A Dynamical and Kinematic Model of the Galactic Stellar Halo and Possible Implications for Galaxy Formation Scenarios

摘要

We reanalyze the kinematics and dynamics of the system of blue horizontal-branch field (BHBF) stars in the Galactic halo (in particular the outer halo), fitting the kinematics with the model of radial and tangential 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 available. 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, σr, of the stellar halo's velocity ellipsoid decreases fairly rapidly beyond the solar circle. The observed decrease is from σr 140 ± 10 km s-1 at the Sun to an asymptotic value of σr = 89 ± 19 km s-1 at large r. The rapid decrease in σr is matched by an increase in the tangential velocity dispersion, σt, with increasing r. Thus, the character of the stellar halo velocity ellipsoid is shown to change markedly from radial anisotropy at the Sun to tangential anisotropy in the outer parts of the Galactic halo (r 20 kpc). The implications of our results for possible Galactic formation scenarios are discussed. Our results may indicate that the Galaxy formed hierarchically (partly or fully) through the merging of smaller subsystems—the "bottom-up" galaxy formation scenario, which for quite a while has been favored by most theorists and recently also has been given some observational credibility by Hubble Space Telescope observations of a potential group of small galaxies, at high redshift, possibly in the process of merging into a larger galaxy.