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Isotropic at the Break? 3D Kinematics of Milky Way Halo Stars in the Foreground of M31 | | Emily C. Cunningham
; Alis J. Deason
; Puragra Guhathakurta
; Constance M. Rockosi
; Roeland P. van der Marel
; Elisa Toloba
; Karoline M. Gilbert
; Sangmo Tony Sohn
; Claire E. Dorman
; | | Date: |
9 Feb 2016 | | 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 $langle v^2_l
angle^{1/2} = 138^{+43}_{-26}$ km/s,
$langle v^2_b
angle^{1/2} = 88^{+28}_{-17}$ km/s, and $langle
v^2_{
m{LOS}}
angle^{1/2} = 91^{+27}_{-14}$ 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 $eta$ using 3D kinematics
outside of the solar neighborhood. We find $eta=-0.3^{+0.4}_{-0.9}$,
consistent with isotropy and lower than solar neighborhood $eta$ measurements
by 2$sigma$ ($eta_{SN} sim 0.5-0.7$). 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
$eta=0.1^{+0.4}_{-1.0}$, which is still lower than solar neighborhood
measurements by $1sigma$. The potential decrease in $eta$ 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. | | Source: | arXiv, 1602.3180 | | Services: | Forum | Review | PDF | Favorites | |
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