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 Galacto-centric radius r. The basic result is that the radial component sigma(r) of the halo's velocity ellipsoid decreases fairly rapidly beyond the solar circle. The decrease is from sigma(r) congruent-to 140 km s-1 at r = R(.) to an asymptotic value of sigma(r) = 80 +/- 16 km s-1 at large r. The fairly rapid decrease in sigma(r) is matched by an increase in the tangential velocity dispersion sigma(t) with increasing r. Hence the stellar halo velocity ellipsoid is shown to change markedly from radial anisotropy (beta almost-equal-to 1.5) at the Sun to tangential anisotropy (beta almost-equal-to 0.6-0.8) in the outer halo (r less-than-or-similar-to 20 kpc). These observations provide a very interesting new constraint on possible Galactic formation scenarios.