The mass of the Galactic dark matter halo is under vivid discussion .
A recent study by Xue et al .
( 2008 , ApJ , 684 , 1143 ) revised the Galactic halo mass downward by a factor of \sim 2 relative to previous work , based on the line-of-sight velocity distribution of \sim 2400 blue horizontal-branch ( BHB ) halo stars .
The observations were interpreted in a statistical approach using cosmological galaxy formation simulations , as only four of the 6D phase-space coordinates were determined .
Here we concentrate on a close investigation of the stars with highest negative radial velocity from that sample .
For one star , SDSSJ153935.67+023909.8 ( J1539+0239 for short ) , we succeed in measuring a significant proper motion , i.e .
full phase-space information is obtained .
We confirm the star to be a Population II BHB star from an independent quantitative analysis of the SDSS spectrum – providing the first NLTE study of any halo BHB star – and reconstruct its 3D trajectory in the Galactic potential .
J1539+0239 turns out as the fastest halo star known to date , with a Galactic rest-frame velocity of 694 ^ { +300 } _ { -221 } { km s ^ { -1 } } ( full uncertainty range from Monte Carlo error propagation ) at its current position .
The extreme kinematics of the star allows a significant lower limit to be put on the halo mass in order to keep it bound , of M _ { halo } \geq 1.7 _ { -1.1 } ^ { +2.3 } \times 10 ^ { 12 } M _ { \odot } .
We conclude that the Xue et al .
results tend to underestimate the true halo mass as their most likely mass value is consistent with our analysis only at a level of 4 % .
However , our result confirms other studies that make use of the full phase-space information .