A Two Micron All-Sky Survey View of the Sagittarius Dwarf Galaxy. IV. Modeling the Sagittarius Tidal Tails

M giants recovered from the Two Micron All-Sky Survey have recently been used to map the position and velocity distributions of tidal debris from the Sagittarius (Sgr) dwarf spheroidal galaxy around the entire Galaxy. We compare this data set to both test-particle orbits and N-body simulations of satellite destruction run within a variety of rigid Milky Way potentials, and we find that the mass of the Milky Way within 50 kpc of its center should be × 1011 M☉ in order for any Sgr orbit to simultaneously fit the velocity gradient in the Sgr trailing debris and the apocenter of the Sgr leading debris. Orbital pole precession of young debris and leading debris velocities in regions corresponding to older debris provide contradictory evidence in favor of oblate/prolate Galactic halo potentials, respectively, leading us to conclude that the orbit of Sgr has evolved over the past few gigayears. In light of this discrepancy, we consider constraints from only the younger portions of the debris within three models of the flattening of the Galactic potential [q = 0.90 (O), 1.0 (S), and 1.25 (P), i.e., oblate, spherical, and prolate] in our further N-body simulations. On the basis of the velocity dispersion and width along the trailing tidal stream, we estimate the current bound mass of Sgr to be MSgr = × 108 M☉ independent of the form of the Galactic potential; this corresponds to a range of mass-to-light ratios (M/L)Sgr = 14-36(M/L)☉ for the Sgr core. Models with masses in this range best fit the apocenter of leading Sgr tidal debris when they orbit with a radial period of roughly 0.85 Gyr and have perigalactica and apogalactica of about 15 and 60 kpc, respectively. These distances scale with the assumed distance to the Sgr dwarf and the assumed depth of the Galactic potential. The density distribution of debris along the orbit in these models is consistent with the M giant observations, and debris at all orbital phases in which M giants are obviously present is younger (i.e., was lost more recently from the satellite) than the typical age of a Sgr M giant star.

[1]  H. Plummer.  On the Problem of Distribution in Globular Star Clusters: (Plate 8.) , 1911 .

[2]  Jeremiah P. Ostriker,et al.  A self-consistent field method for galactic dynamics , 1992 .

[3]  M. Irwin,et al.  A dwarf satellite galaxy in Sagittarius , 1994, Nature.

[4]  David N. Spergel,et al.  The Disruption of the Sagittarius Dwarf Galaxy. , 1995 .

[5]  S. White,et al.  On the dynamics of the Sagittarius dwarf galaxy , 1995, astro-ph/9503022.

[6]  Rodrigo Ibata,et al.  Sagittarius: the nearest dwarf galaxy , 1995 .

[7]  Kathryn V. Johnston,et al.  Fossil Signatures of Ancient Accretion Events in the Halo , 1995 .

[8]  S. White,et al.  The Structure of cold dark matter halos , 1995, astro-ph/9508025.

[9]  Gerard Gilmore,et al.  The Kinematics, Orbit, and Survival of the Sagittarius Dwarf Spheroidal Galaxy , 1997 .

[10]  D. Edelsohn,et al.  Computer models of the Sagittarius dwarf interaction with the Milky Way , 1997 .

[11]  The APM survey for cool carbon stars in the Galactic halo – I , 1998 .

[12]  K. Johnston.  A Prescription for Building the Milky Way's Halo from Disrupted Satellites , 1997, astro-ph/9710007.

[13]  H. Morrison,et al.  Tracing the Outer Structure of the Sagittarius Dwarf Galaxy:Detections at Angular Distances between 10° and 34° , 1998, astro-ph/9810015.

[14]  Galactic Indigestion: Numerical Simulations of the Milky Way's Closest Neighbor , 1998, astro-ph/9802212.

[15]  S. Majewski,et al.  Starcounts Redivivus. III. A Possible Detection of the Sagittarius Dwarf Spheroidal Galaxy at b = −40° , 1999, astro-ph/9906455.

[16]  Building up the stellar halo of the Galaxy , 1999, astro-ph/9901102.

[17]  S. Majewski,et al.  Constraining the History of the Sagittarius Dwarf Galaxy Using Observations of Its Tidal Debris , 1999, astro-ph/9906457.

[18]  The orbit and mass of the Sagittarius dwarf galaxy , 1999, astro-ph/9908025.

[19]  Accepted... Received...; in original form... , 2000 .

[20]  Interpreting debris from satellite disruption in external galaxies , 2001, astro-ph/0101543.

[21]  R. Ibata,et al.  Great Circle Tidal Streams: Evidence for a Nearly Spherical Massive Dark Halo around the Milky Way , 2000, astro-ph/0004011.

[22]  M. Steinmetz,et al.  The Power Spectrum Dependence of Dark Matter Halo Concentrations , 2000, astro-ph/0012337.

[23]  D. Spergel,et al.  How Lumpy Is the Milky Way’s Dark Matter Halo? , 2001, astro-ph/0111196.

[24]  S. Majewski,et al.  Exploring Halo Substructure with Giant Stars. III. First Results from the Grid Giant Star Survey and Discovery of a Possible Nearby Sagittarius Tidal Structure in Virgo , 2002, astro-ph/0208276.

[25]  Y. Jing,et al.  Triaxial Modeling of Halo Density Profiles with High-Resolution N-Body Simulations , 2002, astro-ph/0202064.

[26]  M. F. Skrutskie,et al.  A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. I. Morphology of the Sagittarius Core and Tidal Arms , 2003, astro-ph/0304198.

[27]  Dynamical friction for dark halo satellites: effects of tidal mass loss and growing host potential , 2004, astro-ph/0403564.

[28]  P. Frinchaboy,et al.  A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. II. Swope Telescope Spectroscopy of M Giant Stars in the Dynamically Cold Sagittarius Tidal Stream , 2004, astro-ph/0403701.

[29]  The distance to the Sagittarius dwarf spheroidal galaxy from the red giant branch tip , 2004, astro-ph/0406350.

[30]  Kathryn V. Johnston,et al.  A TWO MICRON ALL SKY SURVEY VIEW OF THE SAGITTARIUS DWARF GALAXY. III. CONSTRAINTS ON THE FLATTENING OF THE GALACTIC HALO , 2004 .

[31]  TRACING OUT THE NORTHERN TIDAL STREAM OF THE SAGITTARIUS DWARF SPHEROIDAL GALAXY , 2003, astro-ph/0308009.

[32]  Amina Helmi,et al.  Velocity Trends in the Debris of Sagittarius and the Shape of the Dark Matter Halo of Our Galaxy , 2004, astro-ph/0406396.