Blue horizontal-branch stars in the sloan digital sky survey. II. Kinematics of the galactic halo

We carry out a maximum-likelihood kinematic analysis of a sample of 1170 blue horizontal-branch (BHB) stars from the Sloan Digital Sky Survey presented by Sirko and coworkers. Monte Carlo simulations and resampling show that the results are robust to distance and velocity errors at least as large as the errors they estimated. The best-fit velocities of the Sun (circular) and halo (rotational) are 245.9 ± 13.5 km s ^-1 and 23.8 ± 20.1 km s ^-1 but are strongly covariant, so that v _⊙ - v _halo = 222.1 ± 7.7 km s ^-1. If one adopts standard values for the local standard of rest and solar motion, then the halo scarcely rotates. The velocity ellipsoid inferred for our sample is much more isotropic [(σ _r, σ _θ, σ φ) = (101.4 ± 2.8, 97.7 ± 16.4, 107.4 ± 16.6) km s ^-1] than that of halo stars in the solar neighborhood, in agreement with a recent study of the distant halo by Sommer-Larsen and coworkers. The line-of-sight velocity distribution of the entire sample, corrected for the Sun's motion, is accurately Gaussian with a dispersion of 101.6 ± 3.0 km s ^-1.