and Cosmology

5. Active Galactic Nuclei
196
5.4.2 The Broad Emission Lines
Fig. 5.20. Spectrum of the QSO PKS0405−123 at z = 0.57
(data points with error bars) from the NIR and the optical
up to the UV spectral region, plus a model for this spectrum
(solid curve). The latter combines various components:
(1) the radiation from an accretion disk that causes the big blue
bump and whose spectrum is also shown for three individual
radius ranges, (2) the Balmer continuum, and (3) an underlying
power law which may have its origin in synchrotron
emission
Planck spectrum at all radii of the disk is too simple
because the structure of the accretion disk is more complicated.
The spectral properties of an accretion disk
have to be modeled by an “atmosphere” for each radius,
similar to that in stars.
Besides the BBB, an additional maximum exists in
the MIR (IR-bump). This can be described by thermal
emission of warm dust (T 2000 K). Later in this chapter
we will discuss other observations which provide
additional evidence for this dust component.
The optical continuum of blazars is different from
that of Seyfert galaxies and QSOs. It often features
a spectral pattern that follows, to very good approximation,
a power law and is strongly variable and
polarized. This indicates that the radiation is predominantly
non-thermal. The origin of this radiation thus
probably does not lie in an accretion disk. Rather, the
radiation presumably has its origin in the relativistic
jets which we already discussed for the radio domain,
with their synchrotron radiation extending up to optical
wavelengths. This assumption was strongly supported
by many sources where (HST) observations discovered
optical emission from jets (see Fig. 5.12 and Sect. 5.5.4).
Characteristics of the Broad Line Region. One of the
most surprising characteristics of AGNs is the presence
of very broad emission lines. Interpreted as Doppler
velocities, the corresponding width of the velocity distribution
of the components in the emitting region is
of order Δv 10 000 km/s (orΔλ/λ 0.03). These
lines cannot be due to thermal line broadening because
that would imply k B T ∼ m p (Δv) 2 /2 ∼ 1MeV,
or T ∼ 10 10 K – no emission lines would be produced
at such high temperatures because all atoms would be
fully ionized (plus the fact that at such temperatures
a plasma would efficiently produce e + e − -pairs, and
the corresponding annihilation line at 511 keV should
be observable in Gamma radiation). Therefore, the observed
line width is interpreted as Doppler broadening.
The gas emitting these lines then has large-scale velocities
of order ∼ 10 000 km/s. Velocities this high are
indicators of the presence of a strong gravitational field,
as would occur in the vicinity of a SMBH. If the emission
of the lines occurs in gas at a distance r from
a SMBH, we expect characteristic velocities of
√
GM•
v rot ∼
r
= c √
2
( r
r S
) −1/2
.
so for velocities of v ∼ c/30, we obtain a radial distance
of ( ) r
∼ 500 .
r S
Hence, the Doppler broadening of the broad emission
lines can be produced by Kepler rotation at radii of
about 1000 r S . Although this estimate is based on the
assumption of a rotational motion, the infall velocity
for free fall does not differ by more than a factor √ 2
from this rotational velocity. Thus the kinematic state of
the emitting gas is of no major relevance for this rough
estimate if only gravity is responsible for the occurrence
of high velocities.
The region in which the broad emission lines are produced
is called the broad-line region (BLR). The density
of the gas in the BLR can be estimated from the lines that
are observed. To see this, it must be pointed out that allowed
and semi-forbidden transitions are found among
the broad lines. Examples of the former are Lyα, MgII,
and CIV, whereas CIII] andNIV] are semi-forbidden