Reliability of the measured velocity anisotropy of the Milky Way stellar halo

Abstract

Determining the velocity distribution of halo stars is essential for estimating the mass of the Milky Way and for inferring its formation history. Since the stellar halo is a dynamically hot system, the velocity distribution of halo stars is well described by the 3-dimensional velocity dispersions $({\sigma }_r,{\sigma }_{\theta },{\sigma }_{\varphi })$, or by the velocity anisotropy parameter $\beta =1-({\sigma }_{\theta }^2+{\sigma }_{\varphi }^2)/(2{\sigma }_r^2)$. Direct measurements of $({\sigma }_r,{\sigma }_{\theta },{\sigma }_{\varphi })$ consistently suggest $\beta =0.5$-$0.7$ for nearby halo stars. In contrast, the value of $\beta$ at large Galactocentric radius $r$ is still controversial, since reliable proper motion data are available for only a handful of stars. In the last decade, several authors have tried to estimate $\beta$ for distant halo stars by fitting the observed line-of-sight velocities at each radius with simple velocity distribution models (local fitting methods). Some results of local fitting methods imply $\beta <0$ at $r\gtrsim 20\mathrm{kpc}$, which is inconsistent with recent predictions from cosmological simulations. Here we perform mock-catalogue analyses to show that the estimates of $\beta$ based on local fitting methods are reliable only at $r\le 15\mathrm{kpc}$ with the current sample size ($\sim 10^3$ stars at a given radius). As $r$ increases, the line-of-sight velocity (corrected for the Solar reflex motion) becomes increasingly closer to the Galactocentric radial velocity, so that it becomes increasingly more difficult to estimate tangential velocity dispersion $({\sigma }_{\theta },{\sigma }_{\varphi })$ from line-of-sight velocity distribution. Our results suggest that the forthcoming Gaia data will be crucial for understanding the velocity distribution of halo stars at $r\ge 20\mathrm{kpc}$.

[Deleted non-relevant figures] ApJ submitted. Comments welcome. 20 pages (14 pages + 6 pages for Appendix). 13 figures. Main result: Fig 7. Schematic diagram: Fig 9. Companion paper to Loebman et al. (2017)

References in corpus (8)

• Two Stellar Components in the Halo of the Milky Way
• Gaia Data Release 1: Astrometry - one billion positions, two million proper motions and parallaxes
• Kinematics of the stellar halo and the mass distribution of the Milky Way using BHB stars
• The Mass Profile of the Galaxy to 80 kpc
• The velocity dispersion and mass profile of the Milky Way
• Satellites of Simulated Galaxies: survival, merging, and their relation to the dark and stellar halos
• Characterizing stellar halo populations II: The age gradient in blue horizontal-branch stars
• Metallicity Bias in the Kinematics of the Milky Way Stellar Halo

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