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

3.4. Results 75
3.4.6 Ionization structure and excitation
The hardness (fraction of high energy photons) of the radiation and density of the gas
are two of the main factors that drive the different ionization states of the metals. A “hard”
spectrum increases the ionization degree, and this hardness increases with the temperature
of the star. Since at higher number of electrons there are more recombinations and, as a
result, more low ionization species, density is also an important variable.
The ionization degree should be measured estimating the ionic abundance of an element.
Nevertheless, this implies the estimation of temperature, and measurement of very low intensity
lines. To study the ionization degree throughout the nebula, the calculation of the ionic
abundance at each point is not feasible. An alternative is to measure the so-called diagnostic
ratios. These diagnostics involve two or three strong emission lines which result depends
strongly on the ionization degree and, to a lesser extent, on temperature or abundance. The
most common ones are [Oiii] λ 5007/Hβ, [Nii] λ 6584/Hα and, [Sii] λλ 6717,6731/Hα.
The ionization potential of Oiii is high (54.89 eV) as compared with that of Hi (13.6
eV). Thus, the emission of [Oiii] λ 5007 Å is related to highly ionized gas. On the other hand,
the ionization potential of Si is low (10.36 eV) while Sii has a intermediate value (23.40
eV). Therefore, the emission coming from [Sii] λ 6717 Å and [Sii] λ 6731 Å is related to low
ionization and the ratio to Hα will decrease with the degree of ionization. The behaviour
of the [Nii] λ 6584/Hα is similar to [Sii] λλ 6717,6731/Hα, but care has to be taken when
dealing with gas of very low ionization degree. The ionization potential of Ni (14.53 eV) is
slightly higher than that of Hi, so [Nii] λ 6584/Hα could decrease when the ionization degree
increases. Summing up, the flux ratio [Oiii] λ 5007/Hβ is a good indicator of the mean level of
ionization (radiation field strength) and temperature of the gas, whereas [Nii] λ 6584/Hα or
[Sii] λλ 6717,6731/Hα are indicators of the number of ionizations per unit volume (ionization
parameter).
Diagnostic diagrams (BPT, Baldwin et al., 1981; Veilleux and Osterbrock, 1987) can
be used to distinguish two possible ionization sources, AGN or massive stars in star-forming
regions, by comparing the ratios mentioned before. It is known that different objects populate
quite different regions on these diagnostic diagrams depending on the ionization source.
Figure 3.19 shows [Oiii] λ 5007/Hβ versus [Nii] λ 6584/Hα and [Oiii] λ 5007/Hβ versus
[Sii] λλ 6717,6731/Hα nebular diagnostic diagrams for all measurements in all IFU positions.
On these plots, we also show the theoretical “maximum starburst line” derived by Kewley
et al. (2001), indicating a conservative theoretical limit for pure photoionization from the
hardest starburst ionizing spectrum that can be produced. For points to exist above or to
the right-hand side of this threshold, and additional contribution to the excitation from
non-photoionization processes is required. From these classical BPT diagnostic diagrams it
is clear that the line ratios for all the position in the region are located in the general locus
of Hii-region like objects.