We plan to measure IR coronal lines in a sample of AGN with
exceptionally strong forbidden high-ionization lines in their
visual spectra. Observations of coronal lines permit the derivation of
the flux density of energetic photons if the lines are photo-ionized.
Hence these lines allow to measure the photon flux at energies
of up to a few hundred eV, which cannot be measured directly because
of (extragalactic and galactic) absorption of the UV and soft X-ray
radiation. Therefore, such lines provide an efficient calorimeter for
photons in the far-UV regime. The luminosity at energies of the order
100 eV can be probed by sampling lines of different ionization
potentials. Since the strongest forbidden high-ionization lines (FHILs)
are all concentrated in the near- and mid-IR (while the
visual and groundbased IR ranges contain relatively weak FHILs only)
ISO observations can provide particularly important information
on this subject. In detail we wish to observe a set of objects which
(according to a survey carried out by our group) shows exceptionally
strong coronal lines in their visual spectra. Our objects furthermore
cover a range of line widths which may reflect a range of the central
mass and specific orientation. We will combine our IR results with our
groundbased data to derive possible dust obscuration effects and
corrections and we will compare our combined data with existing
model computations to derive the radiation field causing the FHILs.
Morover, we plan to use the line profile information from the IR
spectra to obtain further constraints on the location and geometry of
the coronal line emitting regions within the AGN complex.