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HALO7D II: The Halo Velocity Ellipsoid and Velocity Anisotropy with Distant Main-sequence Stars

Cunningham, Emily C.; Deason, Alis J.; Sanderson, Robyn E.; Sohn, Sangmo Tony; Anderson, Jay; Guhathakurta, Puragra; Rockosi, Constance M.; van der Marel, Roeland P.; Loebman, Sarah R.; Wetzel, Andrew

HALO7D II: The Halo Velocity Ellipsoid and Velocity Anisotropy with Distant Main-sequence Stars Thumbnail


Authors

Emily C. Cunningham

Robyn E. Sanderson

Sangmo Tony Sohn

Jay Anderson

Puragra Guhathakurta

Constance M. Rockosi

Roeland P. van der Marel

Sarah R. Loebman

Andrew Wetzel



Abstract

The Halo Assembly in Lambda Cold Dark Matter: Observations in 7 Dimensions (HALO7D) data set consists of Keck II/DEIMOS spectroscopy and Hubble Space Telescope–measured proper motions of Milky Way halo main-sequence turnoff stars in the CANDELS fields. In this paper, the second in the HALO7D series, we present the proper motions for the HALO7D sample. We discuss our measurement methodology, which makes use of a Bayesian mixture modeling approach for creating the stationary reference frame of distant galaxies. Using the 3D kinematic HALO7D sample, we estimate the parameters of the halo velocity ellipsoid, ávfñ, sr, sf, sq, and the velocity anisotropy β. Using the full HALO7D sample, we find b = - 0.68+0.05 0.04 at árñ = 23 kpc. We also estimate the ellipsoid parameters for our sample split into three apparent magnitude bins; the posterior medians for these estimates of β are consistent with one another. Finally, we estimate β in each of the individual HALO7D fields. We find that the velocity anisotropy β can vary from field-to field, which suggests that the halo is not phase-mixed at árñ = 23 kpc. We explore the β variation across the skies of two stellar halos from the Latte suite of FIRE-2 simulations, finding that both simulated galaxies show β variation over a range similar to that of the variation observed across the four HALO7D fields. The accretion histories of the two simulated galaxies result in different β variation patterns; spatially mapping β is thus a way forward in characterizing the accretion history of the Galaxy.

Citation

Cunningham, E. C., Deason, A. J., Sanderson, R. E., Sohn, S. T., Anderson, J., Guhathakurta, P., …Wetzel, A. (2019). HALO7D II: The Halo Velocity Ellipsoid and Velocity Anisotropy with Distant Main-sequence Stars. Astrophysical Journal, 879(2), Article 120. https://doi.org/10.3847/1538-4357/ab24cd

Journal Article Type Article
Acceptance Date May 24, 2019
Online Publication Date Jul 15, 2019
Publication Date Jul 10, 2019
Deposit Date Aug 2, 2019
Publicly Available Date Mar 28, 2024
Journal Astrophysical Journal
Print ISSN 0004-637X
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 879
Issue 2
Article Number 120
DOI https://doi.org/10.3847/1538-4357/ab24cd

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Copyright Statement
© 2019. The American Astronomical Society. All rights reserved.





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