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

3.5. Discussion 101
1.0
0.8
0.6
0.4
0.2
0.0
log [OIII]5007/Hβ
A
−0.8−0.6−0.4−0.2
B
C
D
−1.2 −0.8 −0.4 0.0
log [NII]6584/Hα
A
B
C
D
−1.2 −0.8 −0.4 0.0
log [SII]6717,31/Hα
Figure 3.27: BPT diagrams including the values of the integrated spectra. The (red) circle points
represents the values of the knots, while the (yellow) square point the position in the diagram of the
whole PPak field.
ratios. As expected, the integrated value of the whole PPak field appears as a relative high
excitation value, implying that Knot A is weighting in the integrated spectrum being the
most important contribution to the total spectrum.
The volume-averaged ionization parameter u is defined as
u =
Q
4πR 2 nc
where Q is the ionizing photon luminosity, R is the radius of the Strömgren sphere, n the
number density of the gas, and c the speed of light. This parameter is essentially the local
ratio of Lyman continuum photons to gas density, which determines the degree of ionization
at any particular location within the nebula. Table 3.12 contains the ionization parameter
as estimated from the [Oii]/[Oiii] and [Sii]/[Siii] ratios. For the last one, Diaz et al. (1991)
gives
log u = −1.68 log([Sii]/[Siii]) − 2.99
For the former ratio, the ionization parameter can be calculated from (Díaz et al., 2000)
log u = −0.80 log([Oii]/[Oiii]) − 3.02
As it can be appreciated, the value of u derived from the [Oii]/[Oiii] ratio is systematically
lower than the one obtained from the [Sii]/[Siii] ratio. This could imply low effective