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Triangulum II
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Title: Triangulum II: Not Especially Dense After All
Author: Evan N. Kirby (1), Judith G. Cohen (1), Joshua D. Simon (2), Puragra Guhathakurta (3), Anders O. Thygesen (1), Gina E. Duggan (1) ((1) Caltech, (2) Carnegie Observatories, (3) UC Santa Cruz)

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. (2015a) identified it as the most dark matter-dominated galaxy known, with a mass-to-light ratio within the half-light radius of 3600 +3500 -2100 M_sun/L_sun. On the other hand, Martin et al. (2016) 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 sigma_v < 3.4 km/s (90% C.L.). Our previous measurement of sigma_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.

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Posts: 128093
Date:
Laevens 2/Triangulum II
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Title: A New Faint Milky Way Satellite Discovered in the Pan-STARRS1 3 pi Survey
Author: Benjamin P. M. Laevens, Nicolas F. Martin, Rodrigo A. Ibata, Hans-Walter Rix, Edouard J. Bernard, Eric F. Bell, Branimir Sesar, Annette M. N. Ferguson, Edward F. Schlafly, Colin T. Slater, William S. Burgett, Kenneth C. Chambers, Heather Flewelling, Klaus A. Hodapp, Nicholas Kaiser, Rolf-Peter Kudritzki, Robert H. Lupton, Eugene A. Magnier, Nigel Metcalfe, Jeffrey S. Morgan, Paul A. Price, John L. Tonry, Richard J. Wainscoat, Christopher Waters

We present the discovery of a faint Milky Way satellite, Laevens 2/Triangulum II, found in the Panoramic Survey Telescope And Rapid Response System (Pan-STARRS 1) 3 pi imaging data and confirmed with follow-up wide-field photometry from the Large Binocular Cameras. The stellar system, with an absolute magnitude of M_V=-1.8 ▒0.5, a heliocentric distance of 30 +2/-2 kpc, and a half-mass radius of 34 +9/-8 pc, shows remarkable similarity to faint, nearby, small satellites such as Willman 1, Segue 1, Segue 2, and Bo÷tes II. The discovery of Laevens 2/Triangulum II further populates the region of parameter space for which the boundary between dwarf galaxies and globular clusters becomes tenuous. Follow-up spectroscopy will ultimately determine the nature of this new satellite, whose spatial location hints at a possible connection with the complex Triangulum-Andromeda stellar structures.

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