Rotating Supermassive Black Holes (5) ...
The following figure illustrates the basic features of the powerhouse at the center of an
active galactic nucleus and summarizes the kinds of radiation emitted by various parts of the AGN.
Use the "Show Photons" and "Show Labels" buttons
to toggle between displaying labels for this figure and showing the dominant types of
radiation being emitted from different parts of the central region.
Ionization Cones
If our model of the central engine for an AGN is correct, there should be evidence in AGNs for
anisotropic ionization. That is, for ionization in the central region of the galaxy
that is not spherically symmetric but concentrated in particular directions from the center.
In the simplest case, there should be cone-shaped regions of ionization corresponding to the directions
from the hot accretion disk that are not blocked off by the dust torus. This ionization cone was
illustrated in the toggle of the previous figure showing the radiation emitted by the
central engine and is displayed schematically again below.
All of the region shaded in violet can "see" the hot central accretion disk and the ionizing
radiation that it is emitting. Therefore this region should be ionized by radiation from the central
engine, whereas regions lying outside the ionization cone can only be ionized by starlight from the
galaxy and not directly by the continuum emitted from the accretion disk.
Observational Evidence for Ionization Cones
The following figure illustrates possible evidence for the ionizing cones expected
in the central regions of AGNs if the unified model is correct. These are two of the
nearest active galaxies to us, lying at distances of about 18 Mpc for NGC 1068 and
about 40 Mpc for NGC 5728.
In both cases we see evidence for highly directional ionization near the center of the active galaxy
that is at least approximately cone-shaped (click the "Show Labels" button
to annotate the images). The favored interpretation is that these regions are the
cones being ionized by high-energy radiation emitted by the central engine of the AGN.
