\documentstyle{preprint}
\begin{document}
\title{HST AND MERLIN OBSERVATIONS OF 3C264---A LABORATORY FOR\\
~\\
JET PHYSICS AND UNIFIED SCHEMES}
\author{Stefi A.\ Baum,\thanks{Space Telescope Science Institute, 3700
San Martin Drive, Baltimore, MD  21218}
Christopher P.\ O'Dea,\samethanks{1}
Gabriele Giovannini,\thanks{Isituto di Radioastronomia del CNR, Bologna,
Italy}\morethanks{Astronomy Department, Bologna University, Italy}
John Biretta,\samethanks{1}\\
\and
William B.\ Cotton,\thanks{National Radio Astronomy Observatory,
Charlottesville, VA}
Sigrid de Koff,\samethanks{1}\morethanks{Sterrewacht, Leiden, The
Netherlands}
Luigina Feretti,\samethanks{2}
Daniel Golombek,\samethanks{1}\\
\and
Lucas Lara,\samethanks{2}\morethanks{IAA, CSIC, Granada, Spain}
Ferdinando D.\ Macchetto,\samethanks{1}\morethanks{On assignment from
the Space Science Department of ESA}
G.~K.\ Miley,\samethanks{5}
William B.\ Sparks,\samethanks{1}\\
\and
Tiziana Venturi,\samethanks{2}
Serguei S.\ Komissarov\/\thanks{Department of Applied Math, University
of Leeds, UK}\\
\\
email: sbaum@stsci.edu, odea@stsci.edu, ggiovannini@astbo1.bo.cnr.it,
biretta@stsci.edu,\\
bcotton@nrao.edu, dekoff@stsci.edu, lferetti@astbo1.bo.cnr.it, golombek@stsci.edu,\\
lucas@astbo1.bo.cnr.it, macchetto@stsci.edu, miley@strw.leidenuniv.nl,
sparks@stsci.edu,\\
tventuri@astbo1.bo.cnr.it, serguei@amsta.leeds.ac.uk}
\tobe{20 June 1997}{ApJ}
\recacc{19 August 1996}{23 January 1997}
\maketitle

\begin{abstract}
We present new HST optical continuum and emission line WFPC2
images and MERLIN radio observations of 3C264 at $\sim0.1$ arcsecond
resolution.  The jet is well resolved in both the optical and radio
images. In addition, we report the discovery of an apparent optical
`ring' at a projected radius of $\sim300$--400 parsecs. The ring is
most likely the manifestation of absorption by a nearly face on
circumnuclear dust disk.  We discuss the evolution of the jet properties
with distance.  The jet collimation, brightness, and orientation change
dramatically as it crosses the outer boundary of the `ring' suggesting
an interaction between the jet and dense circumnuclear gas.

We present a model for the jet propagation in which an initially
relativistic jet decelerates as it crosses through a region of dense
cold gas in the inner region of the galaxy.  We derive the equations for
brightness variations along an adiabatically expending relativistic jet
and we model the jet brightness in 3C264 as the combined effects of
Doppler boosting, and adiabatic losses as traced through the jet velocity
and width.  We find that the data are consistent with a model in which
the jet is initially highly relativistic (v$\sim0.98$c, $\gamma=5$) and
we view it at roughly 50~degrees inclination.  We suggest that 3C264 may
serve as a laboratory for the study of relativistic entraining jets and
may help us to understand the deceleration of jets which is required in
unifying schemes for FRI radio galaxies and BL~Lac objects.
\end{abstract}
\end{document}