\documentstyle{preprint}

\input pub.sty

\begin{document}

\title{A ONE-SIDED IONIZATION CONE\\
~\\
IN THE SEYFERT~2 GALAXY
NGC~5643\thanks{Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by AURA, Inc., under NASA contract NAS5-26555} }

\author{Chris Simpson\\
\\
Space Telescope Science Institute\\
\\
3700 San Martin Drive, Baltimore, MD 21218\\
\and
A.\ S.\ Wilson\/\thanks{Adjunct Astronomer at the Space Telescope Science
Institute}\\
\\
Astronomy Department, University of Maryland\\
\\
College Park, MD 20742\\
\and
Gary Bower, T.\ M.\ Heckman,\samethanks{2} J.\ H.\ Krolik\\
\\
Department of Physics and Astronomy, The Johns Hopkins University\\
\\
Homewood Campus, Baltimore, MD 21218\\
\and
G.\ K.\ Miley\\
\\
Sterrewacht Leiden\\
\\
Postbus 9513, 2300 RA Leiden, The Netherlands}

\tobe{1 January 1997}{Astrophysical Journal}

\recacc{31 May 1996}{18 July 1996}

\maketitle

\end{document}

\abstract{
We present high-resolution (0\farcs1) observations of the Seyfert~2
galaxy NGC~5643 taken with the WFPC2 on board the Hubble Space
Telescope. Images taken in the light of \mbox{[\ion{O}{3}]}~$\lambda$5007 and
H$\alpha$ reveal a halo of emission centered on the nucleus and
high-excitation extranuclear emission which extends eastward for at
least 1.8\,kpc. An excitation map of \mbox{[\ion{O}{3}]}/H$\alpha$ reveals
the high-excitation gas to be distributed within a well-defined
V-shaped structure, presumably the projection of a 3-dimensional
cone. The apex of the cone is coincident with the peak of the red
continuum emission and shows a low \mbox{[\ion{O}{3}]}/H$\alpha$ ratio, which
is probably a result of foreground reddening. There is evidence that
the fine scale structure of the ionized gas is related to the radio
ejecta, which share the same axis as the cone. We detect a dust lane
straddling the nucleus and oriented perpendicular to the radio axis,
and an unusually blue region of $\sim 90$\,pc extent to the east of
the nucleus. Our data strongly support the unified model for Seyfert
galaxies, in which an optically thick structure perpendicular to the
radio axis blocks the nucleus from direct view and allows the optical
and ultraviolet radiation to escape preferentially along and around
this axis.}