THE ASTRONOMICAL JOURNAL       VOLUME 105, NUMBER 5, PAGE 1762         MAY 1993

THE METALLICITY GRADIENT AND DISTRIBUTION FUNCTION OF GLOBULAR CLUSTERS AROUND
                                   NGC 1399

                                 PABLO OSTROV

      Facultad de Ciencias Astronomicas y Geofisicas, La Plata, Argentina

                                 DOUG GEISLER

     National Optical Astronomy Observatories, Cerro Tololo Inter-American
                  Observatory, Casilla 603, La Serena, Chile
                      Electronic Mail: dgeisler@noao.edu

                               JUAN CARLOS FORTE

    Facultad de Ciencias Astronomicas y Geofisicas, La Plata, Argentina and
     Instituto de Astronomia y Fisica del Espacio, Buenos Aires, Argentina
                     Electronic Mail: forte@fcaglp.edu.ar


                                   ABSTRACT

    We have obtained integrated Washington photometry (C,M,T1) of globular
    cluster candidates in the field of NGC 1399. This galaxy, the dominant
    object in the Fornax cluster, shares with NGC 4486 (M87) the property
    of possessing one of the largest known specific frequencies of
    globulars. Conflicting evidence on the presence of a B-V color gradient
    as a function of galactocentric distance has been reported for this
    cluster system. Here we investigate the existence of a gradient using
    the C-T1 index, which is twice as metallicity sensitive as B-V. A small
    gradient is detected. This color gradient, in the sense that outer
    clusters are bluer, amounts to approx. 0.1 mag in C-T1 over a
    galactocentric range from about 0.5 to 3.5 arcmin, corresponding to a
    difference of approx. 0.2 dex in metallicity. This confirms
    qualitatively the B-V gradient suggested by Bridges et al. [AJ, 101,
    469 (1991)]. Photometry for 420 globular cluster candidates is
    presented. Metal abundances internally precise to approx. 0.25 dex and
    accurate to approx. 0.3 dex have been obtained for some 250 of the best
    candidates, after statistically subtracting a field contamination
    estimated at 30%. The mean abundance of -0.9 +/- 0.2 confirms previous
    indications that the NGC 1399 globular cluster system is, on average,
    significantly more metal rich than that of the Galaxy, as expected from
    the relative parent galaxy luminosities. Approximately 10% of the
    clusters exceed solar abundance, similar to the case for NGC 5128. The
    signature of possible globular cluster formation in mergers, as
    suggested by Ashman and Zepf [ApJ, 384, 50 (1992)], is found in the
    distinct peaks in the metallicity distribution function. However,
    their prediction (based on a simple theory designed only for
    application to normal elliptical galaxies and not the more complicated
    cD galaxies represented by NGC 1399) that the color difference between
    the clusters and the background halo light monotonically increases with
    radius is not borne out by the observations. The difference decreases
    with radius from the center to approx. 2 arcmin; beyond this radius,
    the difference does appear to increase, however. At all radii, the mean
    globular cluster abundance is some 0.25-0.5 dex more metal poor than
    that of the background halo.