Absolute Hubble Space Telescope Proper Motion (HSTPROMO) of Distant Milky Way Globular Clusters: Galactocentric Space Velocities and the Milky Way Mass

Sohn, Sangmo Tony; Watkins, Laura L.; Fardal, Mark A.; van der Marel, Roeland P.; Deason, Alis J.; Besla, Gurtina; Bellini, Andrea

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.

Citation

Sohn, S. T., Watkins, L. L., Fardal, M. A., van der Marel, R. P., Deason, A. J., Besla, G., & Bellini, A. (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), Article 52. https://doi.org/10.3847/1538-4357/aacd0b

• Journal Article Type: Article
• Acceptance Date: Jun 14, 2018
• Online Publication Date: Jul 23, 2018
• Publication Date: Jul 23, 2018
• Deposit Date: Aug 9, 2018
• Publicly Available Date: Aug 9, 2018
• Journal: Astrophysical Journal
• Print ISSN: 0004-637X
• Electronic ISSN: 1538-4357
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 862
• Issue: 1
• Article Number: 52
• DOI: https://doi.org/10.3847/1538-4357/aacd0b
• Public URL: https://durham-repository.worktribe.com/output/1323827