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

4.3. Results 129
and the N/H ratio as:
N
O = N +
O +
log N H = log N O + log O H
Neon is only visible in all the spectra via the [Neiii] emission line at λ3868 Å, then Ne 2+
has been derived using this line. For this ion we have taken the electron temperature of
[Oiii], as representative of the high excitation zone ((T e ([Neiii]) ≈ T e ([Oiii]); Peimbert and
Costero, 1969). Classically, the total abundance of neon has been calculated assuming that
Ne
O
= Ne2+
O 2+
Izotov et al. (2004) point out that this assumption can lead to an overestimate of Ne/H
in objects with low excitation, where the charge transfer between O 2+ and H 0 becomes
important. Thus, we have used the following expression for the ICF (Pérez-Montero et al.,
2007):
ICF (Ne 2+ ) ≈ 0.753 + 0.142 ·
O 2+
O + + O 2+ + 0.171 · O+ + O 2+
O 2+
Given the high excitation of the observed objects there are no significant differences
between the total neon abundance derived using this ICF and those estimated using the
classical approximation: Ne/O ≈ Ne 2+ /O 2+ .
The only accessible emission lines of argon in the optical spectra of ionized regions correspond
to Ar 2+ and Ar 3+ . The abundance of Ar 2+ has been calculated from the measured
[Ariii] λ 7136 Å line emission assuming that T e ([Ariii]) ≈ T e ([Siii]) (Garnett, 1992). No [Ariv]
line has been found in the spectra.
The total abundance of Ar was then calculated using the ICF(Ar 2+ ) derived from photoionization
models by Pérez-Montero et al. (2007):
(
ICF (Ar 2+ ) = 0.749 + 0.507 · 1 −
O 2+ )
(
O + + O 2+ + +0.0604 · 1 −
O 2+
O + + O 2+ ) −1
Finally, for iron (only measured in Knot A of J1657) we have used the emission line of
[Feiii] λ 4658 Å to calculate Fe 2+ assuming T e ([Feiii]) ≈ T e ([Oiii]).
The ICF for iron twice ionized has been taken from Rodríguez and Rubin (2004):
ICF (F e 2+ ) =
( O
+ ) 0.09
·
[1
O 2+ + O2+ ]
O +