Kinematics of the system of blue horizontal branch field stars of the outer Galactic halo

We analyse the kinematics and dynamics of the system of blue horizontal branch field (BHBF) stars in the outer Galactic halo, fitting the kinematics with a simple model of the radial and tangential velocity dispersions in the halo as functions of Galactocentric radius r. The basic result is that the radial component σr of the halo’s velocity ellipsoid decreases fairly rapidly beyond the solar circle. The decrease is from σr ≃ 140 km s – 1 at r = R⊙ to an asymptotic value of σr = 80 ± 16 km s–1 at large r. The fairly rapid decrease in σr is matched by an increase in the tangential velocity dispersion σt with increasing r. Hence the stellar halo velocity ellipsoid is shown to change markedly from radial anisotropy (β ≈ 1.5) at the Sun to tangential anisotropy (β ≈ 0.6–0.8) in the outer halo (r ≥ 20 kpc). These bservations provide a very interesting new constraint on possible Galactic formation scenarios.