Lessons from Non-X-ray Photoionized Spectroscopy

Gary J. Ferland

Physics Department
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.