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Published November 20, 2015 | Published + Submitted
Journal Article Open

Triangulum II: Possibly a Very Dense Ultra-faint Dwarf Galaxy


Laevens et al. recently discovered Triangulum II (Tri II), a satellite of the Milky Way. Its Galactocentric distance is 36 kpc, and its luminosity is only 450 L_⊙. Using Keck/DEIMOS, we measured the radial velocities of six member stars within 1'.2 of the center of Tri II, and we found a velocity dispersion of σ_v = 5.1_(-1.4)^(+4.0) km s^(-1). We also measured the metallicities of three stars and found a range of 0.8 dex in [Fe/H]. The velocity and metallicity dispersions identify Tri II as a dark matter-dominated galaxy. The galaxy is moving very quickly toward the Galactic center v_(GSR) = -262 km s^(-1). Although it might be in the process of being tidally disrupted as it approaches pericenter, there is no strong evidence for disruption in our data set. The ellipticity is low, and the mean velocity, 〈|v_(helio)〉 =-382.1 ± 2.9 km s^(-1), rules out an association with the Triangulum–Andromeda substructure or the Pan-Andromeda Archaeological Survey stellar stream. If Tri II is in dynamical equilibrium, then it would have a mass-to-light ratio of 3600_(-2100)^(+3500) M}_⊙ L_⊙^(-1), the highest of any non-disrupting galaxy (those for which dynamical mass estimates are reliable). The density within the 3D half-light radius would be 4.8_(-3.5)^(+8.1) M⊙ pc^(-3), even higher than Segue 1. Hence, Tri II is an excellent candidate for the indirect detection of dark matter annihilation.

Additional Information

© 2015 The American Astronomical Society. Received 2015 October 12; accepted 2015 November 3; published 2015 November 16. We thank Gina Duggan for obtaining LRIS images, Emily Cunningham for helpful statistics advice, and the anonymous referee for helpful feedback. P.G. acknowledges support from NSF grants AST-1010039 and AST-1412648. We are grateful to the many people who have worked to make the Keck Telescope and its instruments a reality and to operate and maintain the Keck Observatory. The authors wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented herein would have been possible. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Facilities: Keck:I (LRIS) - , Keck:II (DEIMOS). - KECK II Telescope

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Published - Kirby-ApJL-2015pdf.pdf

Submitted - 1510.03856v3.pdf


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August 20, 2023
October 25, 2023