Kinematics of Metal-poor Stars in the Galaxy. III. Formation of the Stellar Halo and Thick Disk as Revealed from a Large Sample of Nonkinematically Selected Stars

Abstract

We present a detailed analysis of the space motions of 1203 solar-neighborhood stars with metal abundances [Fe/H] ≤ -0.6, on the basis of a catalog, of metal-poor stars selected without kinematic bias recently revised and supplemented by Beers et al. This sample, having available proper motions, radial velocities, and distance estimates for stars with a wide range of metal abundances, is by far the largest such catalog to be assembled to date. We show that the stars in our sample with [Fe/H] ≤-2.2, which likely represent a "pure" halo component, are characterized by a radially elongated velocity ellipsoid (σ U , σ V , σ W ) = (141 ± 11, 106 ± 9, 94 ± 8) km s -1 and small prograde rotation ⟨ V ϕ ⟩ = 30 to 50 km s -1 , consistent with previous analysis of this sample by Beers and Sommer-Larsen based on radial velocity information alone. In contrast to the previous analysis, we find a decrease in ⟨ V ϕ ⟩ with increasing distance from the Galactic plane for stars that are likely to be members of the halo population (Δ⟨ V ϕ ⟩/Δ| Z | = -52 ± 6 km s -1 kpc -1 ), which may represent the signature of a dissipatively formed flattened inner halo. Unlike essentially all previous kinematically selected catalogs, the metal-poor stars in our sample exhibit a diverse distribution of orbital eccentricities, e , with no apparent correlation between [Fe/H] and e . This demonstrates, clearly and convincingly, that the evidence offered in 1962 by Eggen, Lynden-Bell, & Sandage for a rapid collapse of the Galaxy, an apparent correlation between the orbital eccentricity of halo stars with metallicity, is basically the result of their proper-motion selection bias. However, even in our nonkinematically selected sample, we have identified a small concentration of high- e stars at [Fe/H] ∼ -1.7, which may originate, in part, from infalling gas during the early formation of the Galaxy. We find no evidence for an additional thick disk component for stellar abundances [Fe/H] ≤ -2.2. The kinematics of the intermediate-abundance stars close to the Galactic plane are, in part, affected by the presence of a rapidly rotating thick disk component with ⟨ V ϕ ⟩ ≃200 km s -1 (with a vertical velocity gradient on the order of Δ⟨ V ϕ ⟩/Δ| Z | = -30 ± 3 km s -1 kpc -1 ) and velocity ellipsoid (σ U , σ V , σ W ) = (46 ± 4, 50 ± 4, 35 ± 3) km s -1 . The fraction of low-metallicity stars in the solar neighborhood that are members of the thick disk population is estimated as ∼10% for -2.2 < [Fe/H] ≤ -1.7 and ∼30% for -1.7 < [Fe/H] ≤ -1. We obtain an estimate of the radial scale length of the metal-weak thick disk of 4.5 ± 0.6 kpc. We also analyze the global kinematics of the stars constituting the halo component of the Galaxy. The outer part of the halo, which we take to be represented by local stars on orbits reaching more than 5 kpc from the Galactic plane, exhibits no systematic rotation. In particular, we show that previous suggestions of the presence of a "counter-rotating high halo" are not supported by our analysis. The density distribution of the outer halo is nearly spherical and exhibits a power-law profile that is accurately described as ρ ∝ R -3.55±0.13 . The inner part of the halo is characterized by a prograde rotation and a highly flattened density distribution. We find no distinct boundary between the inner and outer halo. We confirm the clumping in angular-momentum phase space of a small number of local metal-poor stars noted in 1999 by Helmi et al. We also identify an additional elongated feature in angular-momentum phase space extending from the clump to regions with high azimuthal rotation. The number of members in the detected clump is not significantly increased from that reported by Helmi et al., even though the total number of the sample stars we consider is almost triple that of the previous investigation. We conclude that the fraction of halo stars that may have arisen from the precursor object of this clump may be smaller than 10% of the present Galactic halo, as previously suggested. The implications of our results for the formation of the Galaxy are discussed, in particular in the context of the currently favored cold dark matter theory of hierarchical galaxy formation.