Physical Conditions in the Inner Narrow Line Region of the Seyfert 2
  Galaxy Markarian 573
  We have examined the physical conditions within a bright emission-line knot
in the inner narrow-line region (NLR) of the Seyfert 2 galaxy Mrk 573 using
optical spectra and photoionization models. The spectra were obtained with the
Hubble Space Telescope/Space Telescope Imaging Spectrograph with the G430L and
G750M gratings. Comparing the spatial emission-line profiles, we found [Fe X]
6734 barely resolved, [O III] 5007 centrally peaked, but broader than [Fe X],
and [O II] 3727 the most extended. The spectra reveal that [Fe X] is broader in
velocity width and blue-shifted compared with lines from less ionized species.
Our estimate of the bolometric luminosity indicates that the active galactic
nucleus (AGN) is radiating at or above its Eddington Luminosity, which is
consistent with its identification as a hidden Narrow-Line Seyfert 1. We were
able to successfully match the observed emission line ratios with a
three-component photoionization model. Two components, one to account for the
[O III] emission and another in which the [Fe X] arises, are directly ionized
by the AGN, while [O II] forms in a third component, which is ionized by a
heavily absorbed continuum. Based on our assumed ionizing continuum and the
model parameters, we determined that the two directly-ionized components are ~
55 pc from the AGN. We have found similar radial distances for the central
knots in the Seyfert 2 galaxies Mrk 3 and NGC 1068, but much smaller radial
distances for the inner NLR in the Seyfert 1 galaxies NGC 4151 and NGC 5548.
Although in general agreement with the unified model, these results suggest
that the obscuring material in Seyfert galaxies extends out to at least tens of
parsecs from the AGN.
