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

90 3 • IFS of a GEHR in NGC 6946
PPak-field Knot A Knot B Knot C Knot D
n([Sii]) 55: 61 ± 20 98: 77: 73: a
t([Oiii]) b 0.78 ± 0.06 0.87 ± 0.06 0.80 ± 0.13 0.78 ± 0.10 · · ·
t([Oii]) 0.83 ± 0.11 0.80 ± 0.07 0.87 ± 0.13 c 0.78 ± 0.15 · · ·
t([Siii]) 0.61 ± 0.13 d 0.71 ± 0.12 0.63 ± 0.20 d 0.61 ± 0.17 d · · ·
a Assuming a mean temperature from the temperature measured in the other knots
b [Oiii] temperatures measured in the mosaic
c From a relation with T([Oiii]) based on photoionization models
d From an empirical relation with T([Oiii])
Table 3.7: Electron densities and temperatures for the integrated spectra of the PPak-field and the
four knots. Densities in cm −3 and temperatures in 10 4 K.
when required, the unseen ionization stages of each element, resorting to the most widely
accepted ICFs for each species:
X
H = ICF (Xi ) X+i
H +
Helium abundance
We have measured emission fluxes for 3 lines of Hei and one of Heii. The spectra presented
other helium lines, although all of them were blended with another emission line or were so
weak that they cannot be used to derive the helium abundance with the necessary accuracy.
We have therefore used the Hei λλ 4471 (only found in knot A), 5876, 6678 and 7065, and
Heii λ 4686 lines to estimate the abundances of helium once and twice ionized, respectively.
Helium lines arise mainly from pure recombination; however, they could have some contribution
from collisional excitation as well as be affected by self-absorption and, if present,
by underlying stellar absorption (see Olive and Skillman, 2001, for a complete treatment).
We have taken the electron temperature of [Oiii] as representative of the zone where the He
emission arises and we have used the equations given by Olive and Skillman (2001) to derive
the He + /H + value (see Appendix B). We have not taken into account, however, the underlying
stellar population and the corrections for fluorescence, since the involved helium lines
have a negligible dependence with optical depth effects and the observed objects have low
densities. As we have seen in section 3.4.7, the integrated spectra of the PPak-field and three
of the four knots show in their spectra the signature of the presence of WR stars by the blue
bump around λ 4600. For this reason, we have taken care when measuring the emission-line
flux of Heii λ 4686. To calculate the abundance of twice ionized helium, we have resorted
to the work by Kunth and Sargent (1983). Then, for the total abundance, we have directly
added the two ionic abundances: