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

128 4 • Long-slit spectrophotometry of multiple knots of Hii galaxies
J1657 Knot A J1657 Knot B J1657 Knot C
He + /H + (λ4471) 0.093±0.007 0.085±0.009 0.094±0.009
He + /H + (λ5876) 0.085±0.003 0.079±0.002 0.093±0.003
He + /H + (λ6678) 0.086±0.005 0.077±0.003 0.084±0.005
He + /H + (λ7065) 0.093±0.005 0.086±0.006 0.105±0.010
He + /H + (Adopted) 0.087±0.005 0.080±0.005 0.092±0.009
He 2+ /H + (λ4686) 0.0011±0.0001 0.0028±0.0006 0.0015±0.0002
(He/H) 0.088±0.008 0.080±0.008 0.092±0.009
Table 4.6: Ionic and total chemical abundances for helium for the knots of J1657.
Ionic and total chemical abundances from forbidden lines
The oxygen ionic abundance ratios, O + /H + and O 2+ /H + , have been derived from the
[Oii] λλ 3727,29 Å and [Oiii] λλ 4959, 5007 Å lines respectively using for each ion its corresponding
temperature 3 .
At the temperatures derived for our observed galaxies, most of the oxygen is in the form
of O + and O 2+ , therefore the approximation:
has been used.
O
H = O+ + O 2+
H +
In the same way, we have derived S + /H + and S 2+ /H + , abundances using t([Sii]) and
t([Siii]) values and the fluxes of the [Sii] emission lines at λλ 6717,6731 Å and the near-IR
[Siii] λλ 9069, 9532 Å lines respectively. For IIZw71, we have assumed that T([Sii]) ≈ T([Oii]).
Unlike oxygen, for sulphur a relatively important contribution from S 3+ may be expected
depending on the nebular excitation. The total sulphur abundance has been calculated using
an ICF for S + +S 2+ according to the formula by Barker (1980), which is based on the photoionization
models by Stasińska (1978):
ICF (S + + S 2+ ) =
[ (
O 2+ ) α ] −1/α
1 − 1 −
O + + O 2+
where α = 2.5 gives the best fit to the scarce observational data on S 3+ abundances (Pérez-
Montero et al., 2006). Taking this ICF as a function of the ratio O 2+ /O instead of O + /O
reduces the propagated error for this quantity.
The ionic abundance of nitrogen, N + /H + has been derived from the intensities of the
λλ 6548,6584 Å lines and along with the assumption T e ([Nii]) ≈ T e ([Oii]).
The N/O abundance ratio has been derived under the assumption that
3 As in Chapter 3, we have used fittings to the ionic task results. Again, see Appendix B for details.