and Cosmology

5. Active Galactic Nuclei
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whether they are infalling or streaming outwards, or
whether their motion is rather chaotic. It is also possible
that different regions within the BLR exist with
different kinematic properties.
Reverberation Mapping. A direct method to examine
the extent of the BLR is provided by reverberation mapping.
This observational technique utilizes the fact that
heating and ionization of the gas in the BLR are both
caused by the central continuum source of the AGN.
Since the UV radiation of AGNs varies, we expect corresponding
variations of the physical conditions in the
BLR. In this picture, a decreasing continuum flux should
then lead to a lower line flux, as is demonstrated in Fig.
5.21. Due to the finite extent of the BLR, the observed
variability in the lines will be delayed in time compared
to the ionizing continuum. This delay Δt can be identified
with the light travel time across the BLR, Δt ∼ r/c.
In other words, the BLR feels the variation in the continuum
source only after a delay of Δt. From the observed
correlated variabilities of continuum and line emission,
Δt can be determined for different line transitions, and
so the corresponding values of r can be estimated.
Such analyses of reverberation mapping are extremely
time-consuming and complex because one
needs to continuously monitor the continuum light and,
simultaneously, the line fluxes of an AGN over a long
period. The relevant time-scales are typically months
for Seyfert 1 galaxies (see Fig. 5.22). To perform such
measurements, coordinated campaigns involving many
observatories are necessary because the light curves
have to be observed without any gaps, and one should
not depend on the local weather conditions at any observatory.
From the results of such campaigns and the
correlation of the light curves in the UV continuum
and the different line fluxes (Fig. 5.23), the picture
of an inhomogeneous BLR is obtained which extends
over a large range in r and which consists of different
“layers”. The extent of the BLR scales with the
luminosity of the AGN. Its ionization structure varies
with r; the higher the ionization energy of a transition,
the smaller the corresponding radius r. For the
Fig. 5.21. In the left-hand
panel, the UV spectrum
of the Seyfert 1 galaxy
NGC 5548 is plotted for
two different epochs in
which the source radiated
strongly and weakly, respectively.
It can clearly
be seen that not only
does the continuum radiation
of the source vary
but also the strength of
the emission lines. The
right-hand panels show
the flux of the continuum
at ∼ 1300 Å, the CIV line
at λ = 1549 Å, and the
HeII line at λ = 1640 Å, as
a function of the near-UV
flux at different epochs
during an eight-month
observational campaign
with the IUE