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-Levy, D. Brooks, E. Buckley-Geer,, D. L. Burke, A. Carnero Rosell, M. Carrasco Kind

TL;DR
This study presents spectroscopic analysis of the Milky Way satellite Tucana III, suggesting it may be a tidally-stripped dwarf galaxy with low metallicity and ambiguous dark matter content, highlighting its proximity and bright member stars.
Contribution
First detailed spectroscopic characterization of Tucana III, providing velocity, metallicity, and mass estimates to inform its classification as a dwarf galaxy or globular cluster.
Findings
Tucana III has a very low velocity dispersion, consistent with a dark matter-dominated dwarf galaxy.
Metallicity spread is small, indicating a possibly simple stellar population.
Tucana III is one of the closest external galaxies, with bright member stars.
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, a velocity dispersion of 0.1^+0.7_-0.1 km/s, and a mean metallicity of [Fe/H] = -2.42^+0.07_-0.08. The upper limit on the velocity dispersion is sigma < 1.5 km/s 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 Msun. We cannot rule out mass-to-light ratios as large as 240 Msun/Lsun 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…
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