THE ASTRONOMICAL JOURNAL    VOLUME 113, NUMBER 2, PAGE 634     FEBRUARY 1997

         THE KINEMATICS, ORBIT, AND SURVIVAL OF THE SAGITTARIUS DWARF
                               SPHEROIDAL GALAXY

                               RODRIGO A. IBATA

     Department of Physics and Astronomy, University of British Columbia,
                       2219 Main Mall, Vancouver, Canada
                    Electronic mail: ibata@astro.ubc.ca

                              ROSEMARY F. G. WYSE

      Department of Physics and Astronomy, Johns Hopkins University,
                         Baltimore, Maryland 21218
                     Electronic mail: wyse@pha.jhu.edu

                                GERARD GILMORE

   Institute of Astronomy, Madingley Road Cambridge, CB3 0HA, United Kingdom
                    Electronic mail: gil@ast.cam.ac.uk

                               MICHAEL J. IRWIN

               Royal Greenwich Observatory, Madingley Road,
                    Cambridge, CB3 0EZ, United Kingdom
                    Electronic mail: mike@ast.cam.ac.uk

                             NICHOLAS B. SUNTZEFF

           Cerro Tololo Inter-American Observatory, Casilla 603,
                             La Serena, Chile,
                    Electronic mail: nick@ctio.noao.edu

    The Sagittarius dwarf spheroidal galaxy, the closest satellite galaxy
    of the Milky Way, has survived for many orbits about the Galaxy. Extant
    numerical calculations modeled this galaxy as a system with a
    centrally-concentrated mass profile, following the light, and found
    that it should lose more than one-half of its mass every 2-4 orbits and
    be completely disrupted long before now. Apparently the Sagittarius
    dwarf spheroidal, and by implication other dSph galaxies, do not have a
    centrally-concentrated profile for their dark matter. We develop a
    model in which the stars of the Sgr dwarf are embedded in a
    constant-density dark matter halo, representing the core of a
    tidally-limited system, and show that this is consistent with its
    survival. We present new photometric and kinematic observations of the
    Sagittarius dwarf spheroidal and show these data are consistent with
    this explanation for the continued existence of this galaxy. The
    Sagittarius dwarf is being tidally distorted and is tidally limited,
    but is not disrupted as yet. The corresponding minimum total mass is
    10^9 Msun, while the central mass to visual light ratio is ~50 in Solar
    units. Our new photographic photometry allows the detection of
    main-sequence stars of the Sagittarius dwarf over an area of 22 x 8
    deg. The Sagittarius dwarf is prolate, with axis ratios ~3:1:1. For an
    adopted distance of 16+/-2 kpc from the Galactic center on the opposite
    side of the Galaxy to the Sun, the major axis is ~>9 kpc long and is
    aligned approximately normal to the plane of the Milky Way Galaxy,
    roughly following the coordinate line l=5 deg. The central velocity
    dispersion of giant stars which are members of the Sagittarius dwarf is
    11.4+/-0.7 km/s and is consistent with being constant over the face of
    the galaxy. The gradient in mean line-of-sight velocity with position
    along the major axis, dv/db, is ~0 km/s/degree in the central regions
    and increases in amplitude to dv/db=-3 km/s/degree over the outermost
    three degrees for which we have data. A first measurement of the proper
    motion of the Sagittarius dwarf determines the component of its space
    velocity parallel to its major axis to be 250+/-90 km/s, directed
    towards the Galactic Plane. We model these kinematic data to determine
    the orbit of the Sagittarius dwarf. Our best fit model has an orbital
    period of ~<1 Gyr and has the Sagittarius dwarf spheroidal close to
    perigalacticon. This period is shorter, by about a factor of ~>10, than
    the age of the bulk of its stellar population. (Copyright) 1997
    American Astronomical Society.