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

We have obtained moderate resolution (~6 km s-1) spectroscopy of several hundred M giant candidates selected from Two Micron All Sky Survey photometry. Radial velocities are presented for stars mainly in the southern Galactic hemisphere, and the primary targets have Galactic positions consistent with association to the tidal tail system of the Sagittarius (Sgr) dwarf galaxy. M giant stars selected from the apparent trailing debris arm of Sgr have velocities showing a clear trend with orbital longitude, as expected from models of the orbit and destruction of Sgr. A minimum 8 kpc width of the trailing stream about the Sgr orbital midplane is implied by verified radial velocity members. The coldness of this stream (σv ~ 10 km s-1) provides upper limits on the combined contributions of stream heating by a lumpy Galactic halo and the intrinsic dispersion of released stars, which is a function of the Sgr core mass. We find that the Sgr trailing arm is consistent with a Galactic halo that contains one dominant, LMC-like lump; however, some lumpier halos are not ruled out. An upper limit to the total mass-to-light ratio of the Sgr core is 21 in solar units. Evidence for other velocity structures is found among the more distant (>13 kpc) M giants. A second structure that roughly mimics expectations for wrapped, leading Sgr arm debris crosses the trailing arm in the southern hemisphere; however, this may also be an unrelated tidal feature. Among the bright, nearby (<13 kpc) M giants toward the south Galactic pole are a number with large velocities that identify them as halo stars; these too may trace halo substructure, perhaps part of the Sgr leading arm near the Sun. The positions and velocities of southern hemisphere M giants are compared with those of southern hemisphere globular clusters potentially stripped from the Sgr system. Support for association of the globular clusters Pal 2 and Pal 12 with Sgr debris is found, based on positional and radial velocity matches. Our discussion includes description of a masked-filtered cross-correlation methodology that achieves better than 1/20 of a resolution element velocities in moderate-resolution spectra. The improved velocity resolution achieved allows tighter constraints to be placed on the coldness of the Sgr stream than previously established.

[1]  J. Gunn,et al.  Dynamical studies of globular clusters based on photoelectric radial velocities of individual stars. I. M3. , 1979 .

[2]  J. Tonry,et al.  A survey of galaxy redshifts. I. Data reduction techniques. , 1979 .

[3]  D. Lin,et al.  On the proper motion of the Magellanic Clouds and the halo mass of our Galaxy , 1982 .

[4]  David W. Latham,et al.  A Search for Spectroscopic Binaries in the Globular Cluster M3 , 1988 .

[5]  Steven R. Majewski,et al.  A complete, multicolor survey of absolute proper motions to B of about 22.5 - Galactic structure and kinematics at the north Galactic pole , 1992 .

[6]  J. Kuhn.  Unbound dwarf spheroidal galaxies and the mass of the Milky Way , 1993 .

[7]  B. Moore.  An Upper Limit to the Mass of Black Holes in the Halo of the Galaxy , 1993, astro-ph/9306004.

[8]  G. Preston,et al.  A Spectroscopic Analysis of 33 of the Most Metal-Poor Stars.I. , 1995 .

[9]  D. Lynden-Bell,et al.  Ghostly streams from the formation of the Galaxy’s halo , 1995 .

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

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

[12]  D. Osterbrock,et al.  NIGHT-SKY HIGH-RESOLUTION SPECTRAL ATLAS OF OH AND O2 EMISSION LINES FOR ECHELLE SPECTROGRAPH WAVELENGTH CALIBRATION , 1996 .

[13]  James E. Hesser,et al.  Spectroscopic Binaries in Globular Clusters. II. A Search for Long-Period Binaries in M22 , 1996 .

[14]  S. White,et al.  A Universal Density Profile from Hierarchical Clustering , 1996, astro-ph/9611107.

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

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

[17]  Tidal Streams as Probes of the Galactic Potential , 1998, astro-ph/9807243.

[18]  George Lake,et al.  Dark Matter Substructure within Galactic Halos , 1999, astro-ph/9907411.

[19]  Terrence M. Girard,et al.  Space Velocities of Globular Clusters. III. Cluster Orbits and Halo Substructure , 1999 .

[20]  Determining the Galactic Mass Distribution Using Tidal Streams from Globular Clusters , 1999, astro-ph/9902226.

[21]  A Search for Tidal Stellar Debris from the Magellanic Clouds: Survey Results from the First Two Years , 1999 .

[22]  Francisco Prada,et al.  Where Are the Missing Galactic Satellites? , 1999, astro-ph/9901240.

[23]  S. R. Majewski,et al.  The Absolute Proper Motion of Palomar 12: A Case for Tidal Capture from the Sagittarius Dwarf Spheroidal Galaxy , 2000, astro-ph/0006314.

[24]  J. Valenti,et al.  Visible and Near Infrared Atlas of the Arcturus Spectrum 3727-9300 A , 2000 .

[25]  Sabine Frink,et al.  A Strategy for Identifying the Grid Stars for the Space Interferometry Mission , 2001 .

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

[27]  J. Navarro,et al.  Halo Substructure and Disk Heating in a Λ Cold Dark Matter Universe , 2001, astro-ph/0106268.

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

[29]  Joachim Stadel,et al.  Tidal debris of dwarf spheroidals as a probe of structure formation models , 2001, astro-ph/0110386.

[30]  Radial velocities of giant M stars near the ecliptic , 2001 .

[31]  Heather A. Rave,et al.  The Ghost of Sagittarius and Lumps in the Halo of the Milky Way , 2001, astro-ph/0111095.

[32]  Uncovering cold dark matter halo substructure with tidal streams , 2001, astro-ph/0110690.

[33]  Yale University,et al.  Remnants of the Sagittarius Dwarf Spheroidal Galaxy around the Young Globular Cluster Palomar 12 , 2002, astro-ph/0203296.

[34]  Christopher Palma,et al.  On the Distribution of Orbital Poles of Milky Way Satellites , 2001, astro-ph/0108474.

[35]  Valentin D. Ivanov,et al.  Red Giant Branch Stars as Probes of Stellar Populations I: 2MASS Calibration and Application to 2MASS GC01 , 2002 .

[36]  P. Frinchaboy,et al.  Exploring Halo Substructure with Giant Stars: Spectroscopy of Stars in the Galactic Anticenter Stellar Structure , 2003, astro-ph/0307505.

[37]  Michele Bellazzini,et al.  Building Up the Globular Cluster System of the Milky Way: The Contribution of the Sagittarius Galaxy , 2003 .

[38]  D. Lamb,et al.  A Low-Latitude Halo Stream around the Milky Way , 2003, astro-ph/0301029.

[39]  R. Ibata,et al.  Tracing the Sgr Stream with 2MASS - Detection of Stream stars around Outer Halo globular clusters , 2003, astro-ph/0304502.

[40]  S. Majewski,et al.  Tracing the Galactic Anticenter Stellar Stream with 2MASS M Giants , 2003 .

[41]  Abundances in Stars from the Red Giant Branch Tip to near the Main-Sequence Turnoff in M5 , 2002, astro-ph/0210245.

[42]  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.