Lessons from other wavelengths

I outline some lessons learned from spectroscopic observations of AGN done at lower energies. First, spectroscopy is subject to powerful selection effects. We can only detect gas that has the right density, temperature, and ionization to effectively emit the light we see. Although a temperature and density can be derived using standard plasma diagnostics, it is often the case that a range of conditions are actually present. The "Locally Optimally-emitting Cloud" (LOC) model, which uses distribution functions to describe a range of cloud parameters, was developed to treat this situation. The second lesson is the need to consider emission at all wavelengths when studying an environment. I discuss two examples - He-like ions seen in the X-ray should also emit UV lines. Although they have not yet been identified, their detections or upper limits would constrain the conditions in the gas. Another example is the very hot dust that emits the continuum near 1 μm. The gas associated with the dust is likely to have T ≈ 10 ⁶ K and emit in the X-rays. These are all examples of the insight that large-scale plasma simulations can bring to understand conditions in a physical environment.