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\begin{document}

\title{\protect\boldmath{$HUBBLE\ SPACE\ TELESCOPE$} OBSERVATIONS OF CANDIDATE 
YOUNG GLOBULAR CLUSTERS AND STELLAR ASSOCIATIONS IN THE RECENT MERGER REMNANT
NGC 3921\thanks{Based on observations with the NASA/ESA \hst,
obtained at the Space Telescope Science Institute, which is operated by
the Association of Universities for Research in Astronomy, Inc., under
NASA Contract NAS\/5--26555.}}

\author{Fran\c cois Schweizer\\
Department of Terrestrial Magnetism, Carnegie Institution of Washington,\\
5241 Broad Branch Road, N.W., Washington, DC 20015-1305\\
Electronic mail: schweizer@dtm.ciw.edu\\
\and
Bryan W.\ Miller\\
Department of Terrestrial Magnetism, Carnegie Institution of Washington,\\
5241 Broad Branch Road, N.W., Washington, DC 20015-1305\\
and\\
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
Maryland 21218\\
Electronic mail: bmiller@dtm.ciw.edu, bmiller@stsci.edu\\
\and
Bradley C.\ Whitmore and S.\ Michael Fall\\
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
Maryland 21218\\
Electronic mail: whitmore@stsci.edu, fall@stsci.edu}

\tobe{November 1996}{The Astronomical Journal}
\recacc{19 June 1996}{24 July 1996}

\maketitle

\abstract{
New, high-resolution images of \n3921 obtained with the Wide Field and
Planetary Camera~2 of the refurbished \hst\ are presented. \n3921 is the
remnant of two merged disk galaxies and thought to be a 0.7$\pm 0.3$~Gyr
old protoelliptical.  A search for star clusters has yielded 102 candidate
globular clusters (GC) and 49 separate, more extended stellar
associations~(A) brighter than $V=26.0$, the estimated completeness limit
(corresponding to \mv $=-8.5$ for \H0). Most of these objects are newly
discovered.  Among the best-observed objects [$V\lesssim 25$, error
\mbox{$\sigma$(\vi)$\,\leq 0.15$]}, the majority are blue and have remarkably 
uniform colors, with median (\vi)$_{\rm Cousins}=0.65$ for GC's and 0.56 
for~A's. The color indices of the GC's yield estimates of 250--750~Myr for the
median age, depending on the adopted metallicity (1.0--0.2~\zsun), and
suggest a prolonged formation period (several 100~Myr) roughly in agreement
with the estimated duration of the merger.  The most luminous candidate
GC's have absolute magnitudes \mv $=-12$ to $-$13 and are predicted to
fade by 3--4~mag over the next 15~Gyr, whence they will then have
luminosities typical of luminous old GC's.  The median half-light radius
of candidate GC's is \reff $\lesssim 5$~pc, compared with a median
\reff $=3$~pc for Milky Way globulars.  Since from their colors most of
these clusters appear to be dozens of core crossing times old, they
must be gravitationally bound. The luminosity functions of both the GC's
and the A's are well represented by a single power law,
$\phi(L) dL \propto L^{-2.1\pm 0.3} dL$ (for \mv $\leq -8.5$).
Various arguments suggest that the progenitors of these objects may have
been normal Giant Molecular Clouds rather than Supergiant Molecular Clouds.
The projected radial distribution of GC's is strongly centrally concentrated
and follows closely the $V$-light profile of the galaxy ($\sim$\rdV-law),
suggesting that these clusters or their progenitors experienced the same
violent relaxation as did the average luminous matter of the two merging
galaxies.  In contrast, the radial distribution of A's appears relatively
flat.  We estimate that the total number of GC's in \n3921 has increased by
$\gtrsim$40\% during the merger, from about $270\pm 60$ GC's before to
$\gtrsim$380 GC's after the merger. The inferred specific GC frequency is
\sn $\gtrsim 0.7$ at present and will, barring further mergers or significant
cluster destruction, increase to \sn $\gtrsim 1.4$, 1.8, and 2.9 over the
next 2, 5, and 15~Gyr as the galaxy fades.  Hence, within $\lesssim$7~Gyr
\n3921 may begin to appear as a normal elliptical with a GC population
typical for E's in low-density environments.}