KINEMATICS OF THE SYSTEM OF BLUE HORIZONTAL-BRANCH FIELD STARS OF THEOUTER GALACTIC HALO

We analyse the kinematics and dynamics of the system of blue horizonta l branch field (BHBF) stars in the outer Galactic halo, fitting the ki nematics with a simple model of the radial and tangential velocity dis persions in the halo as functions of Galacto-centric radius r. The bas ic result is that the radial component sigma(r) of the halo's velocity ellipsoid decreases fairly rapidly beyond the solar circle. The decre ase is from sigma(r) congruent-to 140 km s-1 at r = R(.) to an asympto tic 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 veloc ity dispersion sigma(t) with increasing r. Hence the stellar halo velo city ellipsoid is shown to change markedly from radial anisotropy (bet a almost-equal-to 1.5) at the Sun to tangential anisotropy (beta almos t-equal-to 0.6-0.8) in the outer halo (r less-than-or-similar-to 20 kp c). These observations provide a very interesting new constraint on po ssible Galactic formation scenarios.