Appendix B.
The total abundances have been calculated by taking into account, when required, the
unseen ionization stages of each element, using the appropriate ionization correction factor
(ICF) for each species:
X
H = ICF (X+i ) X+i
H +
Helium
Neither J1657 nor IIZw71 show in their spectra the signature of the presence of Wolf-
Rayet (WR) stars, as traced by the blue ‘bump’ around λ 4600 Å, and for the red ’bump’
around λ 5808 Å.
Not enough signal-to-noise was achieved in the spectra of IIZw71 in order to derive helium
abundances. Thus, this section is devoted to the results of J1657.
We have used the well detected and measured Hei λλ 4471, 5876, 6678 and 7065 Å lines,
to calculate the abundances of once ionized helium. For the three knots also the Heii λ 4686 Å
line was measured allowing the calculation of twice ionized He. The He lines arise mainly
from pure recombination, although they could have some contribution from collisional excitation
and be affected by self-absorption. We have taken the electron temperature of [Oiii]
as representative of the zone where the He emission arises since at any rate ratios of recombination
lines are weakly sensitive to electron temperature. All the details of the equations
used in the calculation of the helium abundance are presented in Appendix B. We have not
made any corrections for fluorescence since three of the used helium lines have a small dependence
with optical depth effects but the observed objects have low densities. Neither for
the presence of an underlying stellar population.
The total abundance of He has been found by adding directly the two ionic abundances:
He
H = He+ + He 2+
H +
The results obtained for each line and the total He abundances, along with their corresponding
errors are presented in Table 4.6. The adopted value for He + /H + that is the
average, weighted by the errors, of the different ionic abundances derived from each Hei
emission line, is also given.