3.2. Near-IR/UV
The optical to UV band continuum emission is affected by dust absorption. Typically, the optical/UV continuum in type II AGN is heavily absorbed by dust and thermally reradiated at longer wavelengths. The observed continuum is dominated by the stellar contribution of the host galaxy and/or scattered emission. This latter component can be disentangled through observations in polarized light (see below for further details).
The emission-line spectrum is dominated by "narrow" emission lines
(typical widths 300 - 800 km s-1) corresponding to forbidden
atomic transitions plus Balmer lines.
This implies that the emitting gas is located farther
from the center than the broad emission line emitter (if the width is
interpreted as Keplerian motion) and that its density is lower than
~ 105 cm-3. The broad emission lines are absent in
objects classified as "pure" type II AGN (except for a weak scattered
component
visible in polarized light). Less obscured AGN are classified as type 1.9
and type 1.8 and show broad components in, respectively, the
H
line and both the H and
H
lines. The
optical spectrum of the prototype Seyfert 2 NGC 1068 is shown in
Figure 10.
 |
Figure 10. Optical spectrum of the
prototype Seyfert 2 galaxy NGC 1068.
Main emission lines are 1:
[OII] |
In heavily obscured AGN, when only emission lines are seen, it can be
hard to distinguish an AGN from a starburst (this will be discussed in
Section 4).
A major indicator of the presence of an AGN is the high ratio of
high ionization lines, such as [OIII]
5007 Å or N V, with
respect to low ionization lines, such as
H or
H.
These narrow emission lines are the result of the reprocessing of
nuclear radiation by gas not covered by the nuclear absorber because
it lies outside the obscuring region. The central emission (continuum
and broad lines) can be scattered by circumnuclear hot gas, or by dust
from some region outide of the obscuring region that lies in a direction
with a clear view of the central source. This component is typically
too weak with respect to the galaxy emission to be seen in the total
spectrum, but it clearly emerges in polarimetric observations. The
implication is that the obscuring region has a flattened distribution.
Historically, the observation of broad lines in the polarized spectrum of NGC 1068 was fundamental for the formulation of the unified model of AGN, which states that type I and type II AGN are intrinsically the same objects and differ only in the orientation of the circumnuclear absorber (Antonucci & Miller 1985).
, 4:
HeII