When interpreting the emission-line spectra, it is important to be able to distinguish emission produced by star-forming regions from other sources such as AGN. The conventional means for distinguishing between gas ionized by stars and nonthermal processes are diagnostic line diagrams.
They make use of reddening-corrected fluxes of the following
line: 
, 
, 
,
, 
, 
,
.
Fig. 3, 4, 5 show that appropriate ratios
of these lines
can clearly separate
extragalactic H II regions from AGNs.
We now try to give a qualitative explanation of why diagnostic
diagrams work so well.
Figure 3: Reddening-corrected 
vs. 
intensity ratios. Symbols for types
of objects are shown in keys to this figures and Fig. 4. Four
short-dahed lines are HII region models of Evans and Dopita (1985)
for 
56000, 45000, 38500 and 37000 K from the top to the bottom
respectively. Long-dahed curve represents HII region models of McCall,
Rybski and Schields (1985). Solid curve divides AGNs from HII
region-like objects.
Figure 4: Reddening-corrected 
vs. 
intensity ratios.
Symbols and curves as in Fig. 3.
Figure 5: Reddening-corrected 
vs. 
intensity ratios. Symbols and curves
as in Fig. 3 and 4.
As we have seen, AGN are characterized by
the existence of a partially ionized zone.
In this partly ionized region, 
, H, and free electrons coexist
with neutral atoms of other elements, as well as with ions having an
ionization potential similar to that of H.
The dominant forms of O, S, and N in the partly ionized zone are
, 
and 
, while smaller fractions of 
and
are also present. Hot free electrons produced in
this region by X-ray photoionization will increase the
strengths of lines produced by collisional excitation.
Important lines such 
,
, 
are
of this type.
Therefore, intensities of 
, 
and 
are larger with respect to
in narrow-line AGNs than in H II region-like objects
because collisional excitation of these lines is more important
in objects with extended partly ionized zones.
Since the ionization potential of 
matches the ionization
potential of H very well we should expect a large difference
between the 
ratio of the H II
region-like objects and that of narrow-line AGNs.
The effect is also important for
but the fact
that 
can also exist within the 
zone of H II regions
somewhat attenuates the difference between the two classes of objects.
Finally, 
is produced predominantly by UV photons
(
eV) well inside the partly ionized zone and close to the
ionizing source. The relatively larger numbers of photons that can ionize
to 
in the
power-law type spectra generally make 
larger in the AGNs than in all but
the highest H II region-like objects.
Therefore, in trying to decide whether an object is an H II region galaxy or a
narrow-line AGN, more weight should be given to the diagram of
versus 
than to the other two diagrams.