Nearest Neighbor: The Low-mass Milky Way Satellite Tucana III

J. D. Simon, T. S. Li, A. Drlica-Wagner, K. Bechtol, J. L. Marshall, D. J. James, M. Y. Wang, L. Strigari, E. Balbinot, K. Kuehn, A. R. Walker, T. M.C. Abbott, S. Allam, J. Annis, A. Benoit-Lévy, D. Brooks, E. Buckley-Geer, D. L. Burke, A. Carnero Rosell, M. Carrasco KindJ. Carretero, C. E. Cunha, C. B. D'Andrea, L. N.Da Costa, D. L. Depoy, S. Desai, P. Doel, E. Fernandez, B. Flaugher, J. Frieman, J. García-Bellido, E. Gaztanaga, D. A. Goldstein, D. Gruen, G. Gutierrez, N. Kuropatkin, M. A.G. Maia, P. Martini, F. Menanteau, C. J. Miller, R. Miquel, E. Neilsen, B. Nord, R. Ogando, A. A. Plazas, A. K. Romer, E. S. Rykoff, E. Sanchez, B. Santiago, V. Scarpine, M. Schubnell, I. Sevilla-Noarbe, R. C. Smith, F. Sobreira, E. Suchyta, M. E.C. Swanson, G. Tarle, L. Whiteway, B. Yanny

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Abstract

We present Magellan/IMACS spectroscopy of the recently discovered Milky Way satellite Tucana III (Tuc III). We identify 26 member stars in Tuc III from which we measure a mean radial velocity of v hel = -102.3 ±0.4 (stat.) ±2.0 (sys.)km s-1, a velocity dispersion of 0.1+0.7 -0.1 km s-1, and a mean metallicity of (Fe/H)= -2.42+0.07 -0.08. The upper limit on the velocity dispersion is σ < 1.5 at 95.5% confidence, and the corresponding upper limit on the mass within the half-light radius of Tuc III is 9.0 × 104 M o. We cannot rule out mass-to-light ratios as large as 240 M oL/o for Tuc III, but much lower mass-to-light ratios that would leave the system baryon-dominated are also allowed. We measure an upper limit on the metallicity spread of the stars in Tuc III of 0.19 dex at 95.5% confidence. Tuc III has a smaller metallicity dispersion and likely a smaller velocity dispersion than any known dwarf galaxy, but a larger size and lower surface brightness than any known globular cluster. Its metallicity is also much lower than those of the clusters with similar luminosity. We therefore tentatively suggest that Tuc III is the tidally stripped remnant of a dark matter-dominated dwarf galaxy, but additional precise velocity and metallicity measurements will be necessary for a definitive classification. If Tuc III is indeed a dwarf galaxy, it is one of the closest external galaxies to the Sun. Because of its proximity, the most luminous stars in Tuc III are quite bright, including one star at V = 15.7 that is the brightest known member star of an ultra-faint satellite.

Original languageEnglish
Article number11
JournalAstrophysical Journal
Volume838
Issue number1
DOIs
StatePublished - Mar 20 2017

Funding

This publication is based upon work supported by the National Science Foundation under grant AST-1108811. We thank Dan Kelson for many helpful conversations regarding IMACS data reduction and the anonymous referee for suggestions that improved the paper. E.B. acknowledges financial support from the European Research Council (ERC-StG-335936).

FundersFunder number
National Science FoundationAST-1108811
Seventh Framework Programme1138766, 335936, 1108811
European Research Council

    Keywords

    • Local Group
    • dark matter
    • galaxies: dwarf
    • galaxies: individual (Tucana III)
    • galaxies: stellar content
    • stars: abundances

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