AGN with strong forbidden high-ionisation lines selected from the Sloan
  Digital Sky Survey
  We have defined a sample of 63 AGN with strong forbidden high-ionisation line
(FHIL) emission. These lines, with ionisation potentials >~ 100eV, respond to a
portion of the spectrum that is often difficult to observe directly, thereby
providing constraints on the EUV-soft X-ray continuum. The sources are selected
from the Sloan Digital Sky Survey (SDSS) on the basis of their [Fe X]6374A
emission, yielding one of the largest and the most homogeneous sample of
FHIL-emitting galaxies. We fit a sequence of models to both FHILs ([Fe XI], [Fe
X] and [Fe VII]) and lower-ionisation emission lines ([O III], [O I], H-alpha,
[N II], [S II]) in the SDSS spectra. These data are combined with X-ray
measurements from Rosat, which are available for half of the sample. The
correlations between these parameters are discussed for both the overall sample
and subsets defined by spectroscopic classifications. The primary results are
evidence that: (1) the [Fe X] and [Fe XI] lines are photoionised and their
strength is proportional to the continuum flux around 250 eV; (2) the
FHIL-emitting clouds form a stratified outflow in which the [Fe X] and [Fe XI]
source regions extend sufficiently close to the BLR that they are partially
obscured in Seyfert 2s whereas the [Fe VII] source region is more extended and
is unaffected by obscuration; (3) narrow-lined Seyfert 1s (NLS1s) tend to have
the strongest [Fe X] flux (relative to lower-ionisation lines); and (4) the
most extreme [Fe X] ratios (such as [Fe X]/[O III] or [Fe X]/[Fe VII]) are
found in the NLS1s with the narrowest broad lines and appear to be an
optical-band indication of objects with strong X-ray soft excesses.
