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\begin{document}

\title{THE SUPERMASSIVE BLACK HOLE OF M87 AND\\
~\\
THE KINEMATICS OF ITS ASSOCIATED GASEOUS DISK\/\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{F.~Macchetto,\thanks{Affiliated to the Astrophysics Division, 
Space Science Department, ESA.}
A.~Marconi,\thanks{Dipartimento di Astronomia e Scienza dello Spazio,
Universit\`a di Firenze, Largo E.~Fermi~5, I--50125, Italy.}
D.~J.\ Axon\/\samethanks{2}\morethanks{On leave from the University of 
Manchester.}\\
\\
Space Telescope Science Institute\\
3700 San Martin Drive\\
Baltimore, MD 21218\\
\and
A.~Capetti\\
\\
Scuola Superiore di Studi Superiori Avanzati\\
Via Beirut 2-4\\
34014 Trieste, Italy\\
\and
W.~Sparks\\
\\
Space Telescope Science Institute\\
\and
P.~Crane\\
\\
European Southern Observatory\\
Karl-Schwarzschild-Str. 2\\
D-85748 Garching bei M\"unchen, Germany}

\pub{Astrophysical Journal}
\recacc{21 April 1997}{16 June 1997}
\maketitle

\begin{abstract}
\smallskip

We have obtained long-slit observations of the circumnuclear region
of M87 at three different locations, with a spatial sampling of 0\farcs028
using the Faint Object Camera f/48 spectrograph on board HST.
These data allow us to determine the rotation  curve of the 
inner $\sim1$\arcsec\ of the ionized gas disk  in \oii\wl 3727 
to a distance as close as 0\farcs07 ($\simeq5$~pc) to the dynamic 
center, thereby significantly improving on both the spatial resolution
and coverage of previous FOS observations. We have modeled the kinematics 
of the gas under the assumption of the existence of both a central  
black hole and an extended  central mass distribution, taking into account 
the effects of the instrumental PSF, the intrinsic luminosity distribution 
of the line, and the finite size of the slit.
We find that the central mass must be concentrated within a sphere
whose maximum radius is 0\farcs05 ($\simeq$3.5pc) and show that both
the observed rotation curve and line profiles are consistent with a thin--disk
in keplerian motion. We conclude that the most likely explanation for
the observed motions is the presence of a supermassive black hole and
derive a value of $M_{BH} = (3.2\pm 0.9)\xten{9}$~\Mo\ for its mass.
\end{abstract}