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Triangulum II. Not Especially Dense After All

Kirby, Evan N. and Cohen, Judith G. and Simon, Joshua D. and Guhathakurta, Puragra and Thygesen, Anders O. and Duggan, Gina E. (2017) Triangulum II. Not Especially Dense After All. Astrophysical Journal, 838 (2). Art. No. 83. ISSN 0004-637X. doi:10.3847/1538-4357/aa6570.

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Among the Milky Way satellites discovered in the past three years, Triangulum II has presented the most difficulty in revealing its dynamical status. Kirby et al. identified it as the most dark-matter-dominated galaxy known, with a mass-to-light ratio within the half-light radius of 3600_(-2100)^(+3500)M_☉L_☉^(-1). On the other hand, Martin et al. measured an outer velocity dispersion that is 3.5 ± 2.1 times larger than the central velocity dispersion, suggesting that the system might not be in equilibrium. From new multi-epoch Keck/DEIMOS measurements of 13 member stars in Triangulum II, we constrain the velocity dispersion to be σ_v < 3.4 km s^(−1) (90% C.L.). Our previous measurement of σ_v, based on six stars, was inflated by the presence of a binary star with variable radial velocity. We find no evidence that the velocity dispersion increases with radius. The stars display a wide range of metallicities, indicating that Triangulum II retained supernova ejecta and therefore possesses, or once possessed, a massive dark matter halo. However, the detection of a metallicity dispersion hinges on the membership of the two most metal-rich stars. The stellar mass is lower than galaxies of similar mean stellar metallicity, which might indicate that Triangulum II is either a star cluster or a tidally stripped dwarf galaxy. Detailed abundances of one star show heavily depressed neutron-capture abundances, similar to stars in most other ultra-faint dwarf galaxies but unlike stars in globular clusters.

Item Type:Article
Related URLs:
URLURL TypeDescription
Kirby, Evan N.0000-0001-6196-5162
Cohen, Judith G.0000-0002-8039-4673
Guhathakurta, Puragra0000-0001-8867-4234
Thygesen, Anders O.0000-0002-4912-1183
Duggan, Gina E.0000-0002-9256-6735
Additional Information:© 2017 American Astronomical Society. Received 2017 January 7. Accepted 2017 March 8. Published 2017 March 29. We thank D. Stern, S. Hemmati, and D. Masters for observing the TriIIc slitmask. 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. Facility: Keck:II (DEIMOS).
Group:Astronomy Department
Subject Keywords:galaxies: abundances – galaxies: dwarf – Local Group
Issue or Number:2
Record Number:CaltechAUTHORS:20170329-095524041
Persistent URL:
Official Citation:Evan N. Kirby et al 2017 ApJ 838 83
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:75507
Deposited By: Ruth Sustaita
Deposited On:29 Mar 2017 19:43
Last Modified:15 Nov 2021 16:34

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