| Annu. Rev. Astron. Astrophys. 1999. 37:
487-531
Copyright © 1999 by Annual Reviews. All rights
reserved
|
2.2. Origin of the Broad Emission Lines
Quasar emission-line research is an example of the inverse problem in
astrophysics. We know the answer - the observed spectrum of a quasar,
and we are trying to understand the question - the conditions that
created it. Any model of the line-forming regions will have
uncertainties related to uniqueness, but these can be minimized by
considering the astrophysical context and by limiting the models to
essential properties. The essential properties of the BEL region
(BELR) are as follows:
- The BELR is photoionized. The main evidence for photoionization is
that the emission-line spectra change in response to changes in the
continuum, with lag times corresponding to characteristic radii of the
BELR (Peterson 1993).
The shape of the ionizing continuum is a fundamental parameter and is in
itself an area of active research (e.g.
Zheng et al. 1997,
Korista et al. 1997a,
Brunner et al. 1997,
Laor 1998).
Below we present calculations using simple power laws
between 1 µm and 100 keV, and we describe results that do
not depend strongly on the continuum shape.
- The BELR spans a range of distances from the central
object. The line variability or reverberation studies just mentioned
find different lag times for different ions. Highly ionized species
tend to tie closer to the continuum source. Overall, the radial
distances scale with luminosity, such that
R 0.1
(L / 1046
ergs s-1)1/2 pc is a typical value
(Peterson 1993).
- The BELR has a wide range of densities and ionization
states. The range in ionization follows simply from the lines detected,
from OI 1303 to at
least NeVIII 774
(Hamann et al. 1998).
The range in density comes mainly from the estimated
radii and photoionization theory (e.g.
Ferland et al. 1992).
Clouds (1)
with densities from 108 to >1012
cm-3 may be present. [We use the term "cloud" loosely,
referring to some localized part of the BELR but not favoring any
particular model or geometry (see
Arav et al. 1998,
Mathews & Capriotti
1985)].
Any given object could have a broad mixture of BELR properties
(Baldwin et al. 1995,
1996).
- The BELR probably has large column densities. Large column
densities, typically NH
1023
cm-2, were originally used in
BELR simulations to produce a wide range of ionizations in single clouds
(Kwan & Krolick
1981,
Ferland & Persson
1989). These
large column densities might not apply globally because we now know
that different lines form in different regions. In our calculations
below, we truncate the clouds at the hydrogen recombination front,
with the result that different clouds/calculations can have different
total column densities. However, the truncation depths are in all
cases large enough to include the full emission regions of the
relevant lines.
- Thermal velocities within clouds are believed to
dominate the local line broadening and radiative transfer. The
observed line widths are thus caused entirely by bulk motions of the
gas. This issue is important because (a) continuum
photoexcitation (pumping) can overwhelm other excitation processes if the
local line broadening (e.g. microturbulence) is large, and (b)
the line optical depths and thus photon escape probabilities (see below)
vary inversely with the amount of line broadening. The interplay between
these factors makes it hard to predict the behavior of a given line without
explicit calculations.
Shields et al. (1995)
plot some
line-strength behaviors for the particular case of low-column-density
clouds. One argument against significant microturbulence involves the
Ly /
H intensity
ratio. Simple recombination theory predicts a ratio of ~ 34
(Osterbrock 1989),
although the observed value is far smaller, closer to 10
(Baldwin 1977a).
This discrepancy is worsened by microturbulence
(Ferland 1999).
The solution probably requires severely trapped
Ly photons resulting
from large optical depths at thermal line widths (see also
Netzer et
al. 1995).
1 We use the term
"cloud" loosely, referring to some localized part of the
BELR but not favoring any particular model or geometry (see
Arav et al. 1998,
Mathews & Capriotti
1985).
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