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

92 3 • IFS of a GEHR in NGC 6946
6584 Å lines assuming that T([Nii]) ≈ T([Oii]). Then, the total N abundance was derived
under the following assumption:
N
O ≈ N +
O +
Neon is only visible in the spectra via the [Neiii] emission line at λ3869 Å. For this ion,
we took the electron temperature of [Oiii], as representative of the high excitation zone. The
total abundance of neon was calculated using 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+
This formula considers the overestimate of Ne/H in objects with low excitation, where
the charge transfer between O 2+ and H 0 becomes important (Izotov et al., 2004).
The only accessible emission lines of argon in the optical spectra of ionized regions correspond
to Ar 2+ and Ar 3+ . However, in the integrated spectra of the knots only [Ariii] λ 7136 Å
was measured and the abundance of Ar 2+ was calculated assuming that T([Ariii]) ≈ T([Siii])
(Garnett, 1992). As already described in section 3.4.7, [Ariv] λ 4741 Å was present in some
knots and subtracted to their WR blue bump flux, but their contribution was small to yield
any accurate abundance determination. The total abundance of Ar was then calculated using
the ICF(Ar 2+ ) derived from photo-ionization 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 the [Feiii] λ 4658 Å emission line was used and the electron temperature
of [Oiii]. We have taken the ICF(Fe 2+ ) from Rodríguez and Rubin (2004), which yields
ICF (F e 2+ ) =
( ) O
+ 0.09 ( )
O 2+ 1 + O2+
O +
The ionic and total abundances for each observed element for knots A, B, and, C are
given in Table 3.9, along with their corresponding errors.
3.5 Discussion
3.5.1 Mass of the ionized gas
We have calculated the Hα luminosities for the four knots from our observed values,
correcting for extinction according to the values found from the spectroscopic analysis. The