Lessons from Non-X-ray Photoionized Spectroscopy
Gary J. Ferland
U of Kentucky
Lexington, KY 40506
The two greatest lessons I have learned from analysis of optical,
UV, and IR spectra of Active Galactic Nuclei (AGN) are to beware
of the atomic physics selection effects introduced by inhomogeneities,
and to consider the consequences of a particular model derived
from one spectral region for allothers, including the X-ray.
The "Locally Optimally-emitting Clouds" (LOC) model
of AGN was developed in response to variability observations
showing that the emitting gas was spatially extended. Atomic
physics selection effects, together with gas with a broad range
of density and distance from the source of ionizing radiation,
reproduce the observed spectrum, including profiles, intensities,
and variability, without resorting to finely tuned free parameters.
A second example is the long-standing attempts at understanding
the very hot dust, T ~ 1,000K, which is known to exist in AGN.
Most work considered the dust by itself, neglecting the gas which
must also be present. Detailed predictions of the properties
of both the dust and gas show that the gas associated with the
dust is hot, with a temperature of roughly a million degrees,
and so likely to be visible in the X-rays.