Seyfert Galaxies (3) ...
The first step in constructing a plausible model for Seyfert galaxies is to follow the hint of
our earlier characterization of Seyfert 1 galaxies that the broad lines and
narrow lines originate in different physical regions of the galaxy.
Broad Line and Narrow Line Regions
Let us refer to the region
where the broad lines originate as the broad line region or BLR, and the
region where the narrow lines originate as the narrow line region or NLR.
High velocities are required to produce the broad line region, as illustrated in
the animation concerning Doppler broadening that is shown below.
Those high velocities require
a huge mass concentration. Therefore, we hypothesize that the BLR corresponds to a region very near the
black hole at the
center of the galaxy core, while the NLR corresponds to a region lying further from the
black hole (but still in the core of the galaxy).
Properties of the Broad Line Region
More detailed considerations than we shall discuss here suggest that in order to account for
all that we know about Seyferts the BLR must consist of perhaps 10,000 solar masses of highly
ionized gas in a region lying within less than 1 parsec of the black hole. Furthermore, the
details imply that this gas is not distributed continuously in the BLR
but is in the form of small clouds
that are dense enough that they absorb UV radiation rather strongly (astronomers call these
optically thick clouds, because they are thick enough to absorb much of the light
falling on them). These clouds must be whirling around the black hole at
velocities approaching 10,000 km/s to account for the widths of emission lines produced in the
BLR.
Properties of the Narrow Line Region
On the other hand, the detailed considerations imply that the narrow line region is composed
also of clouds, but lying much further from the black hole (perhaps 100 pc up to 1000 pc away),
with lower velocities and lower densities than in the broad line region. These lower
velocities and lower densities favor narrow lines and enhance the possibility of observing
forbidden transitions in the NLR.
The following table summarizes the properties of the broad line and narrow line regions for
Seyfert galaxies.
|
Seyfert Broad Line and Narrow Line Regions
|
| Region |
Electron
Density (cm-3) |
T (K) |
Radii (pc) |
Average Cloud
Velocities (km/s)
|
| BLR |
108 - 1012 |
104 |
0.01 - 1 |
1000 - 10,000 |
| NLR |
103 - 106 |
104 |
100 - 1000 |
100 - 1000 |
|
|
|
|
|
These properties support strongly the idea that the broad line and narrow line regions
correspond to physically different portions of the galaxy core.
