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

166 Appendix B
Argon
For argon, the [Ariii] 7136 Å line is used:
12 + log(Ar 2+ /H + ) = log
( ) I(7136)
+ 6.157 + 0.808 − 0.508 · log t
I(Hβ)
t
In this case, the approximation t([Ariii]) ≈ t([Siii]) is normally followed.
Iron
In the case of iron, the [Feiii] λ 4658 Å emission line is used to derive the ionic abundance
of the iron twice ionized, using the electron temperature of [Oiii]. Care has to be taken
because this line is in the region where the blue bump of the Wolf-Rayet feature lies. Izotov
et al. (1994), using emissivities by Garstang et al. (1978), derived:
N(F e ++ )
N(H + )
= 2.3 × 10 −6 1.387 t −0.983 10 −0.0424/t√ 1.34/t I(λ4658)
t 10
I(Hβ)
which can be written in the functional form given by Pagel et al. (1992) as:
12 + log(F e 2+ /H + ) = log
( ) I(4658)
+ 6.504 + 1.298 − 0.483 · log t
I(Hβ)
t
Helium
Helium lines, as hydrogen ones in the visible spectrum, arise mainly from pure recombination,
although they have some contribution from collisional excitation and be affected
by self-absorption. There are many of them, but usually blended with other lines. The ones
generally used are Hei λλ 4471, 5876, 6678 and 7065Å, and Heii λ 4686Å to estimate the
helium abundances once and twice ionized respectively. Normally, the [Oiii] temperature is
adopted as representative of the helium zone.
Olive and Skillman (2001) carried out a very thorough study to determinate all the
contributions to the helium lines. In that work, the helium intensities are scaled to Hβ and
the singly ionized helium abundance is given by:
y + (λ) =
I(λ) E(Hβ)
I(Hβ) E(λ)
( W (λ) + aHei
W (λ)
) 1 1
1 + γ f(τ)
where E(λ)/E(Hβ) is the theoretical emissivity scaled to Hβ and W(λ) the equivalent
width. The last expression also contains a correction factor for underlying stellar absorption,
parametrized by a Hei , a density dependent collisional correction factor, (1 + γ) −1 , and
a fluorescence correction which depends on the optical depth τ.
The theoretical emissivities scaled to Hβ are taken from Smits (1996):