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\title{THE IONIZATION STRUCTURE OF THE NLR OF NGC 1068\\
~\\
AND ITS RELATIONSHIP TO THE EXTENDED RADIO EMISSION\/\thanks{Based on
observations with the NASA/ESA Hubble Space Telescope, obtained at the
Space Telescope Science Institute, which is operated by AURA, Inc.,
under NASA contract NAS 5-26555 and by ST~ScI grant GO-3594.01-91A.}}

\author{A.\ Capetti,\thanks{Also Scuola Internazionale Superiore di Studi,
Avanzati, Via Beirut 2-4, 34014 Trieste, Italy.}
D.~J.\ Axon\/\thanks{On leave from the University of 
Manchester.}\morethanks{Associated with the Space Science Department of ESA.}
and F.~D.\ Macchetto\/\samethanks{4}\\
\\
Space Telescope Science Institute\\
\\
3700 San Martin Drive, Baltimore, MD 21218}

\tobe{November 1997}{The Astrophysical Journal}

\recacc{December 1996}{March 1997}

\maketitle

\abstract{
HST/WFPC2 imaging has been used to investigate the
emission-line structure of the Narrow Line Region (NLR)
of the Seyfert~2 galaxy NGC~1068 and
its relationship to the extended radio emission.
We find that the morphology and the ionization
properties of the NLR of NGC~1068 are dominated by the interaction with
the radio-jet, in full agreement with similar results obtained for other
Seyfert galaxies with extended radio-emission.

The emission-lines originate in two distinct regions: a large scale cocoon
enveloping the northeastern radio-lobe and a small scale broken network of 
filaments and knots surrounding the radio-jet. 

The material along the radio-jet, although denser than the surrounding gas,
is in a much higher ionization state than the remaining of the NLR. 
We believe that this requires the presence of
a source of ionizing radiation which locally dominates over the nuclear
emission, \eg free-free emission from the gas heated by the interaction
with the radio-jet.
 
At a distance of $\sim4$\arcsec\ ($\sim300$~pc) from the nucleus,
corresponding to the transition in the radio structure from jet-like to
lobe-like, the ionization structure of the NLR shows a sharp and well
defined boundary between an inner low-ionization zone and an outer
higher ionization zone. This ionization change is easily explained if there 
is a density increase where the jet enters the lobe, presumably due to the 
compression of the backflowing radio cocoon at the jet working surface.  

A color image reveals the presence of a nuclear dust lane projected onto
the location of the hidden nucleus. Nuclear dust lane appear to be
ubiquitous in Seyfert~2 galaxies observed with HST and can be naturally
associated with the material responsible for the obscuration of the
active nucleus.}