THE ASTRONOMICAL JOURNAL    VOLUME 109, NUMBER 3, PAGE 960           MARCH 1995

          HUBBLE SPACE TELESCOPE OBSERVATIONS OF YOUNG STAR CLUSTERS
                   IN NGC 4038/4039, "THE ANTENNAE" GALAXIES

                              BRADLEY C. WHITMORE

           Space Telescope Science Institute, 3700 San Martin Drive,
                           Baltimore, Maryland 21218
                      Electronic mail: whitmore@stsci.edu

                              FRANCOIS SCHWEIZER

   Carnegie Institution of Washington, Department of Terrestrial Magnetism,
                5241 Broad Branch Road, NW, Washington DC 20015
                    Electronic mail: schweizer@bmrt.ciw.edu


                                   ABSTRACT

    New high-resolution images of the disks of NGC 4038/4039 obtained with
    the Wide Field Camera of the Hubble Space Telescope (HST) are
    presented.  NGC 4038/4039, nicknamed "The Antennae," is a prototypical
    example of a pair of colliding galaxies believed to be at an early
    stage of a merger.  Down to the limiting magnitude of V ~ 23 mag, the
    HST images reveal a population of over 700 blue pointlike objects
    within the disks.  The mean absolute magnitude of these objects is M_V
    = -11 mag, with the brightest objects reaching M_V ~ -15.  Their mean
    apparent color indices are U-V = -0.7 mag and V-I = 0.8 mag on the
    Johnson UVI passband system, while their mean indices corrected for
    internal reddening are (U-V)0 = -1.0 mag and (V-I)0 = 0.5.  Their mean
    effective radius, determined from slightly resolved images, is 18 pc
    (for H0 = 50 km/s/Mpc). Based on their luminosities and resolution,
    most of these objects cannot be individual stars, but are likely young
    compact star clusters.  The brighter ones are similar to the objects
    found in NGC 1275 and NGC 7252, which appear to be young globular
    clusters formed during recent galaxy mergers.  Based on their U-V and
    V-I colors, the brightest, bluest clusters of NGC 4038/4039 appear to
    be less than 10 Myr old.  Most of these bright clusters are relatively
    tightly clustered themselves, with typically a dozen individual
    clusters belonging to a complex identified as a giant H II region from
    ground-based observations.  The cluster luminosity function (LF) is
    approximately a power law, Phi(L)dL is proportional to
    L^(-1.78+/-0.05)dL, with no hint of a turnover at fainter magnitudes.
    This power-law shape agrees with the LF of Magellanic Cloud clusters
    and Galactic open clusters, but differs from the LF of old globular
    cluster systems that is typically Gaussian with a FWHM of ~3 mag.
    Possible explanations for this apparent difference include: (1) We have
    not observed faint enough to see the turnover, (2) the initial LF of
    star clusters is a power law but the fainter objects dissolve with
    time, (3) conditions at the present epoch favor the formation of a wide
    range of cluster masses while conditions at earlier epochs favored the
    formation of massive clusters, and (4) the NGC 4038/4039 clusters may
    not evolve into normal globular clusters.  Besides the blue clusters,
    we also find about a dozen extremely red objects with V-I > 3.0.  The
    highest number density of these red objects is found in the SE
    quadrant, where star formation appears to be most recent.  We propose
    that these objects may be very young star clusters still embedded in
    their placental dust cocoons.