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

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 $⟨v_l^2{⟩}^{1/2}=138_{-26}^{+43}$ km/s, $⟨v_b^2{⟩}^{1/2}=88_{-17}^{+28}$ km/s, and $⟨v_{\mathrm{LOS}}^2{⟩}^{1/2}=91_{-14}^{+27}$ 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 $\beta$ using 3D kinematics outside of the solar neighborhood. We find $\beta =-0.3_{-0.9}^{+0.4}$, consistent with isotropy and lower than solar neighborhood $\beta$ measurements by 2$\sigma$ (${\beta }_{SN}\sim 0.5-0.7$). 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 $\beta =0.1_{-1.0}^{+0.4}$, which is still lower than solar neighborhood measurements by $1\sigma$. The potential decrease in $\beta$ 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 $r\sim 25$ 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.

9 pages, 6 figures, 2 tables. Accepted for publication in ApJ

References in corpus (14)

• Trigonometric Parallaxes of Massive Star Forming Regions: VI. Galactic Structure, Fundamental Parameters and Non-Circular Motions
• The Milky Way's Circular Velocity Curve to 60 kpc and an Estimate of the Dark Matter Halo Mass from Kinematics of ~2400 SDSS Blue Horizontal Branch Stars
• Two Stellar Components in the Halo of the Milky Way
• Tracing Galaxy Formation with Stellar Halos II: Relating Substructure in Phase- and Abundance-Space to Accretion Histories
• The Dual Origin of Stellar Halos
• Kinematics of the stellar halo and the mass distribution of the Milky Way using BHB stars
• Formation of In Situ Stellar Haloes in Milky Way-Mass Galaxies
• The M31 Velocity Vector. I. Hubble Space Telescope Proper Motion Measurements
• The PAndAS Field of Streams: stellar structures in the Milky Way halo toward Andromeda and Triangulum
• Identifying Contributions to the Stellar Halo from Accreted, Kicked-Out, and In Situ Populations
• TriAnd and its Siblings: Satellites of Satellites in the Milky Way Halo
• Exploring Halo Substructure with Giant Stars: The Nature of the Triangulum-Andromeda Stellar Features
• Discovery of a Stellar Overdensity in Eridanus-Phoenix in the Dark Energy Survey
• Stellar Velocity Dispersion and Anisotropy of the Milky Way Inner Halo

Cited by in corpus (22)

• Co-formation of the Galactic disc and the stellar halo
• Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions
• Ages and kinematics of chemically selected, accreted Milky Way halo stars
• The Halo's Ancient Metal-Rich Progenitor Revealed with BHB Stars
• Absolute HST Proper Motion (HSTPROMO) of Distant Milky Way Globular Clusters: Galactocentric Space Velocities and the Milky Way Mass
• Galactic Archaeology with Gaia
• Aurigaia: mock Gaia DR2 stellar catalogues from the Auriga cosmological simulations
• Anisotropy of the Milky Way's stellar halo using K giants from LAMOST and $Gaia$
• Bayesian Mass Estimates of the Milky Way II: The dark and light sides of parameter assumptions
• Constraints on the assembly history of the Milky Way's smooth, diffuse stellar halo from the metallicity-dependent, radially-dominated velocity anisotropy profiles probed with K giants and BHB stars using LAMOST, SDSS/SEGUE, and Gaia
• The Halos and Environments of Nearby Galaxies (HERON) I: Imaging, Sample Characteristics, and Envelope Diameters
• Characterizing stellar halo populations II: The age gradient in blue horizontal-branch stars
• Elemental Abundances in M31: A Comparative Analysis of Iron and Alpha Element Abundances in the Outer Disk, Giant Stellar Stream, and Inner Halo of M31
• The velocity anisotropy of the Milky Way satellite system
• HALO7D II: The Halo Velocity Ellipsoid and Velocity Anisotropy with Distant Main Sequence Stars
• Beta Dips in the Gaia Era: Simulation Predictions of the Galactic Velocity Anisotropy Parameter for Stellar Halos
• Elemental Abundances in M31: Alpha and Iron Element Abundances from Low-Resolution Resolved Stellar Spectroscopy in the Stellar Halo
• Galactic googly: the rotation-metallicity bias in the inner stellar halo of the Milky Way
• HALO7D I: The Line of Sight Velocities of Distant Main Sequence Stars in the Milky Way Halo
• Reliability of the measured velocity anisotropy of the Milky Way stellar halo
• On the robustness of the velocity anisotropy parameter in probing the stellar kinematics in Milky Way like galaxies: Take away from TNG50 simulation
• HALO7D III: Chemical Abundances of Milky Way Halo Stars from Medium Resolution Spectra