Kinematics and Galactic Structure

Abstract

A radial velocity and photometric survey of ∼ 900 high proper motion stars in the solar neighbourhood, recently completed at Mount Wilson, is combined with the earlier subdwarf literature to review the dependencies of kinematics on metallicity. The data confirm the conclusions of Roman and of ELS that the oldest stars, with only a very few exceptions, are moving on highly eccentric orbits, and have higher W velocities than younger stars. The interpretation is again put forward for a collapse from a larger volume as the formation process of the Galaxy.

A Galactic component, missed by ELS because of their selection criteria, is a population with the intermediate metallicity of $< \left[ {\frac{{\text{Fe}}} {\text{H}}} \right]\, > \, = \, - 0.6$, with an asymmetric drift of < V > = −30 km s⁻¹ and σw = 42 km s⁻¹ -higher by a factor of 2.5 from the σw = 17 km s⁻¹ of the old disk. This component is identified as the Gilmore-Reid-Wyse thick disk. An interpretation of its formation is discussed as a change in the collapse rate (due to partial pressure support after the main halo phase) relative to the metal enrichment rate, giving the appearance of a separate spatial, kinematic, and metallicity structure.

A rotation of the halo stars, defined by the kinematic criterion that |W| ≥60 km s⁻¹, decreases monotonically with decreasing [Fe/H], indicating spin-up as the Galactic collapse proceeded. This result, contary to that of Norris (1986a), is not due to our sample being kinematically selected. Rather, it could arise either from an error in our photometric distances of the individual stars that increases with decreasing [Fe/H] (i.e., an error that is systematic with metallicity), or it could be real.

The density normalization at the sun, derived in two ways, is 200:22:1 for the thin disk to the thick disk to the halo. From these values, the mass of stars in a spherically symmetric halo is 5 × 10⁹M_⊙, which is less than ∼ 2 percent of the total mass of the Galaxy.

The oldest stars in the thin disk are nearly as old as the globular clusters, showing that the disk began to form near the earliest episodes of Galactic history as in ELS, rather than as a delayed formation at ∼ 1/2 the age of the Galaxy, as in some previous models.