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

Simon, J D and Li, T S and Drlica-Wagner, A and Bechtol, K and Marshall, J L and James, D J and Wang, M Y and Strigari, L and Balbinot, E and Kuehn, K and Walker, A R and Abbott, T M C and Allam, S and Annis, J and Benoit-Lévy, A and Brooks, D and Buckley-Geer, E and Burke, D L and Rosell, A C and Kind, M C and Carretero, J and Cunha, C E and D’Andrea, C B and Costa, L N da and DePoy, D L and Desai, Shantanu and Doel, P and Fernandez, E and Flaugher, B and Frieman, J and García-Bellido, J and Gaztanaga, E and Goldstein, D A and Gruen, D and Gutierrez, G and Kuropatkin, N and Maia, M A G and Martini, P and Menanteau, F and Miller, C J and Miquel, R and Neilsen, E and Nord, B and Ogando, R and Plazas, A A and Romer, A K and Rykoff, E S and Sanchez, E and Santiago, B and Scarpine, V and Schubnell, M and Sevilla-Noarbe, I and Smith, R C and Sobreira, F and Suchyta, E and Swanson, M E C and Tarle, G and Whiteway, L and Yanny, B (2017) Nearest Neighbor: The Low-mass Milky Way Satellite Tucana III. Astrophysical Journal, 838 (1). p. 11. ISSN 1538-4357

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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)(+0.7) km s(-1), and a mean metallicity of = -[Fe/H] = -2.42(-0.08)(+0.07). The upper limit on the velocity dispersion is sigma < 1.5 km s(-1) at 95.5% confidence, and the corresponding upper limit on the mass within the half-light radius of Tuc. III is 9.0. x. 10(4) M-circle dot. We cannot rule out mass-to-light ratios as large as 240. Me/Le 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.

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