and Cosmology

5.4 Components of an AGN
by magnetic fields, or even gravitationally. One possibility
is that the clouds are the extended atmospheres
of stars; this would, however, imply a very high (too
high?) total mass of the BLR.
5.4.3 Narrow Emission Lines
Besides the broad emission lines that occur in QSOs,
Seyfert 1 galaxies, and broad-line radio galaxies, most
AGNs (with exception of the BL Lacs) show narrow
emission lines. Their typical width is ∼ 400 km/s. This
is considerably narrower than lines of the BLR, but
still significantly broader than characteristic velocities
in normal galaxies. In analogy to the BLR, the region in
which these lines are produced is known as the narrow
line region (NLR). The strongest line from the NLR
is, besides Lyα and CIV, the forbidden [OIII] line at
λ = 5007 Å. The existence of forbidden lines implies
that the gas density in the NLR is significantly lower
than in the BLR. From estimates analogous to those
for the BLR, the characteristic properties of the NLR
are determined. It should be noted that no reverberation
mapping can be applied, since the extent of the NLR
is ∼ 100 pc. Because of this large extent, no variability
of the narrow line intensities is expected on time-scales
accessible to observation, and none has been found.
The line ratios of allowed and forbidden lines yield
n e ∼ 10 3 cm −3 for the typical density of the gas in which
the lines originate. The characteristic temperature of the
gas is likewise obtained from line ratios, T ∼ 16 000 K,
which is slightly lower than in the BLR. The filling
factor here is also significantly smaller than one, about
10 −2 . Hence, the geometrical picture of clouds in the
NLR also emerges. Like in the BLR, the properties of
the NLR are not homogeneous but vary with r.
Since the extent of the NLR in Seyfert galaxies is of
the order of r ∼ 100 pc, it can be spatially resolved for
nearby Seyfert galaxies. The morphology of the NLR is
very interesting: it is not spherical, but appears as two
cone-shaped regions (Fig. 5.24). It seems as if the ionization
of the NLR by the continuum radiation of the
AGN is not isotropic, but instead depends strongly on
the direction.
5.4.4 X-Ray Emission
The most energetic radiation of an AGN is expected to
be produced in the immediate vicinity of the SMBH.
201
Fig. 5.24. Image of the Seyfert galaxy NGC 5728. Left:
a large-scale image showing the disk galaxy; right: an HST
image of its central region taken through a filter with a small
bandwidth (narrow-band filter) centered on a narrow emission
line. This image shows the spatially resolved NLR. We can
see that it is not spherical but consists of two cones. From this,
it is concluded that the ionizing radiation of the AGN is not
isotropic, but is emitted in two preferred directions perpendicular
to the disk of the Galaxy (and thus probably perpendicular
to the central accretion disk)