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Title: Nearest neighbor: The low-mass Milky Way satellite Tucana III

Here, 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.) $$\mathrm{km}\,{{\rm{s}}}^{-1}$$, a velocity dispersion of $${0.1}_{-0.1}^{+0.7}$$ $$\mathrm{km}\,{{\rm{s}}}^{-1}$$, and a mean metallicity of $${\rm{[Fe/H]}}=-{2.42}_{-0.08}^{+0.07}$$. The upper limit on the velocity dispersion is σ < 1.5 $$\mathrm{km}\,{{\rm{s}}}^{-1}$$ at 95.5% confidence, and the corresponding upper limit on the mass within the half-light radius of Tuc III is 9.0 × 10 4 M ⊙. We cannot rule out mass-to-light ratios as large as 240 M ⊙/L ⊙ 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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Publication Date:
Report Number(s):
arXiv:1610.05301; FERMILAB-PUB-16-454-AE
Journal ID: ISSN 1538-4357; KJ0402000; KJ0503000; ERKJ311; ERKJEPM; TRN: US1700600
Grant/Contract Number:
AC05-00OR22725; AC02-76SF00515; AC02-07CH11359
Type:
Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 838; Journal Issue: 1; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Research Org:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); SLAC National Accelerator Lab., Menlo Park, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
Contributing Orgs:
The DES Collaboration
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; dark matter; galaxies: dwarf; galaxies: individual (Tucana III); galaxies: stellar content; Local Group; stars: abundances
OSTI Identifier:
1349626
Alternate Identifier(s):
OSTI ID: 1368581; OSTI ID: 1390198