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Absolute Hubble Space Telescope proper motion (HSTPROMO) of distant Milky Way globular clusters : galactocentric space velocities and the Milky Way mass.

Sohn, Sangmo Tony and Watkins, Laura L. and Fardal, Mark A. and van der Marel, Roeland P. and Deason, Alis J. and Besla, Gurtina and Bellini, Andrea (2018) 'Absolute Hubble Space Telescope proper motion (HSTPROMO) of distant Milky Way globular clusters : galactocentric space velocities and the Milky Way mass.', Astrophysical journal., 862 (1). p. 52.

Abstract

We present Hubble Space Telescope (HST) absolute proper motion (PM) measurements for 20 globular clusters (GCs) in the Milky Way (MW) halo at Galactocentric distances RGC » 10 100 – kpc, with a median per-coordinate PM uncertainty of 0.06 m - as yr 1 . Young and old halo GCs do not show systematic differences in their 3D Galactocentric velocities, derived from combining existing line-of-sight velocities. We confirm the association of Arp 2, Pal 12, Terzan 7, and Terzan 8 with Sgr. These clusters and NGC 6101 have tangential velocity vtan > 290 km s−1 , whereas all other clusters have vtan < 200 km s−1 . NGC 2419, the most distant GC in our sample, is also likely associated with the Sgr stream, whereas NGC 4147, NGC 5024, and NGC 5053 definitely are not. We use the distribution of orbital parameters derived using the 3D velocities to separate halo GCs that either formed within the MW or were accreted. We also assess the specific formation history of, e.g., Pyxis and Terzan 8. We constrain the MW mass via an estimator that considers the full 6D phase-space information for 16 of the GCs from RGC = 10 to 40 kpc. The velocity dispersion anisotropy parameter b = - + 0.609 0.229 0.130. The enclosed mass <=´ - + M M ( ) 39.5 kpc 0.61 10 0.12  0.18 12 , and the virial mass = ´ - + M M vir 2.05 10 0.79  0.97 12 . These are consistent with, but on the high side among, recent mass estimates in the literature.

Item Type:Article
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.3847/1538-4357/aacd0b
Publisher statement:© 2018. The American Astronomical Society. All rights reserved.
Date accepted:14 June 2018
Date deposited:09 August 2018
Date of first online publication:23 July 2018
Date first made open access:09 August 2018

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