In addition to ``extragalactic H II regions'' or ``H II-region galaxies'' there are a few galaxies with ionized gas in their nuclei that is not associated with O and B stars. Examples are Seyfert galaxies, radio galaxies, quasars, collectively called active galactic nuclei (AGN).
To assess the effect of an active nucleus on the integrated spectrum of a galaxy we included in our galaxy sample a Seyfert 2 galaxy. We now briefly summarize the most relevant properties of this class of AGNs.
The narrow emission lines
(Full Width Half Maximum 
) observed in
Seyfert 2
are much the same as the emission lines observed in H II
regions and planetary
nebulae, except that in the AGNs the range of
ionization is considerably greater.
The emission lines typically observed are the forbidden:
, 
and
but also 
, 
,
and frequently 
.
In addition to the forbidden lines, permitted
lines of HI, HeI and He II are moderately strong.
All these narrow lines are emitted by a
highly ionized gas, with roughly normal abundances of the
elements.
The physical difference that distinguishes a narrow-line AGN from an H
II region-like galaxy is the photoionizing continuum. In narrow-line
AGNs the ionizing radiation is approximated by a power-law continuum
with spectral index 
or by a
combination of two such power laws, with 
at higher
energies. So, in contrast with H II region-like objects,
narrow-line AGNs have a significant fraction of their energy in the
X-ray domain. These X-ray photons have important consequences
for the gas ionization structure. Because the absorption
cross section of 
, 
, 
, and all other ions
decrease rapidly with increasing energy, keV X-rays penetrate
deeply into the predominantly neutral region. There they produce a
large partly ionized zone, with characteristic fraction of ionized
hydrogen 
. This extended zone of partly
ionized H does not exist in H II regions photoionized by hot stars.