[An overview of Seyfert Galaxies]

Ionized-absorbers/emitters in AGN

Many Active Galactic Nuclei (AGN) are believed to contain substantial volumes of ionized gas close to their central regions. This gas is primarily observed by the deep absorption features which it imprints on the underlying continuum, and is often refered to as a "warm-absorber".

Under certain (primarily geometrical) conditions, the ionized gas will also give rise to observable narrow emission lines. Occasionally, especially when it is necessary to distinguish it from the absorbing gas, such gas is called the "warm-emitter".

The Gas is Photoionized

For gas close to the central source of ultra-violet, X-ray & Gamma-ray emission (often called the "primary" continuum/source), and hence bathed in its intense radiation field, the ionization state of the gas is dominated by photoionization processes. The ionization state at any given point within a gas cloud is therefore a complex function of
  • the luminosity and shape of the "primary" (photo)ionizing continuum
  • the (radial) distance of the cloud's illuminated surface from the "primary" source(s)
  • the distance of the point of interest within the cloud
  • the density of the gas at that point
  • the elemental abundances within the gas
  • any other clouds between the cloud in question and the the primary source (and, if small, their distribution in sizes compared to the size of the primary source(s))
Furthermore,
  • the luminosity (and possibly/usually shape) of the primary continuum varies on a variety of timescales in most AGN. Thus time-dependent (ie. non-equilibrium ionization) effects can be important.
  • the gas clouds themselves are not static: they are moving with respect to our line-of-sight to the primary source, undoubtedly with both transverse & radial components to their velocity.
Given these complexities, detailed observations coupled with detailed photoionization calculations (using dedicated codes) are required before we can even start to distangle the above issues. Even with these, at the current time many (hopefully reasonable) assumptions must be made.

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