Fading Features Found in the Kinematics of the Far-Reaching Milky Way Stellar Halo

TL;DR

This study investigates the stability of the Milky Way's stellar halo kinematics, revealing that certain velocity features are transient and evolve rapidly, challenging previous assumptions of long-term stability.

Contribution

It demonstrates that observed kinematic features in the stellar halo are transient and evolve quickly, highlighting the need to reconsider models assuming long-term stability.

Findings

Velocity dispersion and anisotropy profiles evolve rapidly within tens of Myr.

The density profile remains largely unchanged despite kinematic evolution.

The observed features are likely transient rather than stable structures.

Abstract

We test the long-term kinematical stability of a Galactic stellar halo model, due to Kafle, et al. (2012), who study the kinematics of approximately 5000 blue horizontal branch (BHB) stars in the Sloan Digital Sky Survey (SDSS). The velocity dispersion $\sigma$ and anisotropy parameter $\beta$ of the stars have been determined as functions of Galactocentric radius, over the range $6<R_\mathrm{GC} < 25$ kpc, and show a strong dip in the anisotropy profile at $R_\mathrm{GC}\sim17$ kpc. By directly integrating orbits of particles in a 3-D model of the Galactic potential with these characteristics, we show that the $\sigma$ and $\beta$ profiles quickly evolve on a time scale of a $\mathrm{few}\times10$ Myr whereas the density $\rho$ profile remains largely unaffected. We suggest that the feature is therefore transient. The origin of such features in the Galactic halo remains unclear.