A DYNAMICAL AND KINEMATICAL MODEL OF THE GALACTIC STELLAR HALO AND POSSIBLE IMPLICATIONS FOR GALAXY FORMATION SCENARIOS

We reanalyze the kinematics and dynamics of the system of blue horizon tal-branch held (BHBF) stars in the Galactic halo (in particular the o uter halo), fitting the kinematics with the model of radial and tangen tial velocity dispersions in the halo as a function of Galactocentric distance r proposed by Sommer-Larsen, Flynn, & Christensen but making use of a much larger sample of BHBF stars than was previously availabl e. The present sample consists of nearly 700 Galactic halo BHBF stars. A very good fit to the observations is obtained. The basic result is that the radial component, sigma(r), of the stellar halo's velocity el lipsoid decreases fairly rapidly beyond the solar circle. The observed decrease is from sigma(r) similar or equal to 140 +/- 10 km s(-1) at the Sun to an asymptotic value of sigma(r) = 89 +/- 19 km s(-1) at lar ge r. The rapid decrease in sigma(r) is matched by an increase in the tangential velocity dispersion, sigma(t), with increasing r. Thus, the character of the stellar halo velocity ellipsoid is shown to change m arkedly from radial anisotropy at the Sun to tangential anisotropy in the outer parts of the Galactic halo (r greater than or similar to 20 kpc). The implications of our results for possible Galactic formation scenarios are discussed. Our results may indicate that the Galaxy form ed hierarchically (partly or fully) through the merging of smaller sub systems-the ''bottom-up'' galaxy formation scenario, which for quite a while has been favored by most theorists and recently also has been g iven some observational credibility by Hubble Space Telescope observat ions of a potential group of small galaxies, at high redshift, possibl y in the process of merging into a larger galaxy.