THE ASTRONOMICAL JOURNAL    VOLUME 109, NUMBER 3, PAGE 1086          MARCH 1995

      A PHOTOMETRIC STUDY OF THE GLOBULAR CLUSTER M54 AND THE SAGITTARIUS
            DWARF GALAXY: EVIDENCE FOR THREE DISTINCT POPULATIONS

                                ATA SARAJEDINI

   Kitt Peak National Observatory, National Optical Astronomy Observatories,
                     P.O. Box 26732, Tucson, Arizona 85726
                         Electronic mail: ata@noao.edu

                               ANDREW C. LAYDEN

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


                                   ABSTRACT

    We present VI CCD color-magnitude diagrams for the globular cluster M54
    and the Sagittarius dwarf galaxy.  These are used to derive the
    following quantities: [Fe/H] = -1.79 +/- 0.08 for M54 and [Fe/H] =
    -0.52 +/- 0.09 for Sagittarius.  We find a metallicity dispersion of
    +/- 0.16 dex in M54, and we infer the possible existence of a [Fe/H] ~
    -1.3 component in Sagittarius, which may have a metallicity dispersion
    as well.  The mean reddening in the direction of M54 is E(B-V) = 0.13
    +/- 0.02.  The distances to M54 and Sagittarius, determined using their
    horizontal branch magnitudes, are identical to within the uncertainties
    of existing RR Lyrae luminosity-abundance relations.  This, together
    with positional and radial velocity arguments, suggests that M54 is
    physically associated with Sagittarius.  We note that M54 is
    substantially brighter than the globular cluster members of the Fornax
    dwarf spheroidal galaxy, and that the metal abundance of Sagittarius is
    quite high for its estimated absolute magnitude.