Isotropic at the Break? 3D Kinematics of Milky Way Halo Stars in the Foreground of M31

Cunningham, E.C.; Deason, A.J.; Guhathakurta, P.; Rockosi, C.M.; van der Marel, R.P.; Toloba, E.; Gilbert, K.M.; Sohn, S.T.; Dorman, C.E.

Abstract

We present the line-of-sight (LOS) velocities for 13 distant main sequence Milky Way halo stars with published proper motions. The proper motions were measured using long baseline (5-7 years) multi-epoch HST/ACS photometry, and the LOS velocities were extracted from deep (5-6 hour integrations) Keck II/DEIMOS spectra. We estimate the parameters of the velocity ellipsoid of the stellar halo using a Markov chain Monte Carlo ensembler sampler method. The velocity second moments in the directions of the Galactic (l,b, LOS) coordinate system are ⟨v2l⟩1/2=138+43−26 km/s, ⟨v2b⟩1/2=88+28−17 km/s, and ⟨v2LOS⟩1/2=91+27−14 km/s. We use these ellipsoid parameters to constrain the velocity anisotropy of the stellar halo. Ours is the first measurement of the anisotropy parameter β using 3D kinematics outside of the solar neighborhood. We find β=−0.3+0.4−0.9, consistent with isotropy and lower than solar neighborhood β measurements by 2σ (βSN∼0.5−0.7). We identify two stars in our sample that are likely members of the known TriAnd substructure, and excluding these objects from our sample increases our estimate of the anisotropy to β=0.1+0.4−1.0, which is still lower than solar neighborhood measurements by 1σ. The potential decrease in β with Galactocentric radius is inconsistent with theoretical predictions, though consistent with recent observational studies, and may indicate the presence of large, shell-type structure (or structures) at r∼25 kpc. The methods described in this paper will be applied to a much larger sample of stars with 3D kinematics observed through the ongoing HALO7D program.

Citation

Cunningham, E., Deason, A., Guhathakurta, P., Rockosi, C., van der Marel, R., Toloba, E., …Dorman, C. (2016). Isotropic at the Break? 3D Kinematics of Milky Way Halo Stars in the Foreground of M31. Astrophysical Journal, 820(1), Article 18. https://doi.org/10.3847/0004-637x/820/1/18

• Journal Article Type: Article
• Acceptance Date: Jan 31, 2016
• Online Publication Date: Mar 10, 2016
• Publication Date: Mar 10, 2016
• Deposit Date: Feb 20, 2017
• Publicly Available Date: Mar 28, 2017
• Journal: Astrophysical Journal
• Print ISSN: 0004-637X
• Electronic ISSN: 1538-4357
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 820
• Issue: 1
• Article Number: 18
• DOI: https://doi.org/10.3847/0004-637x/820/1/18
• Public URL: https://durham-repository.worktribe.com/output/1385940
• Related Public URLs: https://arxiv.org/abs/1602.03180